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Application of Structure-Index Technique to Detect Soil Degradation

Mohamed Fahmy Hussein

Cairo University, Egypt

AbstractAggregate stability in fine-textured soils is a guarantee of good yield. A Structure-Index

technique was applied to 70 layers of 14 profiles in the Nile Delta Apex, north of Cairo, Egypt; inan environment that was traditionally considered as one of the most precious soils of the country.

The used index depends on three fluid treatments: 1- Alcohol followed by water, 2- Benzenethen water, and 3- Water alone handling. The pre- Alcohol treatment leads to increasing aggregatestability, the pre- Benzene treatment puts an extra-explosive power to aggregate destruction, whereasthe Water alone treatment is somewhere between these two organic-fluid actions.

Water regime, chemistry and water table depth closely relate to the structure of heavy-texturedflat irrigated-lands. The Nile Delta, prior to application of perennial irrigation 150years ago, was

historically under basin-irrigation thanks to the annual Nile flood and the endeavor of the Egyptianfarmers who well managed the extra-extensive basins in Upper and Lower Egypt for thousands of years. The State has introduced tile drainage 20 years ago in the study area.

We observed (through time-series model applied to Tritium data of the semi-confined Deltaaquifer) that recharge of 100mm.year -1 of the semi-confined aquifer has resulted in a piezometric-level rise of 10m during the last century. The water table of the unconfined aquifer is ~1m belowsoil-surface at present.

SI data showed degradation of soil-structure in most bottom and some upper layers of half thestudied profiles. The data show that SI values are in the range 212 for the studied area. Linear (ECSI) and (ESPSI) relationships are obtained. The slope of the (ECSI) line is higher than thatof the (ESPSI) indicating structure degradation with early salinity increase. Negative regression is

observed between SI and Ksat.The index is easily realizable and gives meaningful indication on soil structure when related to

other soil parameters. The assessed index is believed suitable for soil-conservation projects.

IntroductionThe agricultural plant yield is largely affected by soil-structure, in particular in the heavy-

textured lands. Soil-structure is the outcome of the factors affecting soil status setup. Soil-structureis a dynamic property that may be improved or degraded. It might be improved through goodmanagement procedures (among which the irrigation application-rate and water-quality stand as themost-important aspects in the irrigated-lands). Soil-structure can be studied in the field (bymorphologically describing the size, form, configuration and orientation of peds), in the lab (on

microscopic-scale in thin sections) and through physical techniques, including wet-sieving andaggregate-size distribution methods. However, soil-structure might be represented by a single-

parameter that may be useful for quick assessment.A simple and rapid method for the evaluation of an easy-to-use Structure-Index, SI is needed

for recognizing structure change with depth in the same profile and matching-up several profiles ina given landscape. No field-instrument is available for providing a structure-index value. Only labdeterminations are applicable. As long as the lab method is readily used, coherent results could beobtained and interpreted. Tedious methods are not encouraging and may impose difficulties for thetreatment of large number of samples in short time. This work presents an appraisal of a newstructure-index as used for the evaluation of soil-structure in a large number of soil-materialssampled from clayey soils in the south of the Nile Delta, Egypt.

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Figure1. Location map

Review of LiteratureAggregation is the product of chemical interaction among organic and inorganic soils particles

(Edward and Bremner, 1967). Stable aggregates are internally held by chemical bonds that resistdisintegration in water. Harris et al. (1966) summarized theories explaining the chemical processesinvolved in soil aggregation. The mechanisms include: (1) linkage of clay particles by water dipoles, (2) cross-bridging and sharing of inter-crystalline forces (due to exchangeable cations)

between clay-particles, (3) envelopment of soil particles by precipitated and irreversibly-dehydratedcolloids (silicates, sesquioxides and humates), (4) presence of hydrophobic organic materials, and5) inter-particle linkage by organic polymers that form bonds (through functional groups) with thesurface of two or more clay plates.

The role of microbial activity on soil-aggregation has been studied by Gasperi-Mago and Troeh(1979) and appears to affect soil-aggregation in three ways: (1) filaments of microbial tissue mayform a network within and around mineral particles, (2) certain soil microorganisms produce

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polysaccharides that have a mucilaginous nature and may cement mineral particles, and (3)microorganisms produce non-polysaccharide organic residues of various kinds that may help in

binding mineral particles (Gabriels and De Boodt, 1976). Soil-aggregation is influenced by agro-technical practices such as tillage and can be improved by appropriate tillage at optimal moisturecontent. Hussein (1975) observed that the Structure Factor (SF) decreases in saline clayey soils inthe north of the Nile Delta. The clay particles deflocculated in water were 5% in surface layers and

up to 46% at 100-150cm depth, and SF decreased with salinity increase mainly due to the sodiumion impact.

Oades (1984) showed that the stability of pores and particles is essential for optimum growthof plants. Two categories of aggregates macro- (>250 m) and micro- (

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Table 1. Maximum and minimum values for some data

ESP ECaP EM gP EKP CEC Clay% K, cm/hr S.I.MAX 32.21 64.78 40.32 8.33 48.85 54.78 1.470 11.52MIN 3.57 39.50 18.89 0.92 29.11 34.99 0.002 1.68ratio MAX/MIN 9.03 1.64 2.14 9.08 1.68 1.57 735.00 6.86

ESP = Exchangeable Sodium%E Ca P = Exchangeable Calcium%,E Mg P= Exchangeable Magnesium%EKP = Exchangeable Potassium%CEC = Cation Exchange Capacity, meq/100 gramK = Hydraulic Conductivity (Saturated)SI = Structure Index

(425) an excludedvalue

1

10

100

1000

1950 1960 1970 1980 1990 2000

TU

Years

Reconstructed Tritium Co ntent in Nile water

Fahmy & R.Michel

Old model

measured

precipitation

Figure 2. Tritium prediction model and data for Nile-water in its head-reaches and in Egypt. (b) Is the Tritium output-function as predicted by 2 dispersion models (Cpp and CJR) with residence time T= 30 and 40y using a relatively high dispersion

parameter (D/vx = 2). (From Hussein, M. F., 1995, and Hussein, et al, 1998.)

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Profile 3

Profile 4

Profile 5

Profile 6

Profile 7

Profile 8

Profile 9

Profile 10

Profile 11

Profile 12

Profile 13

Profile 14

Profile 1

Profile 2

123456789

10111213141516

17181920212223242526272829303132333435363738394041424344454647

4849505152535455565758596061626364656667686970

0 10 20 30 40 50 60 70

e r a n u m e r o a s a m p e s

Stable aggregates, %

(the data-po ints representing the 5 surfa c-layer samplesare shown by t riangle, orthogonal and squ ar symbols)

Alchol

Water

Benzine

Profile 1

Profile 2

Profile 3

Profile 4

Profile 5

Profile 6

Profile 7

Profile 8

Profile 9

Profile 10

Profile 11

Profile 12

Profile 13

Profile 14

123456789

10111213141516

17181920212223242526272829303132333435363738394041424344454647

4849505152535455565758596061626364656667686970

0 1 2 3 4 5 6 7 8 9 10 11 12

e r a n u m e r o a s a m p e s

Structure index

( low = stable structure , high = po or stucture )

indexlowest

highest

mean

Figure 3. (Left) The sampled-layers are arranged in a (1-70) series and the three treatments are comparedFigure 4. (Right) The structure-index values are compared. The lowest, highest and mean values are shown. Four

profiles (#1, 2, 8 and 11) have low values (2-5) (fully-good structure), seven profiles (# 3, 4, 7, 10, 12, 13 and 14) havemoderate values (

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This sharp change in soil-water regime has accompanied the introduction of perennialirrigation and resulted elsewhere in the Nilotic Egypt, Hussein et al, (1998), in soil degradation.That degradation was mainly attributed to water-table rise (Hussein et al, 1998, 1999 and 2001 andMohamed Fahmy Hussein, 2001). To conserve soil and to prevent environmental degradation, ahuge tile-drainage project was initiated 20 years ago and should be accomplished by the year 2017.However, the maintenance of the installed tile-network continually seems a difficult task.

The present work is concerned with soil-structure and its relation to soil-behavior in the studyarea. A host environmental study will be published elsewhere. A new technique, based on fluidtreatment, is applied on seventy soil samples, from fourteen profiles as well as five surface-soilsamples, in the Monofieyah Governorate at the Nile Delta apex, 40km north of Cairo, for theevaluation of the new index. The results are shown in Figures 3-12. Three five air-dry gram massesof each soil sample were treated as follows according to the structure-index of Mounier G.,(1994): 1) Ethyl alcohol, followed by water, 2) Benzene then water, and 3) Water alone. Three setsof stable aggregates are obtained for each sample and a single parameter (structure-index, SI) isderived.

The purpose of adding organic fluids ( Ethyl Alcohol or Benzene ) is to slow-down water-imbibition into soil aggregates. Ethyl alcohol completely mixes with water; it does not decreasecohesion; instead, it artificially increases aggregate-stability (due to air-escape without destroyingthe aggregates, so when water is subsequently added the aggregates do not suddenly explode.Consequently, the poor-structured soils may be compared). Benzene is, on the contrary, a non-polar and uncompressible fluid that does not mix with water. When water is next added, it pressurizes the

Benzene entrapped into soil-pores, so the aggregates abruptly explode and only the very stableaggregates persist. Benzene is used for soils poor in organic-matter in order to increase thedestructive power of water. In the case of organic-matter rich soils, Benzene will be fixed (on theO.M.) and aggregate-stability will not be affected; rather a gradual distribution of the structure-index will be obtained. Benzene application is usually used to study soils with O.M. of differentorigins. Adding water without previous treatment decreases cohesion so air is entrapped andaggregates readily explode. The formula used for the structure index (Mounier G., 1994) is:

The C.S. ratio is the coarse sand percentage in soil sample (determined according to the regular particle-size distribution lab procedure). The factor 0.9 is believed to slightly increasing thedenominator in order to approach soil structure status under field conditions. When this formula

produces a high value, this indicates soil structure deterioration.

Observations and InterpretationFigure 1 demonstrates the study area. Figure 3 is shows the lowest percentages of stable-

aggregates for the Benzene -treatment. These low percentages (min=0.8%, max=11% with oneexception of 19.8% for the surface layer of profile 5, sample #21,) represent the highly stableaggregates. The Alcohol -treatment has exaggerated the stable aggregates (min=16.4%,max=64.2%). The wate r only treatment (min=3.8%, max=33.4%) is between the Benzene and the

Alcohol treatments. These observations show that the three-fold-fluid treatment is working fine.This single-parameter structure indicator is shown in Figures 3, 4 and 5 for the studied profiles.

Each time the sable aggregates (Figure 3) decreases the structure-index (Figure 4) increases. Three profiles (# 2, 8 and 11) generally have low index-values (from ~2 to ~4) that reflect good structure.Eight profiles (#1, 3, 4, 7, 10, 12, 13 and 14) have moderate index-values (from ~3 to ~6) in mostlayers reflecting moderate to sensible structure, and three profiles (#5. 6 an 9) have moderate tohigh index-values (from ~4 to ~12) reflecting a mixture of sensible and poor structure.

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Some profiles (# 1, 5, 10, 11 and 12), Figures 4 and 5, have high index -values in the bottomlayers, whereas other profiles (# 3, 7, 9 and 14) have high index -values also at the soil surface.Profiles #2, 4, and 6, 8, 13 have index-values oscillating between relatively low to relatively highthroughout the sampled depths. Generally, it seems that soil degradation in the study area generallystarts at profile bottom through contact with the water table, reception of the leachate from theupper layers and by the over-burden stresses.

According to the obtained Structure-Index values, we introduce the following limits: (2-4%),(4-6%), (6-8%) and (8-12%) for the following four categories: Good , Moderate , Sensible and P oor structure, respectively.

We used unpublished data, Hussein (1975), for soil structure evaluation of soils in the north of the Nile Delta using the structure-factor (SF) method where only water was used to determine thedeflocculated clay percent. We calculated the term [(1/SF)*100]. A general trend of the increase of this term with depth is observed (Figure 6) with some exceptions. The structure-factor has acurvilinear relationship with the ESP (not shown) whereas the inverse term [(1/SF)*100] has alinear relationship with ESP (Figure 8). It seems that the structure-index method is more reliableand sensitive to show slight changes in soil structure than the structure-factor method; may be dueto the implication of three fluids in the structure-index technique and its improved formula.

The relationship of the structure-index, SI, with the ESP, Figure 7, is showing a linear positivetrend (with the increase of the ESP, soil structure is degraded as shown by the increase of the SIvalue). However, the scatter is high; may be because ESP is just a factor, among others, affectingstructure status. The scatter of data-points sharply increases when the ESP surpasses 15%. Therelationship of SI with EC, Figure 9, is showing that the SI value increases with salinity increase.The scatter of data-points becomes high starting from EC of 4dS/m. When EC surpasses 16dS/m(not shown) the SI value decreases (apparent amelioration of structure probably due to excessivesoluble salt-content that enhances cohesion of soil material). It is to be noticed that the very earlyincrease in soil salinity produces rapid structure degradation. Nonetheless, neither salinity (EC)alone nor ESP alone, nor the two together, could be separately considered in data interpretation.The effect of these two factors could be understood in the general context of the drying-wettingcycles affecting soil moisture, irrigation intervals and chemical composition of soil-solution and allthe other factors that may affect soil aggregation, including the microbial activity, temperaturechanges, degasing of soil air, O.M. content and composition, cultivation, management etc sinceall these factors are continuously acting on the destruction and/or the reconstruction of stableaggregates, in the soil space- and time- continuum.

The linear increase of the ln ESP with ln EC (Figure 10) is a common observation in the salt-affected soils. In that diagram we used non-published data (Hussein, 1975) and the wide-range of values is better appreciated through ln-transformation. The exchangeable (ECa+MgP), Figure 11,has a very sharp slope in the linear relationship with ESP, whereas the other exchangeable cations(ECaP, EMgP and EKP) have a less acute slope, a moderate slope and a very week slope,

respectively. It is obvious that exchangeable Ca is the cation preferentially replaced by Na in thesechemically deteriorated soils, followed by the exchangeable Mg. The CEC is weekly related to claycontent (not shown) probably due to the dominance of clay-mineral type over clay content. TheESP is very weekly related to both clay percentage and CEC (not shown). Seemingly, this reflectsthat the relationship of these variables with the ESP is more defined by clay mineralogy (affectingthe state of the diffuse electrical double-layer and the zeta potential) rather than by anything else.

The log-transformation of (Ksat-ESP) data, Figure 12, resulted in improvement of the linear relationship. In the (logK sat-logESP) plot, linearity is shown with a strong scatter. A negative linear trend is obtained for the (logK sat-log SI) relationship, Figure 12. The high scatter says that the K sat,ESP and SI values are affected by several factors, so a single relationship is a part of a complicatedsetup that cannot be defined by any single factor. Table 1 gives summary of the upper and lower

limits of data. The values of K sat have the highest ratio of the upper/lower limits (700 times). Theratio of the upper/lower limits for ESP and SI is comparable (nine and seven times, respectively.)

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Figure5. (Upper) Fourteen profiles are compared according to the structure-index valuesFigure6. (Lower) Five profiles are compared with respect to the inverse of the structure factor, (1/SF) *100

y = 0.124x + 2.696R = 0.243

0

2

4

68

10

12

0 5 10 15 20 25 30 35

Structureindex

ESP

y = 0.167 x + 0.044

R = 0.401

0

5

1015

20

0 10 20 30 40 50

(1/SF )*100

ESP

Figure7. (Left) The relationship of structure-index with ESP; scatter increases when ESP surpasses 15%Figure8. (Right) [(1/SF)*100] increases with ESP increase

y = 0.295x + 3.250R = 0.233

0

2

4

6

8

10

12

0 2 4 6 8 10 12 14 16

Structureindex

EC, dS.m -1

y = 0.482x + 1.921R = 0.579

0.5

1.0

1.5

2.02.5

3.03.5

4.0

4.5

-0.5 0.0 0.5 1.0 1.5 2.0

ln ESP

ln EC, dS.m -1

this work

unpublisheddatalinear fit

Figure9. (Left) Relationship of the structure index with ECFigure10. (Right) Relationship between ESP and EC is shown by ln-transformation

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0102030405060708090

100

0 20 40 60 80 100

EXP

ESP

X = Ca+Mg

regr. (ECa+MgP)-ESP

X = Ca

regr. ECaP-ESP

X = Mg

regr. EMgP-ESP

X = K

regr. EKP-ESP

0

5

10

15

20

25

30

35

40

45

50

55

60

1

12

23

34

45

increae of Na

SAR

55-60

50-55

45-50

40-45

35-40

30-35

25-30

20-25

15-20

10-15

5-10

0-5

Figure11a and b. Regressions of exchangeable cations. The slope of (EMgP-ESP) is one-third of that of (ESP-ECaP).The slope of (EKP-ESP) is 0.08 of that of (ESP-ECaP). In the SAR diagram, Na is given in series on x-axis from 1 to30, (corresponding to 2 to 60 meq.l -1)

-3.0-2.0-1.00.01.0

0.5 1.0 1.5 2.0

logK sat

log ESP

this work unpublished data a ll Linea r (a ll)

-3.0

-2.0

-1.0

0.0

1.0

0.0 0.5 1.0

log

K sat

log structure index

Figure12 Log-transformations show decrease of (log K sat) with both (log ESP) and (log structure-index) increase

ConclusionsA new single-parameter indicator (the structure index, SI) was applied on 70 soil materials and

proved informative and sensitive to soil structure status. The technique is simple, elegant and can beused on a routine-basis for soil conservation purposes. Soil degradation has several reasons. One of the most outstanding reasons for the alluvial soils in Egypt is related to water-table rise in theunconfined aquifer during the last 150years of perennial irrigation practice without installation of field-drainage network until 20years ago. Consequently, the over-recharge of the Delta semi-confined aquifer has taken place and manifested in rise of piezometric level of the upper (unconfined) aquifer. The present-day water-table level is below soil surface by ~1m (instead of 10m by the mid of the 19 th century). The direct result of such situation is the increase of salt-contentand development of high ESP. So, soil became under increasing chemical stresses that lead todeterioration of yield amount and quality. The obtained data show a positive linear relationship

between EC and the ESP. The values of that structure index also illustrate positive linear relationships between the SI-values and both of the EC and the ESP. The SI surpasses the old(structure-factor) parameter in sensitivity. It is recommended to use the SI as a lab routine as it issuccessful for detecting soil structure degradation in its early and moderate stages.

ReferencesDenef, K., J. Six, R. Merckx, and K. Paustian. 2003. Carbon sequestration in micro-aggregates of no-tillage soils with

different clay mineralogy. Soil Sci. Soc. Am. J. 68:19351944Diaz-Zoritaa,b, M., E. Perfect and J. H. Grove. 2002. Disruptive methods for assessing soil structure.

Elsevier Science, 254-9Edwards, A.P., and J.M. Bremner. 1967. Micro-aggregates in soils. J. Soil Sci. 18: 47-63.Eynard, A., T. E. Schumacher, M. J. Lindstrom, and D. D. Malo. 2004. Aggregate sizes and stability in cultivated south

Dakota prairie Ustolls and Usterts. Soil Sci. Soc. Am. J. 68:13601365Gabriels, D., and M. De Boodt. 1976 Report on the use of petroleum products for soil structure modification and soil

conservation. Soil Physics lab, Rijks Univ. Gent.Gasperi-Mago, R.R., and F.R. Troeh. 1979. Microbial effects on soil erodibility. Soil Sci. Soc. Am. J. 43: 765-768.Harris, R.F., 6. Chester, and O.N. Alien. 1966. Dynamics of soil aggregation. Adv. Agron. 18: 107-169.

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Hussein, M. F, 1975.Morphological, physical and chemical changes associated with salinization process in Egyptian Alluvial Soils.M.Sc. Thesis, Cairo Univ., Fac. of Agric, Soil and Water Dept. 171p.

Hussein, M. F. 1995.RIETHM : Radioisotope environmental tracer hydrology model using the exponential and dispersion methods. In:First International Symposium on Application of Tracers in Arid Zone Hydrology , IAEA-IHA, Vienna, 15-26August 1994. IAHS Publ. N. 232, 1995, Proceedings of the Vienna Symposium, August 1994, 211-224.

Hussein, M. Fahmy and Michel, R. L., Froehlich, K. 1998.Historical isotopic changes in the River Nile and their relation to the study of ground-water hydrology of the NileValley and Delta. In: Gambling with Groundwater: Physical, Chemical, and Biological Aspects of Aquifer-StreamRelationships. International Association of Hydro-geologists. XXVIII Congress and Annual Meeting of theAmerican Institute of Hydrology, Las Vegas, Nevada, USA. September 27-October 2, 1998, 105-110.

Hussein, M. F., Frohlich, K., Nada, A., Atta, S.A. and Tanweer, A., 1998. Environmental isotopes and hydrochemistryof ground water in west El-Fashn and Maghagha regions, Isotope & Rad. Res., 30, 1-10.

Hussein, M. Fahmy, Michel, R. L., Nada A., Froehlich, K., Atta, S. and Tanweer, A. 1999.Use of isotope geochemistry to determine the effectiveness of water management strategies, Nile valley, Egypt. In:International Symposium on Isotope Techniques in Water Resources Development and Management. Book of Extended Synopses, IAEA-SM-361, Vienna, Austria, 10-14 May 1999, pp 63-69.

Hussein, M F, Frehlich, K. and Nada A. 2001.Environmental isotope profiles and evaporation in shallow water-table soils. In: Isotope techniques in water

resources investigations in arid and semi-arid regions, IAEA CRP 303-F3-Egy7902/RB, IAEA-TECDOC-1207,March 2001, 27-31.Marquez, C. O., V. J. Garcia, C. A. Cambardella, R. C. Schultz, and T. M. Isenhart. 2004. Aggregate-size stability

distribution and soil stability. Soil Sci. Soc. Am. J. 68:725735Mohamed Fahmy Hussein, 2001. Monitoring land and water degradation: Environmental dimensions and restoration.

Sym. Nat. Res. & Their Conservation in Egypt & Africa, Inst. African Res. & Stud., Cairo Univ., 19-21 November 2001, 474-495

Monnier G. 1994. Assemblage et Organisation Physique des Particules.. In : Bonneua Maurice et Souchier Bernard(editors). Pdologie, Part 2. Constituants et Proprits du sol, 2nd Edition, Masson, Paris, Milan and Barcelone,665p (la mthode analytique de l'indice structural, page 346).

Oades, J. M. 1984. Soil organic-matter and structural stability: mechanisms and implications for management. Plant andSoil 76, 319-337

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