kompendium perilaku air dalam tanah

PERILAKU AIR DALAM TANAH

IEKE W.A., WIYONO, S. PRIYONO, dan SOEMARNO 2012

What is Soil Moisture? Lengas Tanah?

Diunduh dari: http://wwwghcc.msfc.nasa.gov/landprocess/lp_home.html …… 11/11/2012

Soil moisture is difficult to define because it means different things in different disciplines.

For example, a farmer's concept of soil moisture is different from that of a water resource manager or a

weather forecaster.

Secara umum, lengas tanah adalah air yang ditahan dalam ruang pori tanah.

Surface soil moisture is the water that is in the upper 10 cm of soil, whereas root zone soil moisture is the water

that is available to plants, which is generally considered to be in the upper 200 cm of soil.

Soil moisture – Lengas Tanah

Diunduh dari: http://en.wikipedia.org/wiki/Water_storage…… 11/11/2012

Lengas tanah merupakan air yang idtahan dalam pori tanah dalam zone perakaran tanaman, biasanya dalam profil tanah hingga

kedalaman 200 cm.

Water storage in the soil profile is extremely important for agriculture, especially in locations that rely on rainfall for

cultivating plants. For example, in Africa rain-fed agriculture accounts for 95% of farmed land.

Water storage is a term used within agriculture to define locations where water is stored for later use. These range from natural water

stores, such as groundwater aquifers, soil water and natural wetlands to small artificial ponds, tanks and reservoirs behind

major dams.

SOIL WATER CONTENT – Kadar Air (Lengas) Tanah

Diunduh dari: http://en.wikipedia.org/wiki/Water_content …… 11/11/2012

Kadar air tanah (lengas tanah) adalah jumlah air yang ada di dalam tanah.

Water content is used in a wide range of scientific and technical areas, and is expressed as a ratio,

which can range from 0 (completely dry) to the value of the materials' porosity at saturation. It can be

given on a volumetric or mass (gravimetric) basis.

Diunduh dari: http://www.engineeringtoolbox.com/soil-water-content-d_1643.html …… 11/11/2012

The water content in soil is also known as moisture content and can be expressed as

w = 100 Mw/Ms

Where: w = moisture content (%)

Mw = mass of water in soil (kg, lb)Ms = dry mass of soil (kg, lb)

The water content test according ASTM D 2216-92 consists of determining the mass of the wet soil specimen and then drying the

soil in an oven 12 - 16 hours at a temperature of 110oC.

KADAR LENGAS TANAH

NERACA AIR – NERACA LENGAS

Diunduh dari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html…… 11/11/2012

The water balance is an accounting of the inputs and outputs of water.

The water balance of a place, whether it be an agricultural field, watershed, or continent, can be determined by calculating the

input, output, and storage changes of water at the Earth's surface.

The major input of water is from precipitation and output is evapotranspiration.

The geographer C. W. Thornthwaite (1899-1963) pioneered the water balance approach to water resource analysis.

He and his team used the water-balance methodology to assess water needs for irrigation and other water-related issues.

Diunduh dari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html …… 11/11/2012

The soil water balance

(After Strahler & Strahler, 2006)

Precipitation (P). Precipitation in the form of rain, snow, sleet, hail, etc.

makes up the primarily supply of water to the

surface. In some very dry locations, water can be

supplied by dew and fog.

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Actual Evapotranspiration (AE).

Evaporation is the phase change from a liquid to a gas releasing water from a wet surface into the air above. Similarly, transpiration is represents a phase

change when water is released into the air by plants.

Evapotranspiration is the combined transfer of water into the air by evaporation and transpiration. Actual evapotranspiration is the amount of water delivered to

the air from these two processes. Actual evapotranspiration is an output of water that is dependent on moisture availability, temperature and humidity.

Think of actual evapotranspiration as "water use", that is, water that is actually evaporating and transpiring given the environmental conditions of a place.

Actual evapotranspiration increases as temperature increases, so long as there is water to evaporate and for plants to transpire.

The amount of evapotranspiration also depends on how much water is available, which depends on the field capacity of soils. In other words, if there is

no water, no evaporation or transpiration can occur

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Potential evapotranspiration (PE).

The environmental conditions at a place create a demand for water. Especially in the case for plants, as as energy input increases, so does the demand for

water to maintain life processes. If this demand is not met, serious consequences can occur. If the demand for water far exceeds that which is

actual present, dry soil moisture conditions prevail. Natural ecosystems have adapted to the demands placed on water.

Potential evapotranspiration is the amount of water that would be evaporated under an optimal set of conditions, among which is an unlimited supply of

water. Think of potential evapotranspiration of "water need". In other words, it would be the water needed for evaporation and transpiration given the local environmental conditions. One of the most important factors that determines water demand is solar radiation. As energy input increases the demand for

water, especially from plants increases. Regardless if there is, or isn't, any water in the soil, a plant still demands water. If it doesn't have access to water, the

plant will likely wither and die.

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Soil Moisture Storage (ST). Soil moisture storage refers to the amount of water held in the soil at any

particular time. The amount of water in the soil depends soil properties like soil texture and organic matter content. The maximum amount of water the soil can hold is called the field capacity. Fine grain soils have larger field capacities than

coarse grain (sandy) soils. Thus, more water is available for actual evapotranspiration from fine soils than coarse soils. The upper limit of soil

moisture storage is the field capacity, the lower limit is 0 when the soil has dried out.

Change in Soil Moisture Storage (ΔST).

The change in soil moisture storage is the amount of water that is being added to or removed from what is stored. The change in soil moisture storage falls

between 0 and the field capacity.

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Deficit (D)

A soil moisture deficit occurs when the demand for water exceeds that which is actually available .

In other words, deficits occur when potential evapotranspiration exceeds actual evapotranspiration (PE>AE). Recalling that PE is water demand and AE is actual water use (which depends on how much water is really available), if we demand more than we have available we will experience a deficit. But, deficits only occur

when the soil is completely dried out. That is, soil moisture storage (ST) must be 0. By knowing the amount of deficit, one can determine how much water is

needed from irrigation sources.

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Surplus (S)

Surplus water occurs when P exceeds PE and the soil is at its field capacity (saturated). That is, we have more water than we actually

need to use given the environmental conditions at a place. The surplus water cannot be added to the soil because the soil is at its field capacity so it runs off the surface. Surplus runoff often ends up in nearby streams causing stream discharge to increase. A knowledge of surplus runoff can help forecast potential flooding

of nearby streams.

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Computing a Soil - Moisture Budget

The best way to understand how the water balance works is to actually calculate a soil water budget.

A knowledge of soil moisture status is important to the agricultural economy of this region that produces mostly corn and soy beans.

To work through the budget, we'll take each month (column) one at a time. It's important to work column by column as we're assessing the moisture status in a given month and one month's value may be determined by what happened in

the previous month.

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Water Budget (location of Rockford, Illinois). Field Capacity = 90 mm

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Water Budget - Rockford, ILField Capacity = 90 mm



J F M A M J J A S O N D Year

P 50 49 66 78 100 106 88 84 86 73 56 45 881

PE 0 0 5 40 84 123 145 126 85 44 8 0 531

P-PE 50 49 61 38 16 -17 -57 -42 1 29 48 45

ΔST 0 0 0 0 0 17 57 16 1 29 48 12

ST 90 90 90 90 90 73 16 0 1 30 78 90

AE 0 0 5 40 84 123 145 100 85 44 8 0 634

D 0 0 0 0 0 0 0 26 0 0 0 0 26

S 50 49 61 38 16 0 0 0 0 0 0 33 258

Soil Moisture Recharge . Field Capacity = 90 mm

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Soil Moisture Recharge - Rockford, IL Field Capacity = 90 mm


P 50 49 66 78 100 106 88 84 86 73 56 45 881

PE 0 0 5 40 84 123 145 126 85 44 8 0 531

P-PE 50 49 61 38 16 -17 -57 -42 1 29 48 45

ΔST 0 0 0 0 0 -17 -57 -16 1 29 48 12

ST 90 90 90 90 90 73 16 0 1 30 78 90

AE 0 0 5 40 84 123 145 100 85 44 8 0 634

D 0 0 0 0 0 0 0 26 0 0 0 0 26

S 50 49 61 38 16 0 0 0 0 0 0 33 258

We'll start the budget process at the end of the dry season when precipitation begins to replenish the soil moisture, called soil moisture recharge, in September. At the beginning of the month the soil is considered dry as the storage in August is equal to zero. During

September, 86 mm of water falls on the surface as precipitation. Potential evapotranspiration requires 85 mm. Precipitation therefore satisfies the need for water

with one millimeter of water left over (P-PE=1). The excess one millimeter of water is put into storage (ΔST=1) bringing the amount in storage to one millimeter (August ST =0 so 0 plus the one millimeter in September equals one millimeter). Actual evapotranspiration is

equal to potential evapotranspiration as September is a wet month (P>PE). There is no deficit during this month as the soil now has some water in it and no surplus as it has

not reached its water holding capacity.

During the month of October, precipitation far exceeds potential evapotranspiration (P-PE=29). All of the excess water is added to the existing soil moisture (ST (September) +

29 mm = 30 mm). Being a wet month, AE is again equal to PE.

Calculating the budget for November is very similar to that of September and October. The difference between P and PE is all allocated to storage (ST now equal to 78 mm) and

AE is equal to PE.

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Soil Moisture Recharge - Rockford, IL Field Capacity = 90 mm

Soil Moisture Surplus

During December, potential evapotranspiration has dropped to zero as plants have gone into a dormant period thus reducing their need for water and cold temperatures inhibit evaporation. Notice that P-PE is equal to 45 but not all is

placed into storage. Why? At the end of November the soil is within 12 mm of being at its field

capacity. Therefore, only 12 millimeters of the 45 available is put in the soil and the remainder runs off as surplus (S=33).

Given that the soil has reached its field capacity in December, any excess water that falls on the surface in January will likely generate surplus runoff. According to the water budget table this is indeed true. Note that P-PE is 50 mm and ΔST is

0 mm. What this indicates is that we cannot change the amount in storage as the soil is at its capacity to hold water. As a result the amount is storage (ST)

remains at 90 mm. Being a wet month (P>PE) actual evapotranspiration is equal to potential evapotranspiration. Note that all excess water (P-PE) shows up as

surplus (S=50 mm).

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Soil Moisture Surplus . Field Capacity = 90 mm

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P 50 49 66 78 100 106 88 84 86 73 56 45 881

PE 0 0 5 40 84 123 145 126 85 44 8 0 531

P-PE 50 49 61 38 16 -17 -57 -42 1 29 48 45

ΔST 0 0 0 0 0 -17 -57 -16 1 29 48 12

ST 90 90 90 90 90 73 16 0 1 30 78 90

AE 0 0 5 40 84 123 145 100 85 44 8 0 634

D 0 0 0 0 0 0 0 26 0 0 0 0 26

S 50 49 61 38 16 0 0 0 0 0 0 33 258

Given that the soil has reached its field capacity in December, any excess water that falls on the surface in January will likely generate surplus runoff. According to the water

budget table this is indeed true. Note that P-PE is 50 mm and ΔST is 0 mm. What this indicates is that we cannot change the amount in storage as the soil is at its capacity to hold water. As a result the amount is storage (ST) remains at 90 mm. Being a wet month (P>PE) actual evapotranspiration is equal to potential evapotranspiration. Note that all

excess water (P-PE) shows up as surplus (S=50 mm).

Similar conditions occur for the months of February, March, April, and May. These are all wet months and the soil remains at its field capacity so all excess water becomes surplus. Note too that the values of PE are increasing through these months. This indicates that plants are springing to life and transpiring water. Evaporation is also

increasing as insolation and air temperatures are increasing. Notice how the difference between precipitation and potential evapotranspiration decreases through these months. As the demand on water increases, precipitation is having a harder time satisfying it. As

a result, there is a smaller amount of surplus water for the month.

Surplus runoff can increase stream discharge to the point where flooding occurs. The flood duration period lasts from December to May (6 months), with the most intense

flooding is likely to occur in March when surplus is the highest (61 mm).….

Surplus Lengas Tanah


Soil Moisture Utilization. Field Capacity = 90 mm

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P 50 49 66 78 100 106 88 84 86 73 56 45 881

PE 0 0 5 40 84 123 145 126 85 44 8 0 531

P-PE 50 49 61 38 16 -17 -57 -42 1 29 48 45

ΔST 0 0 0 0 0 -17 -57 -16 1 29 48 12

ST 90 90 90 90 90 73 16 0 1 30 78 90

AE 0 0 5 40 84 123 145 100 85 44 8 0 634

D 0 0 0 0 0 0 0 26 0 0 0 0 26

S 50 49 61 38 16 0 0 0 0 0 0 33 258

By the time June rolls around, temperatures have increased to the point where evaporation is proceeding quite rapidly and plants are requiring more water to keep them healthy. As potential evapotranspiration is approaching its maximum value during these warmer months, precipitation is falling off. During June P-PE is -17 mm. What this means is precipitation no longer is able to meet the demands of potential evapotranspiration. In

order to meet their needs, plants must extract water that is stored in the soil from the previous months. This is shown in the table by a value of 17 in the cell for ΔST (change

in soil storage). Once the 17 m is taken out of storage (ST) it reduces its value to 73.

The month of June is considered a dry month (P<PE) so AE is equal to precipitation plus the absolute value of ΔST (P + |ΔST|). When we complete this calculation (106 mm + 17 mm = 123 mm) we see that AE is equal to PE. What this means is precipitation and what

was extracted from storage was able to meet the needs demanded by potential evapotranspiration. Note that there is no surplus in June as the soil moisture storage has dropped below its field capacity. There is still no deficit as water remains in storage. The

calculations for July is similar to June, just different values. Note that by the time July ends, water held in storage is down to a mere 16 mm.

Soil Moisture Utilization


Soil Moisture DeficitNERACA AIR



P 50 49 66 78 100 106 88 84 86 73 56 45 881

PE 0 0 5 40 84 123 145 126 85 44 8 0 531

P-PE 50 49 61 38 16 -17 -57 -42 1 29 48 45

ΔST 0 0 0 0 0 -17 -57 -16 1 29 48 12

ST 90 90 90 90 90 73 16 0 1 30 78 90

AE 0 0 5 40 84 123 145 100 85 44 8 0 634

D 0 0 0 0 0 0 0 26 0 0 0 0 26

S 50 49 61 38 16 0 0 0 0 0 0 33 258

August, like June and July, is a dry month. Potential evapotranspiration still exceeds precipitation and the difference is a -

42 mm.

Up until this month there has been enough water from precipitation and what is in storage to meet the demands of potential evapotranspiration. However,

August begins with only 16 mm of water in storage (ST of July). Thus we'll only be able to extract 16 mm of the 42 mm of water needed to meet the demands of potential evapotranspiration So, of the 42 mm of water we would need (P-PE) to

extract from the soil. In so doing, the amount in storage (ST) falls to zero and the soil is dried out.

What happens to the remaining 26 mm of the original P-PE of 42? The unmet need for water shows up as soil moisture deficit. In other words, we have not been able to meet our need for water from both precipitation and what we can extract from storage. AE is therefore equal to 100 mm (84 mm of precipitation

plus 16 mm of ΔST).….

Soil Moisture Deficit


Soil Moisture SeasonsFour soil moisture seasons can be defined by the soil moisture conditions.


Recharge The recharge season is a time when water is added to soil moisture storage (+ΔST). The recharge period occurs when precipitation exceeds potential evapotranspiration but the soil has yet to reach its field capacity.

Surplus

The surplus season occurs when precipitation exceeds potential

evapotranspiration and the soil has reached its field capacity. Any additional water

applied to the soil runs off. If this water runs off into nearby streams and rivers it could

cause flooding. Thus, the intensity (amount) and duration (length of season) of surplus

can be used to predict the severity of potential flooding.

http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/Rockford_Water_Budget_Seasons.jpg

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Utilization The utilization season is a time when water is withdrawn from soil moisture storage (-

ΔST). The utilization period occurs when potential evapotranspiration exceeds precipitation but soil storage has yet to reach 0 (dry soil).

DeficitThe deficit season occurs when occurs when potential evapotranspiration exceeds precipitation and soil storage has reached 0. This is a time when there is essentially

no water for plants. Farmers then tap ground water reserves or water in nearby streams and lakes to irrigate their crops. Thus, the intensity (amount) and duration

(length of season) of deficit can be used to predict the need for irrigation water.Whether a place experiences all four seasons depends on the climate and soil

properties. Wet climate and those places with soils having high field capacities are less likely to experience a deficit period. Likewise the duration and intensity of any

season will be determined by the climate and soil properties. Given equal amounts of precipitation, coarse textured soils will generate runoff faster than fine textured soils

and may experience more intense surplus

AIR DALAM TANAH

STRUKTUR & CIRI H2O

Molekul air terdiri atas atom oksigen dan dua atom hidrogen, yang berikatan secara kovalenAtom-atom tidak terikat secara linear (H-O-H), tetapi atom hidrogen melekat pada atom oksigen seperti huruf V dengan sudut 105o.

Molekul air bersifat dipolar: Zone elektro positif

+

H H 105o

Zone elektro negatif -

Lingkaran Tanah-Air-Tanaman

LTAT mrpk sistem dinamik dan terpadu dimana air mengalir dari tempat dengan tegangan rendah menuju tempat dengan

tegangan air tinggi.

Serapan bulu akarPenguapan

Hilang melalui stomata daun (transpirasi)

Air kembali ke atmosfer

(evapo-transpirasi)

Air dikembalikan ke tanah melalui hujan

dan irigasi

Structure of water transport model for the soil-leaf continuum, with the inputs outlined in boxes.

Root and shoot components are represented by a resistance network, each component of which varies according to the inputted K(y) function from

vulnerability curves of xylem.

Layers of roots reach to different soil depths according to an inputted root area profile. Canopy layers reflect an inputted leaf area and Y profile.

Soil is modeled as a rhizosphere resistance connecting roots to bulk soil of an inputted y and K(y).

The model predicts transpiration (E) as a function of the inputs.

SISTEM TANAH-TANAMAN

30

Kekuatan ikatan antara molekul air dengan partikel tanah dinyatakan dengan TEGANGAN AIR TANAH. Ini merupakan fungsi dari gaya-gaya adesi dan kohesi di antara molekul - molekul air dan

partikel tanah

Partikel tanahH2O

Adesi Kohesi

Air terikat Air bebas

Air Tersedia untuk pertumbuhan tanaman

32

).

Fine textured soils with small pores can hold the greatest

amounts of PAW.

Coarse textured sandy soils with large pores can hold the least

amounts of PAW.

Status Air Tanah

Perubahan status air dalam tanah, mulai dari kondisi jenuh hingga titik layu

Jenuh Kap. Lapang Titik layu

100g 8g udara

Padatan Pori

100g 20g udara

100g 10 g udara

100g air 40g tanah jenuh air

kapasitas lapang

koefisien layu

koefisien higroskopis

34

TEGANGAN &

KADAR AIR

PERHATIKANLAH proses yang terjadi kalau tanah basah dibiarkan mengering. Bagan berikut melukiskan hubungan antara tebal lapisan air di sekeliling partikel tanah dengan tegangan air

Bidang singgung tanah dan air Koef. Koef. Kapasitaspadatan tanah higroskopis layu lapang

10.000 atm 31 atm 15 atm 1/3 atm

10.000 atm Mengalir krn gravitasi

Tegangan air

1/3 atm

tebal lapisan air

Representasi bola air yang menyelubungi partikel padatan tanah

The amount of soil water is usually measured in terms of water content as percentage by volume or mass, or as soil water potential. Water content

does not necessarily describe the availability of the water to the plants, nor indicates, how the water moves within the soil profile. The only

information provided by water content is the relative amount of water in the soil.

Soil water potential, which is defined as the energy required to remove water from the soil, does not directly give the amount of water present in the root zone either. Therefore, soil water content and soil water potential should both be considered when dealing with plant growth and irrigation.

The soil water content and soil water potential are related to each other, and the soil water characteristic curve provides a graphical

representation of this relationship.

JUMLAH AIR DALAM TANAH

37

TEGANGAN vs

kadar air

Kurva tegangan - kadar air tanah bertekstur lempung

Tegangan air, bar

31 Koefisien higroskopis

Koefisien layu

Kapasitas lapang 0.1 Kap. Lapang maksimum

persen air tanah

Air kapilerAir Air tersediahigros-kopis Lambat tersedia Cepat tersedia Air gravitasi

Zone optimum

Hubungan antara kadar air tanah dan tegangan air tanah untuk tekstur lempung

Kadar air volumetrik, %

Air GravitasiAir tersedia bagi

tanaman

Kapasitas lapang

Titik Layu

Tega

ngan

air

tana

h (b

ar /

atm

39

STRUKTUR &

CIRI

POLARITASMolekul air mempunyai dua ujung, yaitu ujung oksigen yg elektronegatif dan ujung hidrogen yang elektro-positif.Dalam kondisi cair, molekul-molekul air saling bergandengan membentuk kelompok-kelompok kecil tdk teratur.Ciri polaritas ini menyebabkan plekul air tertarik pada ion-ion elektrostatis. Kation-kation K+, Na+, Ca++ menjadi berhidrasi kalau ada molekul air, membentuk selimut air, ujung negatif melekat kation.Permukaan liat yang bermuatan negatif, menarik ujung positif molekul air.

Kation hidrasi Tebalnya selubung air tgtpd rapat muatan pd per-mukaan kation.

Rapat muatan = Selubung air muatan kation / luas permukaan

40

STRUKTUR &

CIRI

IKATAN HIDROGEN Atom hidrogen berfungsi sebagai titik penyambung (jembatan)

antar molekul air.Ikatan hidrogen inilah yg menyebabkan titik didih dan viskositas

air relatif tinggi

KOHESI vs. ADHESIKohesi: ikatan hidrogen antar molekul airAdhesi: ikatan antara molekul air dengan permukaan padatan lainnyaMelalui kedua gaya-gaya ini partikel tanah mampu menahan air dan mengendalikan gerakannya dalam tanah

TEGANGAN PERMUKAAN Terjadinya pada bidang persentuhan air dan udara, gaya kohesi antar molekul air lebih besra daripada adhesi antara air dan udara.

UdaraPermukaan air-udara

air

41

ENERGI AIR TANAH

Retensi dan pergerakan air tanah melibatkan energi, yaitu: Energi Potensial, Energi Kinetik dan Energi Elektrik.

Selanjutnya status energi dari air disebut ENERGI BEBAS, yang merupakan PENJUMLAHAN dari SEMUA BENTUK

ENERGI yang ada.Air bergerak dari zone air berenergi bebas tinggi (tanah basah)

menuju zone air berenergi bebas rendah (tanah kering).

Gaya-gaya yg berpengaruhGaya matrik: tarikan padatan tanah (matrik) thd molekul air; Gaya osmotik: tarikan kation-kation terlarut thd molekul airGaya gravitasi: tarikan bumi terhadap molekul air tanah.

Potensial air tanahKetiga gaya tersebut di atas bekerja bersama mempengaruhi energi bebas air tanah, dan selanjutnya menentukan perilaku air tanah, ….. POTENSIAL TOTAL AIR TANAH (PTAT)PTAT adalah jumlah kerja yg harus dilakukan untuk memindahkan secara berlawanan arah sejumlah air murni bebas dari ketinggian tertentu secara isotermik ke posisi tertentu air tanah.PTAT = Pt = perbedaan antara status energi air tanah dan air murni bebas

Pt = Pg + Pm + Po + …………………………

( t = total; g = gravitasi; m = matrik; o = osmotik)

Hubungan potensial air tanah dengan energi bebas

Energi bebas naik bila air tanah berada pada letak ketinggian yg lebih tinggi dari titik baku pengenal (referensi)

+

0

-

Poten-sial

positif

Poten-sial

negatif

Energi bebas dari air murni Potensial tarikan bumi

Menurun karena pengaruh osmotik

Menurun karena pengaruh matrik

Energi bebas dari air tanah

Potensial osmotik (hisapan)

Potensial matrik (hisapan)

43

POTENSIAL AIR TANAH

POTENSIAL TARIKAN BUMI = Potensial gravitasi

Pg = G.hdimana G = percepatan gravitasi, h = tinggi air tanah di atas posisi ketinggian referensi.Potensial gravitasi berperanan penting dalam menghilangkan kelebihan air dari bagian atas zone perakaran setelah hujan lebat atau irigasi

Potensial matrik dan OsmotikPotensial matrik merupakan hasil dari gaya-gaya jerapan dan kapilaritas.Gaya jerapan ditentukan oleh tarikan air oleh padatan tanah dan kation jerapanGaya kapilaritas disebabkan oleh adanya tegangan permukaan air.Potensial matriks selalu negatifPotensial osmotik terdapat pd larutan tanah, disebabkan oleh adanya bahan-bahan terlarut (ionik dan non-ionik).Pengaruh utama potensial osmotik adalah pada serapan air oleh tanaman

Hisapan dan Tegangan Potensial matrik dan osmotik adalah negatif, keduanya bersifat menurunkan energi bebas air tanah. Oleh karena itu seringkali potensial negatif itu disebut HISAPAN atau TEGANGAN.Hisapan atau Tegangan dapat dinyatakan dengan satuan-satuan positif.Jadi padatan-tanah bertanggung jawab atas munculnya HISAPAN atau TEGANGAN.

Cara Menyatakan

Tegangan Energi

Tegangan: dinyatakan dengan “tinggi (cm) dari satuan kolom air yang bobotnya sama dengan tegangan tsb”.

Tinggi kolom air (cm) tersebut lazimnya dikonversi menjadi logaritma dari sentimeter tinggi kolom air,

selanjutnya disebut pF.

Tinggi unit Logaritma Bar Atmosferkolom air (cm) tinggi kolom air (pF)

10 1 0.01 0.0097 100 2 0.1 0.0967 346 2.53 0.346 1.3 1000 3 110000 4 10 9.674915849 4.18 15.8 1531623 4.5 31.6 31100.000 5 100 96.7492

KANDUNGAN AIR DAN

TEGANGAN

KURVA ENERGI - LENGAS TANAH Tegangan air menurun secara gradual dengan meningkatnya

kadar air tanah.Tanah liat menahan air lebih banyak dibanding tanah pasir pada

nilai tegangan air yang samaTanah yang Strukturnya baik mempunyai total pori lebih

banyak, shg mampu menahan air lebih banyakPori medium dan mikro lebih kuat menahan air dp pori makro

Tegangan air tanah, Bar 10.000

Liat

Lempung

Pasir

0.0110 Kadar air tanah, % 70

Tekstur tanah dan air tersedia

47

Hubungan antara kadar air tanah dengan tegangan air tanah

Kapasitas air tersedia dalam tanah yang teksturnya berbeda-beda

49

Gerakan Air Tanah

Tidak Jenuh

Gerakan tidak jenuh = gejala kapilaritas = air bergerak dari muka air tanah ke atas melalui pori mikro.Gaya adhesi dan kohesi bekerja aktif pada kolom air (dalam pri mikro), ujung kolom air berbentuk cekung.Perbedaan tegangan air tanah akan menentukan arah gerakan air tanah secara tidak jenuh.

Air bergerak dari daerah dengan tegangan rendah (kadar air tinggi) ke daerah yang tegangannya tinggi (kadar air rendah, kering).Gerakan air ini dapat terjadi ke segala arah dan berlangsung secara terus-menerus.

Pelapisan tanah berpengaruh terhadap gerakan air tanah.Lapisan keras atau lapisan kedap air memperlambat gerakan air Lapisan berpasir menjadi penghalang bagi gerakan air dari lapisan yg bertekstur halus.Gerakan air dlm lapisan berpasir sgt lambat pd tegangan

50

Gerakan Jenuh (Perkolasi)

Air hujan dan irigasi memasuki tanah, menggantikan udara dalam pori makro - medium - mikro. Selanjutnya air bergerak ke bawah melalui proses gerakan jenuh dibawah pengaruh gaya gravitasi dan kapiler.Gerakan air jenuh ke arah bawah ini berlangsung terus selama cukup air dan tidak ada lapisan penghalang

Lempung berpasir Lempung berliat

cm 0

15 mnt 4 jam 30 60

90 1 jam 24 jam

120 24 jam 48 jam

150 30 cm 60 cm Jarak dari tengah-tengah saluran, cm

Pola Penetrasi dan Pergerakan Air pada tanah Berpasir dan tanah Lempung-liat

52Pola pergerakan air gravitasi dalam tanah

Pengaruh struktur tanah terhadap pergerakan air tanah ke arah bawah

54

PERKOLASI

Jumlah air perkolasiFaktor yg berpengaruh:

1. Jumlah air yang ditambahkan2. Kemampuan infiltrasi permukaan tanah3. Daya hantar air horison tanah4. Jumlah air yg ditahan profil tanah pd kondisi kapasitas lapang

Keempat faktor di atas ditentukan oleh struktur dan tekstur tanah

Tanah berpasir punya kapasitas ilfiltrasi dan daya hantar air sangat tinggi, kemampuan menahan air rendah, shg perkolasinya mudah dan cepat

Tanah tekstur halus, umumnya perkolasinya rendah dan sangat beragam; faktor lain yg berpengaruh:1. Bahan liat koloidal dpt menyumbat pori mikro & medium2. Liat tipe 2:1 yang mengembang-mengkerut sangat berperan

55

LAJU GERAKAN AIR

TANAH

Kecepatan gerakan air dlm tanah dipengaruhi oleh dua faktor:1. Daya dari air yang bergerak2. Hantaran hidraulik = Hantaran kapiler = daya hantar

i = k.fdimana i = volume air yang bergerak; f = daya air yg bergerak dan k = konstante.

Daya air yg bergerak = daya penggerak, ditentukan oleh dua faktor:1. Gaya gravitasi, berpengaruh thd gerak ke bawah2. Selisih tegangan air tanah, ke semua arah

Gerakan air semakin cepat kalau perbedaan tegangan semakin tinggi.

Hantaran hidraulik ditentukan oleh bbrp faktor:1. Ukuran pori tanah2. Besarnya tegangan untuk menahan air

Pada gerakan jenuh, tegangan airnya rendah, shg hantaran hidraulik berbanding lurus dengan ukuran poriPd tanah pasir, penurunan daya hantar lebih jelas kalau terjadi penurunan kandungan air tanahLapisan pasir dlm profil tanah akan menjadi penghalang gerakan air tidak jenuh

Gerakan air tanah

Gerakan air tanah dipengaruhi oleh kandungan air tanah

Penetrasi air dari tnh basah ke tnh kering(cm) 18

Tanah lembab, kadar air awal 29%

Tanah lembab, kadar air awal 20.2%

Tanah lembab, kadar air awal 15.9%

0 26 156

Jumlah hari kontak, hari

Sumber: Gardner & Widtsoe, 1921.

57

GERAKAN UAP AIR

Penguapan air tanah terjadi internal (dalam pori tanah) dan eksternal (di permukaan tanah)

Udara tanah selalu jenus uap air, selama kadar air tanah tidak lebih rendah dari koefisien higroskopis (tegangan 31

atm).

Mekanisme Gerakan uap airDifusi uap air terjadi dlm udara tanah, penggeraknya adalah perbedaan tekanan uap air.Arah gerapan menuju ke daerah dg tekanan uap rendah

Pengaruh suhu dan lengas tanah terhadap gerapan uap air dalam tanah

Lembab Dingin Kering Dingin

Kering Panas Lembab Panas

58

RETENSI AIR TANAH

KAPASITAS RETENSI MAKSIMUM adalah: Kondisi tanah pada saat semua pori terisi penuh air, tanah jenuh air, dan tegangan matrik adalah nol.KAPASITAS LAPANG: air telah meninggalkan pori makro, mori makro berisi udara, pori mikro masih berisi air; tegangan matrik 0.1 - 0.2 bar; pergerakan air terjadi pd pori mikro/ kapiler

KOEFISIEN LAYU: siang hari tanaman layu dan malam hari segar kembali, lama-lama tanaman layu siang dan malam; tegangan matrik 15 bar.Air tanah hanya mengisi pori mikro yang terkecil saja, sebagian besar air tidak tersedia bagi tanaman.Titik layu permanen, bila tanaman tidak dapat segar kembali

KOEFISIEN HIGROSKOPISMolekul air terikat pada permukaan partikel koloid tanah, terikat kuat sehingga tidak berupa cairan, dan hanya dapat bergerak dlm bentuk uap air, tegangan matrik-nya sekitar 31 bar.Tanah yg kaya bahan koloid akan mampu menahan air higroskopis lebih banyak dp tanah yg miskin bahan koloidal.

59

Klasifikasi Air Tanah

Klasifikasi Fisik:1. Air Bebas (drainase)2. Air Kapiler3. Air Higroskopis

Air Bebas (Drainase):a. Air yang berada di atas kapasitas lapangb. Air yang ditahan tanah dg tegangan kurang dari 0.1-0.5 atmc. Tidak diinginkan, hilang dengan drainased. Bergerak sebagai respon thd tegangan dan tarika gravitasi bumie. Hara tercuci bersamanya

AIR KAPILER: a. Air antara kapasitas lapang dan koefisien higroskopisb. Tegangan lapisan air berkisar 0.1 - 31 atmc. Tidak semuanya tersedia bagi tanamand. Bergerak dari lapisan tebal ke lapisan tipise. Berfungsi sebagai larutan tanah

AIR HIGROSKOPIS : a. Air diikat pd koefisien higroskopisb. Tegangan berkisar antara 31 - 10.000 atmc. Diikat oleh koloid tanahd. Sebagian besar bersifat non-cairane. Bergerak sebagai uap air

Agihan air dalam tanah

Berdasarkan tegangan air tanah dapat dibedakan menjadi

tiga bagian: Air bebas, kapiler dan higroskopis

Koef. Higroskopis Kap. Lapang Jml ruang pori kurang lebih 31 atm kurang lebih 1/3 atm

Lapisan olah

Air higros- Air Kapiler Ruang diisi udara kopik Peka thd gerakan Biasanya jenuh uap air Hampir tdk kapiler, laju pe- Setelah hujan lebat menunjukkan nyesuaian me- sebagian diisi air, sifat cairan ningkat dg me- tetapi air cepat hi- ningkatnya ke- lang krn gravitasi lembaban tanah bumi Lapisan bawah tanah

Karena pemadatan ruang pori berkurang

Strata bawah (jenuh air)

Kolom tanah Jumlah ruang pori

Klasifikasi Biologi

Air tanah

Klasifikasi berdasarkan ketersediaannya bagi tanaman:1. AIR BERLEBIHAN: air bebas yg kurang tersedia bagi tanaman.

Kalau jumlahnya banyak berdampak buruk bagi tanaman, aerasi buruk, akar kekurangan oksigen, anaerobik, pencucian air

2. AIR TERSEDIA: air yg terdapat antara kap. Lapang dan koef. Layu. Air perlu ditambahkan untuk mencapai pertumbuhan tanaman yang optimum apabila 50 - 85% air yg tersedia telah habis terpakai.Kalau air tanah mendekati koefisien layu, penyerapan air oleh akar tanaman tdk begitu cepat dan tidak mampu mengimbangi pertumbuhan tanaman

3. AIR TIDAK TERSEDIA: AIR yg diikat oleh tanah pd TITIK LAYU permanen, yaitu air higroskopis dan sebagian kecil air kapiler.

KH KL KP 100 % pori 31 atm 15 atm 1/3 atm Air Air Ruang udara dan Higroskopis Kapiler air drainase

Tdk tersedia Tersedia Berlebihan Daerah Optimum

Faktor yg mempengaruhi Air Tersedia

Faktor yg berpengaruh:1. Hubungan tegangan dengan kelengasan2. Kedalaman tanah3. Pelapisan Tanah

TEGANGAN MATRIK : tekstur, struktur dan kandungan bahan organik mempengaruhi jumlah air yg dapat disediakan tanah bagi tanaman

TEGANGAN OSMOTIK: adanya garam dalam tanah meningkatkan tegangan osmotik dan menurunkan jumlah air tersedia, yaitu menaikkan koefisien layu.

Persen air Sentimeter air setiap 30 cm tanah

1018 Kap. Lapang

Air tersedia

Koef. Layu 5 6 Air tidak tersedia

Pasir Sandy loam Loam Silty-loam Clay-loam Liat

Tekstur semakin halus

KEHILANGAN UAP AIR

DARI TANAH

HADANGAN HUJAN OLEH TUMBUHANTajuk tumbuhan mampu menangkap sejumlah air hujan, sebagian air ini diuapkan kembali ke atmosfer.Vegetasi hutan di daerah iklim basah mampu menguapkan kembali air hujan yg ditangkapnya hingga 25%, dan hanya 5% yg mencapai tanah melalui cabang dan batangnya.

Awan hujan Pembentukan Awan

Tanah permukaan

Groundwater Batuan

Sungai - laut

presipitasi

infiltrasi

perkolasi

Run off

transpirasi

evaporasi

Pengendalian Penguapan

MULSA & PENGELOLAAN Mulsa adalah bahan yg dipakai pd permukaan tanah untuk

mengurangi penguapan air atau untuk menekan pertumbuhan gulma.

Lazimnya mulsa spt itu digunakan untuk tanaman yang tidak memerlukan pengolahan tanah tambahan

MULSA KERTAS & PLASTIKBahan mulsa dihamparkan di permukaan tanah, diikat spy tdk terbang, dan tanaman tumbuh melalui lubang-lubang yg telah disiapkanSelama tanah tertutup mulsa, air tanah dapat diawetkan dan pertumbuhan gulma dikendalikan

MULSA SISA TANAMAN Bahan mulsa berasal dari sisa tanaman yg ditanam sebelumnya, misalnya jerami padi, jagung, dan lainnyaBahan mulsa dipotong-potong dan disebarkan di permukaan tanahCara WALIK DAMI sebelum penanaman kedelai gadu setelah padi sawah

MULSA TANAH Pengolahan tanah Efektivitas mulsa tanah dalam konservasi air-tanah (mengendalikan evaporasi) masih diperdebatkan, hasil-hasil penelitian masih snagat beragam

Olah Tanah vs Penguapan Air Tanah

Alasan pengolahan tanah: 1. Mempertahankan kondisi fisika tanah yg memuaskan2. Membunuh gulma3. Mengawetkan air tanah.

Pengendalian Penguapan vs Pemberantasan Gulma

Perlakuan Hasil jagung (t/ha) Kadar air tanah (%) hingga kedalaman 1 m

Tanah dibajak dg persiapan yg baik 1. Dibebaskan dari gulma 2.9 22.3 2. Gulma dibiarkan tumbuh 0.4 21.8 3. Tiga kali pengolahan dangkal 2.5 21.9Persiapan Buruk 4. Dibebaskan dari gulma 2.0 23.1

Sumber: Mosier dan Gutafson, 1915.

Pengolahan tanah yg dapat mengendalikan gulma dan memperbaiki kondisi fisik tanah akan berdampak positif thd produksi tanaman

Pengolahan tanah yg berlebihan dapat merusak akar tanaman dan merangsang evaporasi, shg merugikan tanaman

Soil "holds" water available for crop use, retaining it against the pull of

gravity.

This is one of the most important physical facts for agriculture.

If the soil did not hold water, if water was free to flow downward with the pull of gravity as in a river or canal, we would have to constantly irrigate,

or hope that it rained every two or three days.

There would be no reason to pre-irrigate. And there would be no such

thing as dryland farming. www.icsu-scope.org/.../scope56/Chapter05.html

KAPASITAS SIMPANAN AIR TANAH

Hubungan antara Potensial Air Tanah dnegan Air Tersedia pada tiga macam tekstur tanah

Potensial air tanah (-bar)

Air tersedia (%

)

Kapasitas lapang

Titik layu permanen

The soil's ability to hold water depends on both the soil texture and

structure. Texture describes the relative

percentages of sand, silt, and clay particles.

The finer the soil texture (higher percentage of silt and clay), the more

water soil can hold.

Gravity is always working to pull water downwards below the plant's

root zone. To counteract the pull of gravity, soil is

able to generate its own forces, commonly called "matric forces"

("matric" because of the soil "matrix" structure that forms the basis for the

forces).

www.ctahr.hawaii.edu/MauiSoil/a_comp03.aspx

69

An important fact about the soil's water-holding forces is that as the level

of soil moisture goes down, the soil generates more force.

This is the reason that some water will move up into the root zone from a

shallow ground water table. As the plant extracts water in the root zone, the soil pulls water up from the area with more

water to the area with less.

As you would expect, the rate at which the water-holding forces go up with

decreasing soil moisture is different for different soils. In a coarse soil, they will

go up slowly. This means that plants can extract a

great amount of water from coarse soils before they stress. In contrast, these

forces rise quickly in finer soils. www.fao.org/docrep/R4082E/r4082e03.htm

Lapisan olah

Lapisan olah dalam

Lapisan subsoil

Lapisan bahan induk

HUBUNGAN TANAH – AIR - TANAMAN

www.fao.org/docrep/R4082E/r4082e03.htm

The role of soil in the soil-plant-atmosphere continuum is unique. It has been demonstrated that soil is not essential for plant growth and

indeed plants can be grown hydroponically (in a liquid culture).

However, usually plants are grown in the soil and soil properties directly affect the availability of water and nutrients to plants.

Soil water affects plant growth directly through its controlling effect on plant water status and indirectly through its effect on aeration,

temperature, and nutrient transport, uptake and transformation.

The understanding of these properties is helpful in good irrigation design and management.

HUBUNGAN TANAH - AIR

73

The soil system is composed of three major components: solid particles (minerals and organic

matter), water with various dissolved chemicals, and air.

The percentage of these components varies greatly with

soil texture and structure.

An active root system requires a delicate balance between the

three soil components; but the balance between the liquid and gas phases is most critical, since

it regulates root activity and plant growth process.

The amount of soil water is usually measured in terms of

water content as percentage by volume or mass, or as soil

water potential.

Water content does not necessarily describe the

availability of the water to the plants, nor indicates, how the water moves within the soil

profile.

The only information provided by water content is the relative

amount of water in the soil.

Jumlah air tersedia dipengaruhi tekstur tanah

Air Tersedia

Air Tidak Tersedia

Kapasitas Lapang

Titik Laytu

Persen Air

Inchi Air per foot tanah

Soil water potential, which is defined as the energy required to remove water from the soil,

does not directly give the amount of water present in the

root zone either.

Therefore, soil water content and soil water potential should

both be considered when dealing with plant growth and

irrigation.

The soil water content and soil water potential are related to each other, and the soil water

characteristic curve provides a graphical representation of this

relationship. Air dalam tanah (% berat kering)

Pote

nsia

l air

tana

h (M

Pa)

Kapasitas lapang

Titik layu permanen -1.5MPa

76

The nature of the soil characteristic curve depends on the physical properties of the soil namely, texture and structure. Soil

texture refers to the distribution of the soil particle sizes.

The mineral particles of soil have a wide range of sizes classified as sand, silt, and

clay.

The proportion of each of these particles in the soil determines its texture.

All mineral soils are classified depending on their texture. Every soil can be placed

in a particular soil group using a soil textural triangle .

For example a soil with 60% sand and 10% clay separates is classified as a Sandy

loam

There are limits on the amount of water that soil holds for crop use. The upper limit is termed "field capacity".

During an irrigation, or whenever excess water is added to soil, water drains down through the soil due to the pull of gravity.

At first, this internal drainage is relatively rapid.

However, it soon slows to almost nothing. (The increasing soil water-holding forces finally start to counteract

gravity.) At this point we would say the soil is at field capacity.

KAPASITAS LAPANG

You can demonstrate field capacity using a visualization of a sponge (like soil, a porous material that will hold

water). Using a pan of water, hold a sponge

under water until it is saturated. Now, pull the sponge out of the water.

It will immediately start to drip water, quickly at first, then slower and

slower. At some point it will essentially stop

dripping. The internal drainage has stopped and

the sponge is at field capacity.

It is very important to note that you can soak more water into soil that is

already at field capacity. There will be open soil pores that will take the water. However, the excess

water will not be held. It will just drain down until the soil moisture returns to field capacity.

79

Field capacity is a soil-based concept.

That is, it depends on the texture and structure of the soil as well as the physical

conditions in the field.

Coarse soils have lower field capacities than fine soils.

If there is a high water table or severe stratification that would restrict drainage, the

field capacity would be higher than normal.

KAPASITAS LAPANG

The water held by the soil between field capacity and permanent wilting point is termed the "available water holding capacity" of the

soil.

It is water that is "available" for the plant to use. Water added to the soil in excess of field capacity will drain down, below the active root

system.

Water held by the soil that is below the permanent wilting point is of no use, the plant has died.

As a crop manager you are concerned with the soil moisture throughout the depth of the plant's active root system, the "effective

root zone".

AIR TERSEDIA & ZONE AKAR

The effective root zone is that depth of soil where you want to control soil moisture (just as you control fertility

and weed/pest pressures).

The effective root zone may or may not be the actual depth of all active

roots. It may be shallower because of concerns for crop quality or

development (as with many vegetable crops).

For example, with cotton you may estimate the effective root zone as 6 feet for a preirrigation, 2 feet for the

first seasonal irrigation, 4 feet for the second seasonal, and 6 feet

thereafter. For an almond orchard, you may estimate the effective root

zone as four feet for the entire season. With onions, the major concern is with the top 2 feet.

The soil is composed of three major parts: air, water, and solids . The solid component forms the framework of the soil and consists of mineral and

organic matter. The mineral fraction is made up of sand, silt, and clay particles. The

proportion of the soil occupied by water and air is referred to as the pore volume.

The pore volume is generally constant for a given soil layer but may be altered by tillage and compaction. The ratio of air to water stored in the

pores changes as water is added to or lost from the soil. Water is added by rainfall or irrigation, as shown in Figure 2.

Water is lost through surface runoff, evaporation (direct loss from the soil to the atmosphere), transpiration (losses from plant tissue), and either

percolation (seepage into lower layers) or drainage.

HUBUNGAN AIR & TANAH

83

Saturated (wet) soil. All pores (light areas) are filled with water. The dark areas represent soil solids.

Water distribution in a soil at field capacity. Capillary water (lightly shaded areas ) in soil pores is available to plants. Field capacity represents the upper limit of

plant-available water.

Water distribution in a soil at thw wilting point. This water is held tightly in thin films around soil particles and is unavailable to plants. The wilting point

represents the lower limit of plant-available water.

HUBUNGAN ANTARA AIR-TERSEDIA DAN DISTRIBUSI AIR DALAM TANAH .

Kapasitas tanah menyimpan air

Jumlah air tanah pada tiga macam tekstur tanah

Air

dala

m ta

nah

(in/ft

)

89

Jumlah air tersedia dalam tanah yang teksturnya berbeda-beda

Kalau tanaman menyerap air dari tanah , jumlah air tersedia yang tersisa dalam tanah menjadi berkurang.

The amount of PAW removed since the last irrigation or rainfall is the depletion volume.

Irrigation scheduling decisions are often based on the assumption that crop yield or quality will not be reduced as long as the amount of water used by

the crop does not exceed the allowable depletion volume. The allowable depletion of PAW depends on the soil and the crop. For

example, consider corn growing in a sandy loam soil three days after a soaking rain.

Even though enough PAW may be avai1able for good plant growth, the plant may wilt during the day when potential evapotranspiration (PET) is high.

AIR TANAH & STRES TANAMAN

Evapotranspiration merupakan proses hilangnya air tanah ke atmosfer, melalui evaporasi dari permukaan tanah dan proses transpirasi dari

tanaman yang tumbuh di tanah .

Potential evapotranspiration is the maximum amount of water that could be lost through this process under a given set of atmospheric conditions,

assuming that the crop covers the entire soil surface and that the amount of water present in the soil does not limit the process.

Potential evapotranspiration is controlled by atmospheric conditions and is higher during the day. Plants must extract water from the soil that is

next to the roots. As the zone around the root begins to dry, water must move through the

soil toward the root (Figure 7). Daytime wilting occurs because PET is high and the plant takes up water faster than the water can be replaced.

AIR TANAH & STRES TANAMAN

Hubungan antara distribusi air dalam tanah dan konsep jadwal irigasi ketika 50 percent air tersedia telah habis

Ketersediaan air tanah bagi tanaman

Jumlah air tanah tersedia dalam berbagai tipe tanah

Pengaruh Potensial Air tanah thd konduktivitas hidraulik tanah

Kapasitas lapang

Tititk layu permanen

Efek potensial air tanah thd konduktivitas hidraulik

Kon

dukt

ivita

s hi

drau

lik ta

nah

Potensial air tanah

Pola penyerapan air oleh tanaman yang tumbuh pada profil tanah yang tidak mempunyai lapisan penghambat dan suplai air tersedia cukup di

seluruh zone perakaran tanaman

Soil is made up of soil particles in crumb-form (peds), and pore spaces around the soil crumbs.

In a well-structured soil, these crumbs are nice and stable....but in a poorly structured soil, the crumbs are unstable which often limits

pore-space.

The pore-spaces are necessary for holding water, and for the free gaseous exchange of oxygen and carbon dioxide between the plant

roots and the soil surface (respiration process).

There are three types of soil water (ie. water in the soil).

AIR DALAM TANAH

98

AIR GRAVITASI

This is the water which is susceptible to the forces of gravity. It exists after significant rainfall, and after substantial irrigation. This is the water which fills

all the pore-space, and leaves no room for oxygen and gaseous exchange. In "light" soils, this tends to drain away quickly. In heavy soils, this can take time.

AIR KAPILERThis is the water which is held with the force of SURFACE TENSION by the soil

particles, and is resistent to the forces of gravity. This is the water which is present after the gravitational water has drained away, leaving spaces free for

gaseous exchange. When the soil is holding it's MAXIMUM capillary water (after the gravitational water has drained), this is called FIELD CAPACITY. At

this point, the plant is able to take up water easily, and has the oxygen that it needs in the root zone.

AIR -TANAH

Adalah air yang diikat sedemikian kuat (oleh tegangan permukaan) ke partikel tanah sehingga akar tanaman tidak dapat menyerapnya.

Sehingga air ini tidak tersedia bagi tanaman.

At this stage there's generally sufficient oxygen, but there just isn't enough available water.

The plant wilts, and will eventually die if it doesn't get water.

When the plant wilts and is unable to recover, this is called the TITIK LAYU PERMANEN

AIR HIGROSKOPIS

The closer to the soil particle the water is held, the tighter it's held. And the further from the particle, the looser it's held. It takes little energy for the plant roots to take up the

water that's far from the particle and is present at the field capacity point. By contrast, as the water is used up (or evaporates), it takes more and more energy for the plant to take

up water.

I often use the analogy of drinking through a straw. A short straw, ie. when a cup is 15 cm away from you, is easy to use. A one-metre long straw takes a lot of energy to suck up a drink. A twenty-metre straw is impossible to use. It works much the same with plants. The more the soil dries out, the more energy the plant needs to output in order to get a

decent drink. The effect of increased soil salinity (due to high soil salinity, high soil-water salinity, or both) has basically the same effect as a soil drying out. Salt in the soil has as osmotic

effect, and causes the water to be held more tightly around the soil particles.

Semakin tinggi tingkat salinitas tanahnya, tanaman semakin sulit menyerap air, meskipun air itu ada dalam tanah.

TITIK LAYU PERMANEN

Representasi ketersediaan air dalam tanah bagi pertumbuhan tanaman

Tegangan air tanah (kPa, sekala log)

Kadar air tanah (%; mm/100mm)

In other words, Plant Available Water (PAW) is the amount of water held in a soil between the limits of Field Capacity and Permanent

Wilting Point.

However, only the water near to Field Capacity may be Readily Available Water (RAW).

This is particularly so for fine textured, clayey soils because a high proportion of PAW is held in small pores and as thin films and

plants need to 'do more work' to extract this fraction of water from soils.

AIR –TANAH TERSEDIA

http://www.westone.wa.gov.au/toolbox6/hort6/html/resources/glossary/glossary_fs.htm

Not all PAW is equally available to plants.

As soils dry out and PAW approaches PWP, plants will come under water-stress and wilt. It is the objective of irrigators to avoid this

situation.

They prefer to irrigate when the soil water content is about 50% of FC or about 100kPa.

These limits, however, are set by the irrigator to suit the business enterprise. For example, if growth rates are to be restricted then

the trigger for an irrigation event may be 300kPa.

As the name suggests, Readily Available Water or RAW is the amount and availability of water in soils that is readily available to

plants.

AIR –TANAH MUDAH TERSEDIA

Following rainfall, or irrigation, all the pores in soil will be filled with water; this is the Saturation Water Content (SWC). With time the water in the largest pores

will drain to depth due to gravitational forces.

In coarser textured, sandy and loamy soils this drainage will take place in less than a day and will, therefore, be unavailable to plants.

Fine-textured, clayey soils, however, may be somewhat poorly drained and all pores may remain filled with water for several days.

In these cases some of the SWC may be available for EvapoTranspiration and would need to be considered in calculations of soil water balances and

irrigation scheduling. Poorly drained soils, however, are less suitable for irrigation.

They are difficult to manage and may be waterlogged for times that can cause damage to plants for reasons of anaerobic root environments.

AIR –TANAH TERSEDIA

During long-continued heavy rains, infiltration of soil water continues under the force of gravity, carrying the water down to successively greater

depths. Soil pores become filled with water, with only a small amount of free air remaining entrapped in bubbles.

The soil may, for a time, become almost completely saturated with water. Downward percolation continues beyond the soil water belt into the

intermediate belt, a zone too deep to be reached by plat roots. Water may ultimately reach the ground-water zone below .

After the rain has ceased, water continues to drain downward under the influence of gravity, but some remains held in the soil, clinging to the soil

grains in thin films, by the force of capillary tension. This is the same force that causes ink to be drawn upward in a piece of

blotting paper and which permits small water droplets to cling to the side of a vertical pane of glass. Films of capillary water in the soil remain held

in place until gradually dissipated by evaporation or drawn into root systems.

PERGERAKAN AIR TANAH

http://www.nwroc.umn.edu/Cropping_issues/2004/issue4/soil_txtr.jpg

After soil has been saturated by prolonged rains and then drains until no more water moves downward under the force of gravity, the soil is said to

be holding its field capacity of water. Most excess water drains out in a day’s time; usually not more that two or three days are required for

gravity drainage to cease. Soil-moisture content can be stated in terms of the equivalent depth in

inches of water in a given thickness of soil. At field capacity, soil-moisture content ranges from 1 to 4 inches per foot of soil, depending upon soil

texture . Sandy soils have low field capacity, which is rapidly reached because of the

ease with which the water penetrates the large openings (macro pores). Clay soils, on the other hand, have a high field capacity, but require much

longer periods to attain it because of the slow rate of water penetration due to the much smaller openings (micro pores).

A comparable, but lower value of soil moisture is the wilting point, below which foliage wilts because of the inability of the plants to extract the

remaining moisture .

PERGERAKAN AIR TANAH

Air tanah pada berbagai kondisi kelengasan (kadar air)

WATER STORAGE IN

SOIL

Proses Simpanan lengas dalam tanah.

Diunduh dari: http://www.floodsite.net/juniorfloodsite/html/en/student/thingstoknow/hydrology/waterstorage2.html …… 11/11/2012

Rain water can also be stored in the ground. Soils consist of particles and pores. Those pores can be filled with air but also with water. The amount of pores is a soil is different for different types of soil. The pores in a clay soil

account for 40% to 60% of the volume. In fine sand this can be 20%–45%

The soil particles have small pores in them where water can enter (soil water) and between the particles are larger pores that can be filled. The soil is filled

with water up a certain level. This level goes up and down with changing weather conditions. This water level is the ground water level.

The process of water entering the soil is called infiltration. When the soil has taken up all the water it can, we say that it is saturated. If you walk over a saturated soil, you feel that it is wet and soggy, like biscuits dipped in tea.

Part of the water that infiltrates, will move on. It will go to underground storage reservoirs or to underground rivers and may, through ground water flows,

eventually reach a river or a lake. Another part will be used by plants or will evaporate.

Diunduh dari: http://www.floodsite.net/juniorfloodsite/html/en/student/thingstoknow/hydrology/waterstorage2.html …… 11/11/2012

Proses Simpanan lengas dalam tanah.

http://www.floodsite.net/juniorfloodsite/images/content/waterstoragesoil.png

KAPASITAS SIMPANAN LENGAS TANAH

Diunduh dari: http://www.agf.gov.bc.ca/resmgmt/publist/600Series/619000-1.pdf …… 11/11/2012

For irrigation the soil water storage (SWS) capacity is defined as the total amount of water that is stored in the soil within the plant’s root zone. The soil

texture and the crop rooting depth determine this. A deeper rooting depth means there is a larger volume of water stored in the soil and therefore a larger

reservoir of water for the crop to draw upon between irrigations.

Only a portion of the total soil water is readily available for plant use. Plants can only extract a portion of the stored water without being stressed. An availability coefficient is used to calculate the percentage of water that is readily available

to the plant. The maximum soil water deficit (MSWD) (also referred to as the management allowable deficit) is the amount of water stored in the soil that is

readily available to the plant.

The crop should be irrigated once this amount of moisture has been removed from the soil. Once depleted this is the amount that must be replenished by

irrigation. It is also the maximum amount that can be applied at one time, before the risk of deep percolation occurs. However, in some cases leaching of salts is

desirable and extra irrigation would be desired.


HOW TO DETERMINE THE SOIL WATER STORAGE AND THE MAXIMUM SOIL WATER DEFICIET

Step 1 Determine the crop rooting depth, RD (m)Step 2 Determine the available water storage capacity of the soil, AWSC (mm/m), Table 2Step 3 Calculate the total soil water storage, SWS (mm)

SWS (mm) = RD (m) x AWSC (mm/m) …………… (Equation 1)

Step 4 Determine the availability coefficient of the water to the crop, AC (%)Step 5 Calculate the maximum soil water Deficit, MSWD (mm)

MSWD = SWS (mm) x AC (%) …………….. (Equation 2)

Effective Rooting Depth of Mature Cropsfor Irrigation System Design


A Guide to Available Water Storage Capacities of Soils


SOIL WATER STORAGE CAPACITY.


Availability Coefficients

Infiltration and Soil Water StoragePidwirny, M. (2006). "Infiltration and Soil Water Storage". Fundamentals of Physical Geography, 2nd Edition.

Infiltration

Infiltration refers to the movement of water into the soil layer. The rate of this movement is called the infiltration rate. If rainfall intensity is greater than the infiltration rate, water will accumulate on the surface and runoff will begin.

Movement of water into the soil is controlled by gravity, capillary action, and soil porosity. Of these factors soil porosity is most important. A soil's porosity

is controlled by its texture, structure, and organic content. Coarse textured soils have larger pores and fissures than fine-grained soils and therefore allow for

more water flow. Pores and fissures found in soils can be made larger through a number of factors that enhance internal soil structure. For example, the

burrowing of worms and other organisms and penetration of plant roots can increase the size and number of macro and micro-channels within the soil.

The amount of decayed organic matter found at the soil surface can also enhance infiltration. Organic matter is generally more porous than mineral soil

particles and can hold much greater quantities of water.Diunduh dari: http://www.physicalgeography.net/fundamentals/8l.html…… 11/11/2012

Infiltration and Soil Water Storage

Infiltration

The rate of infiltration normally declines rapidly during the early part of a rainstorm event and reaches a constant value after several hours of rainfall.

A number of factors are responsible for this phenomena, including:

1. The filling of small pores on the soil surface with water reduces the ability of capillary forces to actively move water into the soil.

2. As the soil moistens, the micelle structure of the clay particles absorb water causing them to expand. This expansion reduces the size of soil pores.

3. Raindrop impact breaks large soil clumps into smaller particles. These particles then clog soil surface pores reducing the movement of water into the soil.

Diunduh dari: http://www.physicalgeography.net/fundamentals/8l.html…… 11/11/2012

SIMPANAN LENGAS TANAH


Within the soil system, the storage of water is influenced by several different forces. The strongest force is the molecular force of elements and compounds found on the

surface of soil minerals.

The water retained by this force is called hygroscopic water and it consists of the water held within 0.0002 millimeters of the surface of soil particles. The maximum limit of this water around a soil particle is known as the hygroscopic coefficient.

Hygroscopic water is essentially non-mobile and can only be removed from the soil through heating.

Matric force holds soil water from 0.0002 to 0.06 millimeters from the surface of soil particles. This force is due to two processes: soil particle surface molecular

attraction (adhesion and absorption) to water and the cohesion that water molecules have to each other. This force declines in strength with distance from the soil

particle. The force becomes nonexistent past 0.06 millimeters.



Capillary action moves this water from areas where the matric force is low to areas where it is high. Because this water is primarily moved by capillary

action, scientists commonly refer to it as capillary water. Plants can use most of this water by way of capillary action until the soil

wilting point is reached.

Water in excess of capillary and hygroscopic water is called gravitational water.

Gravitational water is found beyond 0.06 millimeters from the surface of soil particles and it moves freely under the effect of gravity.

When gravitational water has drained away the amount of water that remains is called the soil's field capacity.

TEGANGAN AIR TANAH

The relationship between the thickness of water film around soil particles and the strength

of the force that holds this water. Force is measured in units called bars. One bar is

equal to a 1000 millibars.

The graph also displays the location of hygroscopic water, the hygroscopic coefficient, the wilting point, capillary water, field capacity, and

gravitational water along this line.


http://www.physicalgeography.net/physgeoglos/b.html

Soil Water DynamicsO'Geen, A. T. (2012) Soil Water Dynamics. Nature Education Knowledge 3(6):12.

Diunduh dari: http://www.nature.com/scitable/knowledge/library/soil-water-dynamics-59718900…… 11/11/2012

Stored water in soil is a dynamic property that changes spatially in

response to climate, topography and soil properties, and temporally as a

result of differences between utilization and redistribution via

subsurface flow.

Changes in soil moisture storage can be generalized with a mass

balance equation , as a result of the difference between the amount of water added and that which is lost

(Hillel 1982).

Conceptual diagram of a soil profile illustrating the multiple flow paths

through which water moves through soil (Modified from O’Geen et al.

2010)

Change in soil moisture storage = inputs – outputs.

Diunduh dari: …… 11/11/2012

Water content increases (positive change in storage) when inputs including precipitation or irrigation exceed outputs. Water content decreases (negative change in storage) when outputs such as deep percolation, surface runoff,

subsurface lateral flow, and evapotranspiration (ET) exceed inputs.

Water storage and redistribution are a function of soil pore space and pore-size distribution, which are governed by texture and structure. Generally speaking,

clay-rich soils have the largest pore space, hence the greatest total water holding capacity. However, total water holding capacity does not describe how

much water is available to plants, or how freely water drains in soil. These processes are governed by potential energy. Water is stored and redistributed

within soil in response to differences in potential energy. A potential energy gradient dictates soil moisture redistribution and losses, where water moves

from areas of high- to low-potential energy (Hillel 1982). When at or near saturation, soils typically display water potentials near 0 MPa. Negative water potentials arise as soil dries resulting in suction or tension on

water allowing the soil to retain water like a sponge.

Water storage and redistribution are a function of soil pore space and pore-size distribution, which are governed by texture and structure.


Water content and water potential at saturation,

field capacity and permanent wilting point.

The difference in water content between field

capacity and permanent wilting point is plant

available water.

Drainable porosity is the amount of water that

drains from macropores by gravity between saturation to field capacity typically

representing three days of drainage in the field.

.

Influence of Texture and Structure.

Diunduh dari: http://www.nature.com/scitable/knowledge/library/soil-water-dynamics-59718900 …… 11/11/2012

Texture and structure determine pore size distribution in soil, and therefore, the amount of

PAW. Coarse textured soils (sands and loamy sands)

have low PAW because the pore size distribution consists mainly of large pores with limited ability to retain water. Although fine textured soils have the highest total water storage capacity due to large porosity values, a significant fraction of

water is held too strongly (strong matric forces/low, negative water potentials) for plant

uptake. Fine textured soils (clays, sandy clays and silty clays) have moderate PAW because their pore

size distribution consists mainly of micropores. Loamy textured soils (loams, sandy loams, silt loams, silts, clay loams, sandy clay loams and

silty clay loams) have the highest PAW, because these textural classes give rise to a wide range in

pore size distribution that results in an ideal combination of meso- and micro-porosity.

Soil structure can increase PAW by increasing porosity.

PERMEABILITAS TANAH

Diunduh dari: http://www.nature.com/scitable/knowledge/library/soil-water-dynamics-59718900 …… 11/11/2012

Soil structure is highly relevant to water management in soils because it is subject to change either through deterioration by improper management, or to improvement through additions of soil organic

matter. In contrast, it is usually infeasible to change texture.

Permeability Class Permeability(cm/hr) Textural class

Very slow <0.13 clay

Slow 0.13–0.5 sandy clay, silty clay

Moderately slow 0.05–2.0 clay loam, sandy clay loam, silty clay loam

Moderate 2.0–6.3very fine sandy loam,

loam, silt loam, silty clay loam, silt

Moderately rapid 6.3–12.7 sandy loam, fine sandy loam

Rapid 12.7–25.4 sand, loamy sand

Very Rapid >25.4 coarse sand

Total Soil Water Storage Capacity

Diunduh dari: http://nrcca.cals.cornell.edu/soil/CA2/CA0212.4.php…… 11/11/2012

The total soil water storage capacity refers to when all the soil pores or voids are filled with water. This occurs when the soil is saturated or flooded.

A peat soil usually has the highest total soil water storage capacity of around 70 to 85% by volume. Sands and gravels will have the lowest total porosity of around 30 to 40% by

volume. Total porosity for silt soils ranges from 35 to 50%, and clay soils typically range from 40

to 60%. Restricted drainage conditions can cause the soil to attain its total porosity water content, at which time free water is observed and perched water tables develop (in

layered soils) or the apparent water table is found near the surface.

When the total soil water storage capacity is reached, air is pushed out of the pores or void spaces and oxygen and other gaseous diffusion in the soil is severely restricted.

Most agricultural plants cannot tolerate this condition very long (usually no more than a day or two) as plant root respiration requires some oxygen diffusion to the roots. Without air-filled pores, the concentration of carbon dioxide and other gases like

ethylene increase, producing toxic conditions and limiting plant growth.

Soil Water Storage.

Diunduh dari: http://lawr.ucdavis.edu/classes/ssc219/biogeo/sws.htm…… 11/11/2012

Soil water storage is a function of the surface area of soil particles (i.e., particle-size) and the amount of porosity occurring between these particles (i.e., soil structure). Soil pores occur across a wide range of diameters and are often categorized as macropores (>60 µm) and micropores (<60 µm).

Water is present in macropores following precipitation events and is drained by the force of gravity. after water has freely drained due to the force of gravity, the soil is at field capacity and has a soil water potential generally between 0.01 to 0.03 mpa. water in macropores is not available for plant

use because it freely drains from the soil profile and is lost from the rooting zone. water held in very small micropores (<0.2 µm) is held so tightly that plants are not able to extract if for use.

The permanent wilting point is the soil water potential to which plants can effectively utilize

water and corresponds to a soil water potential of approximately 1.5 mpa. thus, the pores in the diameter range 0.2 to 60 µm are the primary storage pores for plant available water (i.e., water held between approximately

0.01 and 1.5 mpa). the distribution of pore sizes is primarily a function of the soil texture and structure. the amount of water storage as a function of soil texture is illustrated in this

figure.

Soil Water and its Availability.


Figure 2.33 indicates the availability of soil water. A soil is at saturation or near saturation following a heavy irrigation or rainfall in which most or all of the spaces between soil particles are filled by water. The force of gravity is greater than the force with which soil particles hold water, so between saturation and field capacity (see below), water is free to drain through the soil by the force of gravity.Field capacity (FC) is the amount of water that a soil can hold against drainage by gravity.Permanent wilting point (PWP) is the moisture content in a soil at which plants permanently wilt and will not recover.Available water (AW) is the water content that the soil can hold between field capacity and wilting point.


Diunduh dari: http://nrcca.cals.cornell.edu/soil/CA2/CA0211.5.php …… 13/11/2012

The soil water storage or soil water content can be quantified on the basis of its volumetric or gravimetric water content.

The volumetric water content is the volume of water per unit volume of soil, expressed as a percentage of the volume.

The gravimetric water content is the mass of water per unit mass of dry (or wet) soil.

The volumetric water content is equal to the gravimetric water content times the soil's bulk density (on a dry soil basis).

Factors that affect the soil water storage are:

1. Total Porosity or Void Space2. Pore-size and Distribution and Connectivity3. Soil Water Pressure Potential or Energy Status of the Soil Water



The total porosity or void space ultimately establishes the upper limit of how much water can be stored in a

given volume of soil. When all the pores are filled with water the soil is saturated, and cannot store any more

water. The total porosity is a function of the soil's particle size, particle uniformity and packing or structure because the void space that remains

between the solid particles determines the extent and distribution of pore sizes and their connectivity.

If one fills the same volume with sand and clay sized particles, the total porosity of the clay is somewhat

higher, about 50-55% of the volume compared to about 35-40% for sand. The spaces between the sand

particles will have larger voids, but there will be fewer of them. The total porosity of medium textured loamy soils is generally around 50% because the smaller silt and clay particles fill some of the voids between the larger sand particles. Soils with good structure will

have somewhat higher total porosity than soil that has been compacted (i.e., where the soil particles are

forced closer together).

The important influence of pore-size and distribution on soil water storage

is in regards to how different pore sizes respond to energy forces or the soil water pressure potential. Under

saturated conditions, large pores drain more easily in response to

gravity potential. Also, when the soil is unsaturated, large pores are less

subject to capillary (or matric potential) forces.

In unsaturated soil conditions, the soil water pressure potential

becomes negative (suction), and the degree to which this occurs greatly

influences the soil water storage (retention) or water content in

different sized pores.

SIMPANAN LENGAS TANAH.


The soil water characteristic (retention) curve defines the

relationship between the soil water pressure potential or energy status (matric or suction potential) and the

soil water content.

It's important to note that soil water moves in direct response to the

energy or pressure potential forces acting upon it (i.e., moving from a

higher to lower energy status), and not necessarily in response to

different soil moisture contents (i.e., from higher to lower soil moisture

content).

SIMPANAN LENGAS TANAH.


The soil water characteristic curve(s) and

definitions are used to establish and further refine

and quantify the general availability of soil water

which is often referred to as :

1. Gravitational water (water subject to drainage),

2. Capillary water (water available to plants), and

3. Hygroscopic water (water that is not available to plants).

Sumber: http://www.tutorvista.com/content/biology/biology-i/natural-resources/water.php#

PERGERAKAN LENGAS TANAH.

Diunduh dari: http://www.cmg.colostate.edu/gardennotes/213.html …… 13/11/2012

Water movement is directly related to the size of pores in the soil. In the small pores of

clayey soils, water slowly moves in all directions by

capillary action. The lack of large pore space leads to

drainage problems and low soil oxygen levels.

On sandy soils with large pores, water readily drains downwards by gravitational

pull. Excessive irrigation and/or precipitation can leach water-soluble nutrients, like

nitrogen, out of the root zone and into ground water.


Diunduh dari: http://nrcca.cals.cornell.edu/soil/CA2/CA0211.5.php

…… 13/11/2012

The water table is defined as the upper surface of groundwater

(saturated zone) or that level in the ground below the soil surface where

the water is at (and in equilibrium with) atmospheric pressure. At the water table reference, the pressure

potential is set equal to zero.

Thus, below the water table, the pressure potential becomes positive,

and above the water table the pressure potential becomes

negative. This negative pressure in unsaturated soil is termed matric,

tension or suction pressure potential so as not to confuse it with positive

pressures.Water infiltration through the soil-water unsaturated zone and into the water table

Sumber: http://iowacedarbasin.org/runoff/showMan.php?c1=2E-1

SOIL WATER STORAGE.


Available Water Capacity

The total available water (holding) capacity is the portion of water that can be absorbed by plant roots. By definition it is the amount of water available, stored, or released

between field capacity and the permanent wilting point water contents.

The soil types with higher total available water content are generally more conducive to high biomass productivity because they can supply adequate moisture to plants during

times when rainfall does not occur. Sandy soils are more prone to drought and will quickly (within a few days) be depleted of their available water when evapotranspiration rates are high. For example, for a plant growing on fine sand with most of its roots in the

top foot of soil, there is less than one inch of readily available water.

A plant transpiring at the rate of 0.25 inches per day will thus start showing stress symptoms within four days if no rainfall occurs. Shallow rooted crops have limited access to the available soil water, and so shallow rooted crops on sandy soils are

particularly vulnerable to drought periods. Irrigation may be needed and is generally quite beneficial on soils with low available water capacity.

SOIL WATER STORAGE.


Soil Type Total Available Water, % Total Available Water, in/ft

coarse sand 5 0.6 fine sand 15 1.8

loamy sand 17 2.0 sandy loam 20 2.4

sandy clay loam 16 1.9 loam 32 3.8

silt loam 35 4.2 silty clay loam 20 2.4

clay loam 18 2.2 silty clay 22 2.6

clay 20 2.4 peat 50 6.0

SOIL WATER STORAGE.



The total soil water storage capacity refers to when all the soil pores or voids are filled with water. This occurs when the soil is saturated or flooded.

A peat soil usually has the highest total soil water storage capacity of around 70 to 85% by volume. Sands and gravels will have the lowest total porosity of

around 30 to 40% by volume.

Total porosity for silt soils ranges from 35 to 50%, and clay soils typically range from 40 to 60%. Restricted drainage conditions can cause the soil to attain its total porosity water content, at which time free water is observed and perched water tables develop (in layered soils) or the apparent water table is found near

the surface.

When the total soil water storage capacity is reached, air is pushed out of the pores or void spaces and oxygen and other gaseous diffusion in the soil is

severely restricted.

SOIL WATER STORAGE.



Most agricultural plants cannot tolerate this condition very long (usually no more than a day or two) as plant root respiration requires some oxygen

diffusion to the roots.

Without air-filled pores, the concentration of carbon dioxide and other gases like ethylene increase, producing toxic conditions and limiting plant growth.

Root cells switch to anaerobic respiration, which is much less efficient than aerobic respiration in converting glucose molecules to ATP (adenosine

triphosphate, the chemical energy within cells for metabolism and cell division).

SOIL WATER STORAGE.



As anaerobic (reduced) conditions develop in the soil, nitrification ceases and denitrification is enhanced.

Corn plants will quickly yellow in response to this saturated soil state as nitrogen becomes limiting, and the plant tries to adjust by producing more

adventitious roots.

Prolonged anaerobic conditions in the soil starts to reduce manganese, iron (causing phosphorus to be more soluble), sulfur (producing hydrogen sulfide),

and eventually methane gases.

Hydrophytic (wetland type) plants are adapted to saturated soils because they are able to obtain oxygen through other forms of plant structure adaptations

(i.e. pneumataphores, lenticels, aerenchyma).

SOIL WATER STORAGE.

Diunduh dari: http://nrcca.cals.cornell.edu/soil/CA2/CA0211.5.php

…… 13/11/2012

Drainable Porosity

The drainable porosity is the pore volume of water that is removed (or

added) when the water table is lowered (or raised) in response to

gravity and in the absence of evaporation.

Consider a soil that is saturated with the water table at the surface.

If this soil has a subsurface drainage pipe (tile) buried several feet down

and it is discharging to the atmosphere at some lower elevation, the drainable porosity water content will be released to the tile drain until

the water table is lowered to the depth of the drain.

Drainable porosity is the amount of water that drains from macropores by gravity between saturation to field capacity typically representing three days of

drainage in the field.

Diunduh dari: http://www.nature.com/scitable/knowledge/library/soil-water-

dynamics-59718900…… 13/11/2012

.

SOIL WATER STORAGE.


Drainable Porosity

Any nutrients or pesticides dissolved or suspended in this readily drainable pore space will also be carried along with this water, either flowing to the tile

drain or continuing downward to the water table via deep percolation if no drainage restriction exists.

In large pores, nutrients that might otherwise adsorb to the soil particles (ammonium or phosphate) will bypass the soil because of limited time for

contact and chemical reactions to occur with the soil surface area.

Soils with a wide range of different pore sizes (sandy loams) or soils with mostly small sized pores are better at filtering nutrients and pesticides as they

leach through the soil profile.

SOIL WATER STORAGE.

Diunduh dari: http://nrcca.cals.cornell.edu/soil/CA2/CA021

1.5.php …… 13/11/2012

Drainable Porosity

The combined aspect of low available water holding

capacity and high drainable porosity for sandy soils causes

these soils to have a high leaching potential.

It will not take much rain or irrigation (or application of

liquid manure) to replenish the available soil water and to raise

the soil water content to a drainable state.

Applying the proper amount (depth) of irrigation to these

soils will both conserve water and enhance irrigation and

nutrient use efficiency.

The variability of drainable porosity with soil texture and structure

Soil TextureField

Capacity (% by vol.)

Wilting Point (% by vol.)

Drainable Porosity (%

by vol.)

clays, clay loams, silty

clays30-50% 15-24% 3-11%

well structured

loams20-30% 8-17% 10-15%

sandy 10-30% 3-10% 18-35%

Diunduh dari: http://www.extension.umn.edu/distribution/cropsystems/dc7644.ht

ml ………..13/11/2012

SOIL WATER STORAGE.


The soil texture and structure fundamentally determines the number and sizes of soil pores, which will influence the fate and transport of air

(gas) and water exchange.

The figure provides an illustration of how

various parameters of soil water storage may

be influenced by different texture and structure

aspects

(From H.M. van Es).

SOIL POROSITY

Diunduh dari: http://passnano.com/8.html…… 13/11/2012

Saturation: all soil pores are filled

Gravitational water:

drainable water 0 to -33 kPa.

Field capacity (after 1-2 days of drainage)

-33 kPa, usually 1/2 of saturation water.

Permanent wilting point: -1500 kPa

Plant available water: difference in water% at FC and PWP.

SOIL PORE

Diunduh dari: http://saret.ifas.ufl.edu/publications/bsbc/chap6.htm…… 11/11/2012

A well aggregated soil has a range of pore sizes. This medium size soil

crumb is made up of many smaller ones. Very large pores occur

between the medium size aggregates.

The ideal soil can be found in a well aggregated medium-textured loam

soil. Such a soil has enough large pore spaces between the aggregates to

provide adequate drainage and aeration during wet periods, but also

has adequate amounts of small pores and water-holding capacity to

provide sufficient water to plants and soil organisms between rainfall or

irrigation events.

Types of soil pores

Diunduh dari: http://www.landfood.ubc.ca/soil200/interaction/water_air.htm …… 13/11/2012

1. Macropores (d>0.08mm) occur between aggregates (interped pores) or individual grains in coarse textured soil (packing pores) and may be formed by soil organisms (biopores). They allow ready movement of air and the drainage of water and provide space for roots and organisms to inhabit the soil.

2. Micropores (d<0.08mm) occur within aggregates. They are usually filled with water and are too small to allow much movement of air. Water movement in micropores is extremely slow and much of the water held by them is unavailable to plants.

Pore space can be filled with either water or air. The volume of soil water and soil air, however, cannot exceed the total soil porosity. As soil

water increases, soil air must decrease and vice versa.

SOIL STRUCTURE

Diunduh dari: http://vro.dpi.vic.gov.au/dpi/vro/vrosite.nsf/pages/soilhealth_soil_structure…… 13/11/2012

Soil structure is the arrangement of pores and fissures (porosity) within a

matrix of solid materials (soil particles and organic matter). The solid materials bond and aggregate to give the pores

and fissures. The quantity, distribution and

arrangement of pores determines water holding

capacity, infiltration, permeability, root

penetration, and, respiration.Only about 50% of soil is solid material. The remainder is

pore space. Small pores within the aggregates provide storage and

refuge. The larger pores (and fissures) between the aggregates are the pathways for liquids, gases, roots and

organisms.

SOIL POROSITY.

Diunduh dari: http://www.soilandhealth.org/01aglibrary/010112krasil/010112krasil.ptII.html…… 11/11/2012

(A) Porosity of cultivated structured soil (schematic): 1--thin, predominantly capillary pores in aggregates, which fill with water on wetting; 2--medium-sized pores (cells, channels), upon wetting they will fill with water for a short period and subsequently, after the resorption of water, with air; 3--capillary pores; 4--large pores, between aggregates almost always filled with air (according to Kschinakii, 1956);

(B) Visible porosity of soil aggregate (reproduction from a microsection). Thin chernozem (southern): 1--micro-aggregates; 2--visible pores (according to Kachinskii et al., 1950).

The total porosity is determined according to its volume and specific gravity. It is determined by the following formula:

P = (1 - vol/sp) x 100

where P--porosity, sp--the specific gravity of the solid phase, vol--weight by volume.

SOIL AGGREGATION

Diunduh dari: http://faculty.yc.edu/ycfaculty/ags105/week05/soil_physical_properties/soil_physical_properties_print.html …… 13/11/2012

Soil management practices that promote good soil structure include:Minimizing tillage; Timing tillage for optimum moisture conditions (approximately field capacity

or a little drier); Maintaining plant litter/residues on the soil surface; Incorporating a constant supply of decomposable organic material; Using sod crops whenever possible; Using green

manure and cover crops whenever possible; Applying gypsum and soil conditioners

AGGREGATE STABILITY

Diunduh dari: http://ipmguidelines.org/FieldCrops/Chapters/CH02/CH02-5.aspx …… 13/11/2012

Relationship between aggregate stability

and soil organic matter in some

selected soils from the Cornell

University research sites in NY.

SOIL POROSITY.

Diunduh dari: http://www.turfdiag.com/porosity.htm…… 13/11/2012

Porosity refers to the amount of space between the solid soil particles. Pore space can be filled with either water or air.

Smaller pores tend to be filled with water.

The amount of water-filled pores is referred to as capillary porosity. Large pores are typically filled with air.

These air-filled pores are referred to as non-capillary porosity.

When a growing media has approximately equal amounts of water-filled and air-filled pore space, the soil is said to have balanced porosity.

Soils text books indicate that growing media with balanced porosity provide an ideal environment where beneficial microbes, nutrients, water,

and air can interact and thrive. This provides advantageous conditions for desirable soil gas exchange, good mineral/water holding, vigorous root growth, and healthy plants.

SOIL POROSITY.

Diunduh dari: http://www.certifiedorganic.bc.ca/rcbtoa/training/soil-article.html…… 13/11/2012

A vertical cross sectional view of a highly structured soil. The

largest soil units shown are macroaggregates (~ 2 mm

diameter). They are composed of

microaggregates (~ 0.1 mm) and sand grains, as shown in

the center left of the macropore.

Four hierarchical classes of soil pore space are illustrated:

(1) macropore, (2) intermacroaggregate, (3)

intermicroaggregate (includes intramacroaggregate space,

see arrow) and (4) tramicroaggregate space. (Illustration by S. L. Rose).

SOIL POROSITY.

Diunduh dari: http://www.certifiedorganic.bc.ca/rcbtoa/training/soil-article.html…… 13/11/2012

Water is retained in many of these pores when the soil is at field capacity and pore space

is large enough to be inhabited by nematodes.

The pores between microaggregates but within macroaggregates are large

enough to accommodate small nematodes and protozoa and

may be the chief habitat of fungi.

The smallest class of pores, those within microaggregates,

may be only about 1 mm, maximally, and may be

inhabited mostly by bacteria (Kilbertus 1980).

Soil Pore spaces

Diunduh dari: http://www.swtafe.vic.edu.au/toolbox/turf/html/pages/office/grass_roots/soil_structure.html…… 13/11/2012

Soil particles do not fit together snugly. There are spaces between particles. These spaces are called

pore spaces and contain water and air.

The pore spaces provide the route for the downward movement of water and

allow roots to grow into them. They also provide air space, which is

essential for plant growth.

The larger the pore spaces the better the drainage of water and the less

water retained in the soil. Conversely, the smaller the pore spaces the less

water drains away and the more water is retained in the soil.

Water holding capacity.

Diunduh dari: http://www.swtafe.vic.edu.au/toolbox/turf/html/pages/office/grass_roots/soil_structure.html…… 13/11/2012

After a soil has been completely soaked by a downpour of rain all its pore spaces are filled with water and it is regarded as being saturated. Any air in the pore spaces

is forced out and the soil is said to be waterlogged.

Once the rain stops and the water has a chance to drain away, the total amount of water that remains in the soil, against the force of gravity, is called the field capacity of the soil. Due to capillary forces, water will drain away from the large pore spaces first and remain in some of the smaller pore spaces. So, at field capacity, the larger

pores will contain air whilst the smaller pores hold water.

Plant roots in the soil are able to suck the water out of many of the small pores. However, eventually water in the small pore spaces is held too tightly by capillary

forces for the roots to take it up. This stage, when the turf plants are unable to extract any more water from the soil and begin to wilt, is called the permanent

wilting point.

Due to differences in particle and pore space size, soils differ in their field capacity and permanent wilting point.

Have a look at the different water holding capacities of sand, loam and clay.

What is water holding capacity of soil?.

Diunduh dari: http://wiki.answers.com/Q/What_is_water_holding_capacity_of_soil …… 13/11/2012

Water holding capacity of soil is just that, the specific ability of a particular type of soil to hold water against the force of gravity.

Different types of soils have difference capacities, for example a sand soil had a lower capacity to hold water when compared to a clay soil. The

nature of the soil, composition of the soil, amount of organic component and size of the soil particles determine its ability to retain water. Water molecules are held closely to the individual soil particles by forces of

cohesion.

The maximum amount of water a soil can hold before it is saturated and starts to loose water by gravity is known as "field capacity“.

A plant's available water holding capacity for soils with different textures.

Diunduh dari: http://www.extension.org/pages/33617/soils-and-water-availability …… 13/11/2012

The texture of a soil is important for soil water availability because it controls not only how

well a soil can hold water but also how well water

is absorbed into the soil.

Any water that infiltrates into a soil does so

primarily through large pores in the soil called “macropores” that are created by plant roots, microorganisms, and

physical processes such as freezing and thawing and drying and wetting.

WHC = WATER HOLDING CAPACITY

Diunduh dari: http://www.nativerevegetation.org/learn/manual/ch_5.aspx …… 13/11/2012

General relationship between soil texture and available

water-holding capacity. As clays increase in a soil, so

does water-holding capacity. Typically, clay loam soils hold

more than twice as much water as sandy textured soils.

The presence of humus in topsoil increases water-

holding capacity of loams and sandy loams at a rate of 2.25% water to each percent rise in

soil humus (Jenny 1980) which equates to approximately

0.75% increase in water for every 1% increase in organic

matter.

Mitigating for Textures with Low Water-Holding Capacities

Diunduh dari: http://www.nativerevegetation.org/learn/manual/ch_5.aspx …… 13/11/2012

Organic Amendments — Incorporation of organic amendments (e.g., compost) can increase the water-holding capacity of a soil.

Because the water-holding capacity of each type of organic matter varies by composition and degree of weathering, the effect on soil water-holding capacity by any organic matter being considered must be

assessed prior to application.

Sandy textured soils benefit most from organic matter additions, especially those with plant available water of 9% or less (Claassen 2006),

which are typically sands, loamy sands, and sandy loam soils.

Testing several different rates of incorporated organic matter on soil moisture-holding capacity should be done to prior to selecting the

source and the amount of material to apply.


Clay — The water-holding capacity of sandy textured soils can be increased by incorporating clay loam, sandy

clay loam, and silty clay loam textures in the soil.

The addition of clays should be at rates that result in new soil textures similar to loams, silt loams, or sandy clay

loams. Higher rates of clay addition are not recommended. It is

always important to test the additions of any soil to another to understand what the effects on water-holding

capacity and structure might be. Ideally this should be in the field in small plots.

Mitigating for Textures with Low Water-Holding Capacities

Soil moisture retention curve

Diunduh dari: http://www.tankonyvtar.hu/hu/tartalom/tamop425/0032_talajtan/ch07s05.html …… 13/11/2012

The soil moisture retention curve (pF curve) gives the relation

between soil moisture suction and soil moisture content.

A soil is at F.C. (field capacity) or has a pF-value of 2, some 2 to 3

days the soil has been saturated by rainfall or irrigation.

When the soil becomes dry and plants cannot take up water

anymore the soil is at W.P (wilting point) or has a pF=4.2.

The amount of water held by a soil in the root zone between F.C. and

W.P. and which can be used by plants is described as available

water. (F.C.- W.P.= available water) For sand, loam and clay the values

are 6, 20 and 17 volume percent respectively.

Plant available water.

Diunduh dari: http://www.tankonyvtar.hu/hu/tartalom/tamop425/0032_talajtan/ch07s05.html …… 13/11/2012

Plant available water is considered the amount of water held between field

capacity and the permanent wilting point.

Limitations to the concept of plant available water:

1. Roots are not distributed throughout the profile.

2. Water content is not uniform throughout the profile.

3. Not all plants have the same wilting point.

4. Water may not be as ‘easily’ obtained (i.e. available) as the soil dries andpotential decreases toward the permanent wilting point.

SOIL

AGGREGATION

Struktur Tanah & Agregasi

Soil may be a loose assemblage of individual and random particles, or consist of distinctly structured aggregates of distinctive size and shape; the particular arrangement of

which is called soil structure.

Most methods of measurement are indirect, and measure various properties that are dependent or at the least

influenced by specific structural properties; e.g., total porosity, pore size distribution, liquid retention/

transmission, and infiltration.

Diunduh dari: www.cabnr.unr.edu/.../10Lectures4and5SoilSt... …… 13/11/2012

Soils may be non-structured (e.g., single grain or massive) or consist of naturally formed units known as peds or aggregates.

The initial stage in the formation of soil structure is the process of flocculation.

Individual colloids typically exhibit a net negative charge which results in an electrostatic repulsion.



Reduction of the forces of electrostatic repulsion allows the particles to come closer together.

Flocculation

This process allows other forces of attraction to become more dominant. The formation of these “flocs” in suspension represents the

early stages of aggregation.



As this process continues, the flocs become larger and larger forming the more refined structural units.

On their own, these units are pretty fragile and the process is easily reversed. But in the presence of natural or artificial binding become more strongly cemented together forming stable soil aggregates.

Bahan perekat (pengikat) dapat berupa :

Inorganic – Fe & Al oxides, carbonates, amorphous gels and sols; or

Organic – polysaccharides, hemicellulose, and other natural or manufactured organic polymers.



The arrangement or organization of individual soil particles (soil separates) into a specific configuration is called “soil structure”. Soil

structure is developed over a geologic time frame, is (or can be) naturally fragile, and is affected by changes in climate, vegetation,

biological activity, and anthropogenic manipulation.

Soil structure influences the mechanical properties of soil such as stability, porosity and compaction, as well as plant growth,

hydrologic function, and erosion.



There are three broad categories of soil structure; single grained, massive, and aggregated.

When particles are entirely unattached the structure is completely loose and such soils are labeled single grained.

When packed into large cohesive blocks the structure is called massive.

Neither have any visible structural characteristics. Between these two extremes particles are present as

aggregates or peds.



The observable shapes of soil structure in the field are classified as:

Platy: Horizontally layered, thin, flat aggregates similar to wafers.

Spherical: Rounded aggregates generally < 2.0 cm in diameter that are often found in loose condition called “granules or

crumbs”.

Blocky: Cube-like blocks, sometimes angular with well-defined sharp faces or sub-angular with rounded faces up to 10cm in

size.

Columnar or Prismatic: Vertically oriented pillars up to 15cm in diameter.


Platy and spherical soil structure is common to the surface soil horizons, blocky and columnar/prismatic are associated with the deeper subsurface soil

horizons


• Non-Structured– Single Grain

– Massive

• Structured

– Platy: horizontal & flat– Spherical (Grannular):

rounded and <2.0 cm– Blocky: cubes up to 10 cm

that are angular (sharp edges) or subangular (rounded)

– Prismatic (Columnar): longer than wide, often 6 sided, sharp or rounded, < 15 cm



Aggregate size distribution also influences the pore size distribution.

Macropores: Inter-aggregate cavities that influence infiltration, drainage, and aeration.

Micropores: Intra-aggregate capillaries important to water and solute retention.

Mesopore: Inbetween.



DISTRIBUSI UKURAN AGREGAT

Similar to particle size distribution, the aggregate size distribution also is determined by sieving.

An index known as the Mean Weight Diameter (X) based on the size and weight distribution of aggregates is derived by

weighing the mass of aggregates within the respective size classes, and characterizing the overall size distribution.

(MWD) X = ∑ xiwi

xi = mean diameter wi = dry mass fraction


Since aggregation and stability is time dependent, another useful characterization is that

of “aggregate stability”.

Aggregate stability expresses the resistance of individual soil aggregates to disruptive forces such as mechanical, wind, and water erosion;

freezing/thawing; wetting/drying; and air entrapment.The level of stability is assessed by determining the fraction of the original

aggregate mass which has withstood disruptive forces. The laboratory approach uses wetting (misting and/or from bottom up with de-aired water)

followed by sieving.


Stabilitas Agregat


TIPE-TIPE STRUKTUR TANAH

Humic-like substances secretionhydrophobic region

Polysaccharide secretion - hydrophobic regionbinding of clay particles

Fungi

Physical entanglement

Soil aggregate

Polysaccharide secretionbinding of clay particles

Bacterial colonies

Cross-section

STRUKTUR TANAH10 structure = soil particles + organic matter (humus) + roots + microorganisms

20 structure = aggregate or ped = stability

Diunduh dari: www.elsevierdirect.com/.../04~Chapter_04_so... ... …… 13/11/2012

C

1 micron

Fissure

Micro-environment =oriented, packedand glued clay

Non perturbed clay

Polysaccharide secretion

Cell wall

Fungal hyphae

Clay Particles

Soil aggregates are formed and stabilized by clay-organic complexes, microbial polysaccharides, fungal hyphae and plant

roots.


AGREGAT TANAH

Interaggregate pore space ( m to mm in size)

Aggregate particle

Intraaggregate pore space(nm to m in size)

Enlargement

Soil aggregates are associated with relatively large inter-aggregate pore spaces that range from um to mm in diameter. Each aggregate also has intra-aggregate pore

spaces that are very small, ranging from nm to um in diameter.

Intra-aggregate pores can exclude bacteria (called micropore exclusion). However, after a spill, contaminants can slowly diffuse into these pores. This creates a long-term sink of pollution as the contaminants will slowly diffuse out again.


AGREGAT TANAH

2 mm

Assume a soil aggregate that is 2 x 2 x 2 mm. Further assume that the volume of the aggregate is 50% pore space. How many pores of diameter 15 um does the aggregate have? How many pores of 50 um?

(the volume of a sphere is: 4/3π r3)

Berapa banyak “Pori” dalam tanah?

2 mm

Calculation for 15 um pores:

The volume of the aggregate is 2 mm x 2 mm x 2 mm = 8 mm3

Pore space is 50% of 8 mm3 = 4 mm3

A pore of 15 um diameter has volume = 4/3 π (7.5 um)3 = 1.77 x 103 um3

4 mm3 (1000 um)3 / 1.77 x 103 um3 = 2.3 x 10 6 pores of 15 um per aggregate! mm3 pore

2 mm


In soil 80 to 90% of the bacteria are attached to surfaces and only 10-20% are planktonic. Cells have a patchy distribution over the solid surfaces, growing in microcolonies. Colony growth allows sharing of nutrients and helps protect against dessication and predation or grazing by protozoa.

Where are the bacteria?


Pergerakan dan Potensial Lengas Tanah

Soil water potential depends on how tightly water is held to a soil surface. This in turn depends on how much water is present.

Surface forces have water potentials ranging from –10,000 to –31 atm.

Capillary forces have water potentials ranging from –31 to –0.1 atm. Optimal microbial activity occurs at approximately -0.1 atm.

At greater distances there is little force holding water to the surface. This is considered free water and moves downward due to the force of gravity.

Soil air

FREE WATER

Gravitationalforces

Cap

illary

forc

es

Surfa

ce fo

rces

Soil

parti

cles

% Saturation ofthe soil pore

100%0%

A m

Soil air

Increasing distance from particle surface


Estimating effects of compaction on pore size distribution of soil aggregates by mercury porosimeter

J. Lipiec, M. Hajnos, R. Świeboda. Geoderma. Volumes 179–180, June 2012, Pages 20–27

Diunduh dari: http://www.sciencedirect.com/science/article/pii/S0016706112000900…… 13/11/2012

The aim of this study was to describe quantitatively the effect of vehicular traffic on pore size distribution (PSD) of topsoil (0.05–0.15 m) and subsoil (0.25–0.35 m) aggregates

(3 mm and 8 mm) of silty loam.

The total aggregate porosity, average pore radius and volume of larger pores, > 1–3 μm at 0.05–0.15 m depth, and > 0.3–0.4 μm at 0.25–0.35 m decreased with increasing soil

compaction, mostly from NC to MC. At 0.25–0.35 m depth this decrease was accompanied by an increase in the volume of smaller pores (< 0.3 μm) mostly from MC to

SC. As a consequence, the volume of pores retaining plant available water (0.1–15 μm radius) decreased in compacted soil.

The differential pore curves exhibited peaks at the pore throat radius of 1–6 μm. At 0.05–0.15 m depth the peaks under SC were lower than under NC and MC, whereas at 0.25–

0.35 m depth they were lower under MC and SC than NC. At all compaction treatments and aggregate fractions the volume of larger pores > 1–3 μm was greater at 0.05–0.15 m depth than at 0.25–0.35 m depth and the inverse was

true for smaller pores (< 0.3 μm). The observed changes in pore size distribution in the subsoil are considered as almost irreversible and thus long-lasting or even permanent.




Cumulative curve of pore

volume vs. equivalent pore

radius of 3–5 mm

aggregates.




Cumulative curve of

pore volume vs. equivalent pore radius of 8–10 mm aggregates.




Differential curve of

pore volume vs. equivalent pore radius of 3–5 mm

aggregates.

Crop-Pasture Rotation for Sustaining the Quality and Productivity of a Typic Argiudoll

Guillermo A. Studdert, Hernan E. Echeverría, Elda M. Casanovas. 2007.

Diunduh dari: http://en.engormix.com/MA-agriculture/articles/croppasture-rotation-sustaining-quality_475.htm …… 13/11/2012

Aggregate Stability Index

The relationship between SOC and ASI. It can be seen that ASI values

under cropping were relatively constant across all the explored range of SOC (29.6-37.7 g kg-1). On the other hand, ASI increased

with increases in SOC (r = 0.64, P < 0.01) when periods under pasture

were analyzed. These results agree with the fact that aggregate stability and SOC are closely related (Greenland,

1981; Oades, 1984). It appears that SOC was not enough

by itself to explain ASI variations because ASI values were different

at the same SOC under cropping or under pasture, respectively.

GEOLISTRIK

PENDUGAAN LENGAS TANAH

GEOLISTRIK.

Sumber: Penggunaan Metode Geolistrik Untuk Mendeteksi Keberadaan Air Tanah . Eva Rolia . TAPAK Vol. 1 No. 1 Nopember 2011 ..... 13/11/2012

Geolistrik merupakan metoda geofisik yang mempelajari sifat aliran listrik di

dalam bumi dan bagaiman cara mendeteksinya di permukaan bumi. Dalam hal ini meliputi pengukuran potensial, arus dan medan elektromagnetik yang terjadi baik secara alamiah ataupun akibat injeksi arus ke dalam bumi.

Ada beberapa macam metoda geolistrik, antara lain : metoda potensial diri, arus telluric, magnetotelluric, IP (Induced Polarization), resistivitas (tahanan jenis)

dan lainnya.

Penyelidikan air tanah dilakukan untuk memperkirakan tempat terjadinya air tanah, kedalaman muka pembentukan (kerikil, pasir, dan lain-lain), serta

ciri-ciri fisik air tanah (suhu, kerapatan, dll).

Penyelidikan air tanah dapat dilakukan dari permukaan tanah maupun dari bawah permukaan tanah.

Penyelidikan air tanah yang biasa dilakukan dari permukaan tanah adalah dengan menggunakan metode Geolistrik.

PENDUGAAN GEOLISTRIK.Geolistrik merupakan salah satu metode geofisika yang mempelajari sifat aliran

listrik di dalam bumi dan untuk mengetahui perubahan tahanan jenis lapisan batuan di bawah permukaan tanah dengan cara mengalirkan arus listrik DC (direct

current) yang mempunyai tegangan tinggi ke dalam tanah. Metode ini lebih efektif jika digunakan untuk eksplorasi yang sifatnya dangkal, contohnya

penentuan kedalaman batuan dasar, pencarian reservoir air, dan juga digunakan dalam eksplorasi geothermal.

Tujuan survey geolistrik tahanan jenis adalah untuk mengetahui resistivitas bawah permukaan bumi dengan melakukan pengukuran di permukaan bumi. Resistivitas bumi berhubungan dengan mineral, kandungan fluida dan derajat saturasi air dalam batuan.

Metode yang bisa digunakan pada pengukuran resistivitas secara umum yaitu dengan menggunakan dua elektroda arus (C1 dan C2), dan pengukuran beda potensial dengan

menggunakan dua elektroda tegangan (P1 dan P2), dari besarnya arus dan beda potensial yang terukur maka nilai resistivitas dapat dihitung menggunakan persamaan:

Dengan k adalah faktor geometri yang tergantung penempatan elektroda permukaan.


Metode Geolistrik Resistivitas

Diunduh dari: http://trisusantosetiawan.wordpress.com/2011/01/04/metode-geolistrik-resistivitas/ …… 13/11/2012

Pendugaan potensi air tanah menggunakan Metode Geolistrik untuk mengetahui nilai resistivitas batuan dan menentukan potensi atau kandungan air tanah. Dari nilai resistivitas batuan dan potensi

atau kandungan air tanah, maka akan diketahui adanya air tanah di Suatu lokasi.

Metode geolistrik resistivitas atau tahanan jenis adalah salah satu dari kelompok metode geolistrik yang digunakan untuk mempelajari

keadaan bawah permukaan dengan cara mempelajari sifat aliran listrik di dalam

batuan di bawah permukaan bumi. Metode resistivitas umumnya digunakan untuk eksplorasi dangkal, sekitar 300 – 500 m.

Prinsip dalam metode ini yaitu arus listrik diinjeksikan ke alam bumi melalui dua

elektrode arus, sedangkan beda potensial yang terjadi diukur melalui dua elektrode potensial. Dari hasil pengukuran arus dan

beda potensial listrik dapat diperoleh variasi harga resistivitas listrik pada lapisan di

bawah titik ukur.

http://trisusantosetiawan.files.wordpress.com/2011/01/skemaresis.jpg

GEOLISTRIK.

1. Geolistrik merupakan metode geofisika yang cukup efektif untuk digunakan dalam mendeteksi keberadaan air tanah dengan memanfaatkan sufat batuan yang mampu mengalirkan arus listrik.

2. Geolistrik merupakan alat alternatif yang dapat digunakan dalam kegiatan teknik sipil untuk mengetahui lapisan tanah di dalam bumi, selain dengan menggunakan metode hand bor, sondir, dan metode lain dalam ilmu teknik sipil.

3. Geolistrik memiliki cara kerja yang efisien karena mudah dioperasikan, mudah dibawa, murah, dan akurasi data yang dapat diandalkan.


METODE GEOLISTRIK

Diunduh dari: http://geofisikaonline.blogspot.com/2009/05/metoda-geolistrik.html…… 13/11/2012

Metoda geolistrik adalah salah satu metoda geofisika yg didasarkan pada penerapan

konsep kelistrikan pada masalah kebumian. Tujuannya adalah untuk memperkirakan sifat

kelistrikan medium atau formasi batuan bawah-permukaan terutama kemampuannya

untuk menghantarkan atau menghambat listrik (konduktivitas atau resistivitas).

Aliran listrik pada suatu formasi batuan terjadi terutama karena adanya fluida elektrolit pada pori-pori atau rekahan batuan. Oleh karena itu resistivitas suatu formasi batuan bergantung

pada porositas batuan serta jenis fluida pengisi pori-pori batuan tsb. Batuan porous

yg berisi air atau air asin tentu lebih konduktif (resistivitas-nya rendah) dibanding batuan yg

sama yg pori-porinya hanya berisi udara (kosong).

http://lh4.ggpht.com/_DUNnpopyxkk/SgwosD9HXKI/AAAAAAAAALg/Li9EZ1-BEBk/s1600-h/resist22.jpg

Temperatur tinggi akan lebih menurunkan resitivitas batuan secara keseluruhan karena meningkatnya mobilitas ion-ion penghantar

muatan listrik pada fluida yg bersifat elektrolit.

Cara kerja metoda geolistrik secara sederhana dapat dianalogikan dengan rangkaian listrik.

Jika arus dari suatu sumber dialirkan ke suatu beban listrik (misalkan kawat seperti terlihat pada gambar) maka besarnya resistansi R dapat diperkirakan berdasarkan besarnya potensial sumber dan besarnya arus yg

mengalir. Dalam hal ini besaran resistansi tidak dapat

digunakan untuk memperkirakan jenis material karena masih bergantung ukuran atau

geometri-nya. Untuk itu digunakan besaran resistivitas yg merupakan resistansi yg telah

dinormalisasi terhadap geometri.

Dalam prakteknya pengukuran geolistrik dilakukan dengan

mengalirkan arus ke dalam tanah melalui 2 elektroda (C1 dan C2)

dan respons-nya (beda potensial) diukur melalui 2

elektroda yg lain (P1 dan P2).

Berdasarkan konfigurasi elektroda dan respons yg terukur

maka sifat kelistrikan medium bawah-permukaan tersebut

dapat diperkirakan.

METODE GEOLISTRIK

Diunduh dari: http://geofisikaonline.blogspot.com/2009/05/metoda-geolistrik.html…… 13/11/2012

kompendium perilaku air dalam tanah

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