AGGREGATE STABILITY IN RELATION TO ORGANIC CARBONCONTENT OF EIGHT DIFFERENT SOILS OF BANGLADESH

A dissertation for the partial fulfillment of the requirements for theDegree of four years Bachelor of Science (Hon’s) in Soil Science

A Project thesis bySharif Sinthia Islam

Roll No: 061331Session: 2006-2007

Supervised byMd. Sadiqul AminAssistant Professor

Soil Science Discipline Khulna University

Chairman of Examination CommitteeAfroza Begum

Associate Professor and HeadSoil Science Discipline

Khulna UniversityKhulna-9208, Bangladesh

Soil Science DisciplineLife Science School, Khulna University

Khulna, Bangladesh.July, 2010

CHAPTER 1: INTRODUCTION

CHAPTER 2: LITERATURE REVIEW 3

2.1. Aggregate stability 32.2. Forces involved in aggregation 32.3. Correlation of aggregate stability with potential causal factors 4

2.3.1. Soil texture 42.3.2. Organic carbon 52.3.3. Chemical dispersing agents 62.3.4. Clay mineralogy 6

2.4. Aggregate hierarchy concept 72.5. Different mechanism of aggregate breakdown 92.6. Aggregate fractionation methods 92.7. Soil organic carbon 102.8. Forms of soil organic carbon 11

2.8.1. Mineral forms 112.8.2. Elemental organic C 112.8.3. Humus 112.8.4. Altered organic residue of plants 12

2.9. Factors influencing organic carbon level in soil 132.9.1. Soil carbon losses 13

2.9.1.1. Type of plant and animal matter entering soil 132.9.1.2. Climatic conditions 132.9.1.3. Management practices 14

2.9.2. Soil carbon inputs 142.9.2.1. Environmental which govern plant production 142.9.2.2. Management practices 15

2.10. Stabilization and destabilization of soil organic matter 162.11. Soil Aggregate –Stability and organic matter 17

2.11.1. Inorganic binding agents 172.11.1.1. Oxides 172.11.1.2. Calcium 18

2.11.2. Soil texture 192.12. Functions of soil organic matter and effect on soil properties 19

2.13. Aggregate stability –time relationship 21-23

CHAPTER 3: MATERIALS AND METHODS

3.1. Sampling sites 243.1.1. Short description of the sampling sites 24

3.2. Sample preparation 253.3. Laboratory Analysis 25

3.3.1. Particle size Analysis 253.3.2. Micro aggregate analysis 253.3.3. Aggregate stability (NSI) 26-273.3.4. Soil reaction (pH) 283.3.5. Electric conductivity (EC) 283.3.6. Organic carbon 283.3.7. Statistical analysis 28

Title Page No.

Acknowledgement vAbstract viContents viiList of Tables viiiList of Figures viii

ContentsCHAPTER 4: RESULTS AND DISCUSSION

4. Results and discussion 294.1. General Analyses of Soil 29

4.1.1. pH 294.1.2. Electrical conductivity(EC) 294.1.3. Organic carbon 304.1.4. Particle size distribution 304.1.5. Micro aggregate analysis 304.1.6. Indices of aggregation 31

4.1.6.1. Degree of aggregation (DA) 344.1.6.2. State of Aggregation (SA) 35

4.1.7. Normalized stability Index (NSI) 354.2. Statistical analysis 364.3. Relationship among soil properties 36

CHAPTER 5 : SUMMARY AND CONCLUSION

5.1. Summary and Conclusion 37

CHAPTER 6 : REFFERENCES 37-45

1.1 Outline

Aggregate stability is an underpinning factor in questions of soil fertility and environmental

problems. Resistance of the aggregate to physical stresses determines soil sensitivity to crusting

and erosion (Le Bissonnais, 1996a), germination and rooting of cultivated plants (Angers and

Caron, 1998) and the ability of a soil to store carbon through the physical protection of organic

molecules (Jastrow and Miller, 1997). The measurement of soil aggregate stability is important

because it can give general information about soil conditions. The aggregate stability is the

ability of the bonds of the aggregates to resist when exposed to stresses causing their

disintegration (e.g. tillage, swelling and shrinking processes, kinetic energy of raindrops, etc.)

Factors that influence aggregate stability have been frequently reviewed (Le Bissonnais, 1996b;

Kay, 1998; Bronick and Lal, 2005). Texture, clay mineralogy, cation content, aluminium and iron

oxides, and soil organic matter are the major soil properties influencing aggregate stability. Soil

organic matter (SOM) content is commonly defined as the percent humus held within a soil.

Organic matter (OM) is a major factor affecting aggregate stability because its abundance and

characteristics can be modified by agricultural practices. In many cultivation systems, fresh OM

is periodically returned to the soil as litter or crop residues but the amounts and quality of the

organic input are variable. This agricultural practice would appear to be an interesting solution

for maintaining soil fertility and for soil rehabilitation in degraded situations. The whole

functioning of soils is profoundly influenced by SOM, its ability to provide conditions for plant

growth, soil biota functioning, reduction of greenhouse gases, modification of pollutants and

maintenance of soil physical condition. SOM influences many soil properties, such as water

retention capacity, extractable bases, capacity to supply macro- and micronutrients, aggregate

stability and aeration.

There is a strong relationship between soil carbon content and aggregate size. An increase in soil

carbon content led to a 134-percent increase in aggregates of more than 2 mm and a 38-percent

decrease in aggregates of less than 0.25 mm (Castro Filho et al., 1998). The active fraction of soil

C (Whitbread and Blair et al., 1998) is the primary factor controlling aggregate breakdown (Bell

et al., 1999). Poor soil structural stability is a serious and increasing problem in several areas of

the world. An appropriate management of organic matter additions to soils may increase

aggregate stability and thus reduce crusting and erosion problems. Soil organic matter can be:

(1) Physically stabilized, or protected from decomposition, through microaggregation, or

(2) Intimate association with silt and clay particles, and

(3) Can be biochemically stabilized through the formation of recalcitrant SOM compounds.

It has been recognized for a long time that organic matter serves as a granulating agent in soils.

Baver (1935) observed a correlation of 0.559 (with 0.21 being significant) between the

percentage of aggregates larger than 0.05mm and the carbon content of a large number of

different soils. This correlation for aggregates larger than 0.1mm was 0.687, which indicates that

organic matter is conducive to the formation of relatively large stable aggregates. Agricultural

management of soil carbon is a recognized means of improving soil fertility, reducing soil

erosion rates, enhancing soil structural stability and assisting carbon sequestration.

1.2. Objectives

Therefore objectives of this study were to correlate aggregate stability of some soils of

Bangladesh in relation to organic carbon content of the particular soil types.

Evaluate the ability to form aggregates or aggregate stability in relation to soil carbon

content in different type of soils in Bangladesh.

2. Review of Literature

Organic materials are important soil additives to improve soil physical properties. This is

important to sustain the productivity of soils particularly in semi-arid regions where there is

low input of organic materials. Usage of organic based materials has gained importance within

the last few years for sustainable agriculture and preventing soil degradation. Soil aggregate

formation and aggregate stability have an important role in crop production and sustainable

agricultural management. Soil aggregate formation has an important role concerning seed–soil

relation, hydraulic conductivity and root respiration, the diffusion of gases within the soil and

plant growth. Furthermore, water-stable aggregates in soil prevent erosion, which is one of the

main factors of soil degradation (Dinel et al. 1991). Structural soil degradation occurs mostly

due to the decrease in soil organic matter caused by excessive soil cultivation (Grandy et al.

2002).

2.1 Soil organic carbon

According to Grace et al. (1995), it is generally accepted that soil organic matter, and principally

carbon is the precursor to sustainable soil management. Soil organic matter (SOM) is as

important as a reservoir of nutrients such as nitrogen, phosphorus and sulphur. It also

contributes significantly to the formation and stabilization of soil structure. Active soil organic

matter (ASOM) is the heterogeneous mix of living and dead organic materials (Wander et al.

1994), including microbial biomass carbon (MBC), easily oxidisable organic carbon (EOC),

water-soluble organic carbon (WSOC), and water soluble carbohydrates (WSC) measured as

anthrone-reactive carbon. As active fractions are usually more easily affected than total soil

organic carbon (TSOC) by management, measures of them can be used as early indicators of

change in SOM status (Gregorich et al. 1994; Bremer et al. 1994).

Hodges (1991) points out that soil organic matter is important as energy for soil organisms, in

soil structure and stability, water relations, soil erosion control, soil warming, availability of

plant nutrients, ion exchange, chelation and buffering, crop growth, and incidence of soil-borne

diseases. However, by world standards, Vagen (2000) suggests that the critical soil organic

matter content for agricultural soils is generally regarded to be between 2 and 4 %. The low

inherent level in surface soils has been exacerbated by up to 39 % of the soil organic carbon

having been lost through erosion and mineralization between 1860 and 1990 (Gifford et al.

2002). These reductions in soil organic matter have led to declining soil structure, made evident

by soil crusting and compaction. The cultivation of soils for crop production commonly leads to a

reduction in soil organic carbon. Rovira and Greacen (1957) found that through shearing soil

aggregates (analogous to cultivation), the soil particles were exposed to increased microbial

activity, thereby oxidising the soil organic matter. To redress soil organic matter decline, it is

therefore necessary to return plant residues to the soil, minimize cultivations and preferably

include a pasture phase within the cropping rotation (Rovira et al. 1957).

2.2 Forms of soil organic carbon

When plant residues are returned to the soil, various organic compounds undergo

decomposition. Decomposition is a biological process that includes the physical breakdown and

biochemical transformation of complex organic molecules of dead material into simpler organic

and inorganic molecules (Juma, 1998). Carbon occurs in soils in 4 forms of mineral and organic

matter:

2.2.1 Mineral forms

Soil organic matter has a very complex and heterogeneous composition and is generally mixed

or associated with the mineral soil constituents to form soil aggregates (Del Galdo et al. 2003).

Carbonate mineral forms (chiefly CaCO3 and MgCO3. CaCO3). However, highly active and

important small amounts also occur as CO2, HCO3- and CO3

- ions of more soluble salts. The

Carbonate fraction is also involved in aggregation, especially soils low in OM (Tisdall and Oades,

1982)

2.2.2 Elemental organic C

Highly condensed, nearly elemental organic C (eg. charcoal, graphite, coal etc.) occupies a little

portion of soil organic carbon.

2.2.3 Humus

Altered and rather resistant organic residues of plants, animals and microorganisms (sometimes

called humus).Largely amorphous and colloidal mixture of complex organic substances no

longer identifiable as tissues. The humus micelles like the particles of clay , carry a swarm of

adsorbed cations (Ca2+, H+,Mg2+,K+) etc, which are changed with cations from the soil solution.

The water holding capacity of humus on a mass basis four to five times that of silicate clays.

Humus plays a role in aggregate formation and stability. The highly complex humus molecules

contain chemical structures that absorb nearly all wavelengths of visible light, giving the

substances a characteristic of black color (Bradey and Weil, 2002). The major components of

SOM implicated in stabilizing soil aggregates are humic substances. Several studies have shown

that the humic substances (mostly humic acids) improve aggregate stability (Fortun et al. 1990)

2.2.4 Altered organic residues of plants

Little altered organic residues of plants, animals, and microorganisms (both living and dead)

subject to rather decomposition in soil. The living part of soil organic matter includes a wide

variety of micro-organisms such as bacteria, viruses, fungi, protozoa and algae. It also includes

plant roots, insects, earthworms, and larger animals such as moles, mice and rabbits that spend

part of their life in the soil. The living portion represents about 5 percent of the total soil organic

matter. Micro-organisms, earthworms and insects help break down crop residues and manures

by ingesting them and mixing them with the minerals in the soil, and in the process recycling

energy and plant nutrients. Sticky substances on the skin of earthworms and those produced by

fungi and bacteria help bind particles together. Earthworm casts are also more strongly

aggregated (bound together) than the surrounding soil as a result of the mixing of organic

matter and soil mineral material, as well as the intestinal mucus of the worm. Thus, the living

part of the soil is responsible for keeping air and water available, providing plant nutrients,

breaking down pollutants and maintaining the soil structure therefore aggregate stabilization.

Organisms occur wherever organic matter occurs (Ingham, 2000). Therefore, soil organisms are

concentrated around roots, in litter, on humus, on the surface of soil aggregates and in spaces

between aggregates.

Soils containing less than 20% organic matter by weight are classified as mineral soil and more

than 20% organic matter is commonly called organic soil. Agricultural soils are essentially

mineral soils and fertile mineral soils on an average contain about 5% organic matter (Johnston,

1991).

2.3 Factors influence organic carbon level in soil

Organic carbon influences many soil characteristics including color, nutrient holding capacity

(cation and anion exchange capacity), nutrient turnover and stability, which in turn influence

water relations, aeration and stability. The amount of carbon in a soil is dependent on the

characteristics of the soil and the balance between inputs and losses. Many factors, such as

rainfall, temperature, vegetation and soil type determine the amount of carbon in soil. Some of

these factors are fixed characteristics of the soil, some are determined by the climate and some

can be influenced by management practices.

2.3.1 Soil carbon losses

Losses of carbon from soil result from decomposition and conversion of carbon in plant residues

and soil organic materials into carbon dioxide. Many factors, such as rainfall, temperature,

vegetation and soil type determine the amount of carbon in soil.

The rate of loss is determined by:

2.3.1.1 Type of plant and animal matter entering the soil

Plant functional types significantly affected the vertical distribution of SOC. The percentage of

SOC in the top 20 cm (relative to the first meter) averaged 33%, 42%, and 50% for shrublands,

grasslands, and forests, respectively. In shrublands, the amount of SOC in the second and third

meters was 77% of that in the first meter; in forests and grasslands, the totals were 56% and

43%, respectively. Globally, the relative distribution of SOC with depth had a slightly stronger

association with vegetation than with climate, but the opposite was true for the absolute amount

of SOC (Johnston, 1991).The importance of these controls switched with depth, climate

dominating in shallow layers and clay content dominating in deeper layers, possibly due to

increasing percentages of slowly cycling SOC fractions at depth. To control for the effects of

climate on vegetation, we grouped soils within climatic ranges and compared distributions for

vegetation types within each range. The percentage of SOC in the top 20 cm relative to the first

meter varied from 29% in cold arid shrublands to 57% in cold humid forests and, for a given

climate, was always deepest in shrublands, intermediate in grasslands, and shallowest in forests

(P < 0.05 in all cases). The effect of vegetation type was more important than the direct effect of

precipitation in this analysis. These data suggest that shoot/root allocations combined with

vertical root distributions, affect the distribution of SOC with depth. (Esteban, et.al., 2000).

2.3.1.2 Climatic conditions

Total SOC content increased with precipitation and clay content and decreased with

temperature. Wind erosion and water erosion increase losses of organic matter. Erosion breaks

down soil aggregates, exposing physically protected organic matter to decomposition and loss.

Organic-rich soil from the surface layer is carried away by runoff or wind. Litter redistribution

by wind or water from or to surrounding rangeland also affects the content of organic matter.

(USDA, May 2001)

2.3.1.3 Management practices

Some management practices which reduce carbon inputs or increase the decomposition of soil

organic matter can also influence carbon losses. In the past, keeping the soil bare was a common

cropping practice. Fallowing was maintained by repeated cultivation for weed control. SOC

declines rapidly under fallowing because of the increased decomposition of organic matter due

to the cultivation operations as well as the higher soil moisture conditions prevailing in the

fallowed soils. Cultivation operations can expose SOC and increase losses by decomposition and

erosion. Historically, excessive cultivation using inappropriate implements resulted in soils

being ‘over-worked’, and the consequent loss of SOC has caused many land degradation

problems such as erosion and soil structural decline. However, it should be noted that some of

the yield increasing practices involve the use of fertilizers and irrigation water which require

large energy consumption and therefore increase carbon dioxide emission. Grazing can increase

the rate of root turnover, but overgrazing reduces the amount of plant energy available for the

growth of new roots. Trampling by livestock can help to incorporate the plant material above

the ground into the soil. In arid ecosystems, however, little plant material is available for

incorporation. Trampling also breaks up soil aggregates, exposing organic matter to

decomposition and loss through erosion.

2.3.2 Soil carbon inputs

Soil organic carbon inputs are controlled by the type and amount of plant and animal matter

being added to the soil. Any practice that enhances productivity and the return of plant residues

(shoots and roots) to the soil enhance the amount of carbon in the soil. The majority of carbon

enters the soil as plant residues.

Plant residue, and thus soil carbon inputs are mainly affected by the

Type of plants being grown

Amount of dry matter the plants accumulate over the growing season

2.3.2.1 Environmental factors which govern plant production

Environmental factors interacting over time affect the amount of organic matter in soil. Rainfall

and temperature affect plant productivity and the rate of organic matter decomposition.

Increasing levels of organic matter promote a higher water holding capacity, which results in

increased plant growth and thus an increased amount of organic matter and plant nutrients .

(USDA, NRCS, May 2001). The influence of mean annual precipitation on the annual SOM

dynamics was demonstrated under a commonly used crop rotation with different initial C

content. A sharp annual decrease of SOM takes place for podzoluvisols with initial C content 2%

when precipitation increases from 200 up to 450 mm, while for soils with a carbon content of

1%, the opposite trend was observed, followed by equilibrium at RV 350 mm (Sirotenko et al.,

2002).

2.3.2.2 Management practices

A variety of management practices can increase soil carbon levels by increasing inputs. In

theory, maximizing productivity also maximizes returns of organic residues to the soil. In

agricultural systems, substantial amounts of plant biomass remain in the field after harvest

(including roots). From a mass balance approach, an increase in soil C can be achieved either by

increasing inputs or decreasing outputs. Decreased output, or conservation of soil C, is a function

of microbial activity, and physical and chemical protection (Figure 2.1). Disturbance of

aggregates, by natural (freeze-thaw, wet-dry cycles) or anthropogenic (tillage) forces increases

the decomposition of the physically protected organic materials (Rice and Angle, 2004). Thus,

conservation tillage practices are important for increasing soil C storage and retaining

sequestered C. Tillage management practices effectively use microbial and physical controls to

increase the amount of C that can be stored, or protected, by a soil. Reduced tillage reduces

carbon losses (from both reduced cultivation and reduced fossil fuel usage) and stubble

retention increases carbon inputs to the soil; both of these lead to SOC increase. Growing cover

crops is one of the best practices for improving organic matter levels and hence, soil quality.

Grasses and cereals are most appropriate for this stage, also because of their intensive rooting

system, which improves the soil structure rapidly. Agro forestry is a collective name for land-use

systems where woody perennials (trees, shrubs, palms, etc.) are integrated in the farming

system (FAO, 1989). Alley cropping is an agro forestry system in which crops are grown

between rows of planted woody shrubs or trees. These are pruned during the cropping season

to provide green manure and to minimize shading of crops (FAO, 1993). Besides adding organic

matter to the system, perennial trees and shrubs recycle plant nutrients from deeper soil layers

through their rooting system.

Fig 2.1: Factors

controlling C conservation in soil.

2.4 Functions of soil organic matter and effects on soil properties

Organic matter affects both the chemical and physical properties of the soil and its overall

health. Properties influenced by organic matter include: soil structure, moisture holding

capacity, diversity and activity of soil organisms, both those that are beneficial and harmful to

crop production and nutrient availability (Fig:2.3). It also influences the effects of chemical

amendments, fertilizers, pesticides and herbicides. The interrelationship between SOC and soil

structure and other physical properties has been extensively studied, and excellent reviews can

be found in Tisdall and Oades (1982). It is well established that addition of SOM can not only

reduce bulk density (Db) and increase water holding capacity, but also effectively increase soil

aggregate stability. Angers and Carter (1996) noted that the amount of water-stable aggregates

(WSA) was often associated with SOC content, and that particularly labile carbon was often

positively related to macro-aggregate stability. Kay and Angers (1999) reported that a minimum

of 2% SOC was necessary to maintain structural stability and observed that if SOC content was

between 1.2-1.5%, stability declined rapidly. Haynes (2000) found that the mean weight

diameter (MWD) of aggregates exhibited a curvilinear increase with carbon content, suggesting

an upper limit of influence of SOC (Fig: 2.2). Carter (1992) found that maximum levels for an

agronomically designed aggregation index (AI) were obtained at SOC contents of > 2.5% and at

microbial biomass carbon contents of 250µg C/g soil, whereas maximum soil structural stability

(determined by MWD) was found at SOC levels of 4.5%. Carter (1992) suggested that 2.5% could

serve as a critical limit to define minimum concentrations of SOC required to provide optimum

structural stability in fine sandy loams.

Figure 2.2: Effect of increasing SOC content on aggregate stability, measured by wet-sieving (MWD, mm), using air-dried (●) and field moist (○) samples (modified after Haynes, 2000).

Boix-Fayos et al. (2001) showed that a threshold of 3-3.5% SOC had to be attained to achieve

increases in aggregate stability, no effects on aggregate stability were observed in soils below

this threshold. Fine-textured soils sequestered more macroaggregate- protected SOC near the

soil surface than coarse textured soils, due largely to greater macroaggregation (Franzluebbers

and Arshad, 1996). The stability of larger macro aggregates, in particular, is largely a function of

active soil organic matter fractions (Tisdall, 1996). These fractions have high turnover rates and

Functions of Soil organic matter

Biological functions-Provides source of energy (essential for biological process)-Provides reservoir of nutrients (N, P, S)-Contributes to resilience of soil /plant system

-contributes to resilience of soil /plant system

Physical functions-Improves structural stability of soils at various scales-Influence water retention properties of soils and thus water –holding capacity-Alter soil thermal properties

Chemical functions-Contribute to the cation exchange capacity-Enhances ability of soils to buffer changes in pH - Reduce concentrations of toxic cations - Promote binding of SOM to soil minerals.

are sensitive to management (Wander et al., 1994). Carter (1992) found that maximum levels for

an agronomically designed aggregation index (AI) were obtained at SOC contents of > 2.5% and

at microbial biomass carbon contents of 250µg C/g soil, whereas maximum soil structural

stability (determined by MWD) was found at SOC levels of 4.5%. Carter (1992) suggested that

2.5% could serve as a critical limit to define minimum concentrations of SOC required to provide

optimum structural stability in fine sandy loams.

Fig 2.3: Functions ascribed to SOM note that interaction occurs between the different soil functions modified from

Badlock and Skjemstad, 1999.

2.5 Stabilization and destabilization of Soil organic matter

Stabilization of a portion of the litter C yields material that resists further transformation.

Stability of the organic C is viewed as resulting from three general sets of characteristics. It is

widely assumed, for example, that fresh plant detritus is converted gradually to more stable

forms (sometimes termed “humus”), and that this stabilization involves a variety of physical,

chemical, faunal, and microbial processes (Ladd et al.,1993).

Recalcitrance comprises molecular-level characteristics of organic substances, including

elemental composition, presence of functional groups, and molecular conformation, that

influence their degradation by microbes and enzymes.

Interactions refers to the inter-molecular interactions between organics and either inorganic

substances or other organic substances, that alter the rate of degradation of those organics or

synthesis of new organics.

Accessibility refers to the location of organic substances as it influences their access by microbes

and enzymes.

Stability is the integrated effect of recalcitrance, interactions, and accessibility. By definition, it

increases with recalcitrance and decreases with accessibility.

Destabilization yields material that is more susceptible to microbial respiration.

Destabilization is the overall process by which soil organic substances become less resistant to

degradation. By definition, it occurs by decreasing recalcitrance or by increasing accessibility.

Decreasing interactions may also promote destabilization.

2.6 Aggregate stability

Aggregate stability is often used as a measurement of soil structure. An aggregate is a group of

primary particles that cohere to each other more strongly than to other surrounding soil

particles. Most adjacent particles adhere to some degree. Stability of aggregates is a function of

whether the cohesive forces between particles withstand the applied disruptive force. However,

aggregate stability is often measured on a specific aggregate size class which is not a

measurement of whole soil structure. Furthermore, aggregate stability measurements are an

important parameter in determining the resistance of soil aggregates against environmental

factors (Hillel, 1982). Soil structure arises from the reciprocal arrangement and placement of

soil particles and aggregates (lal and shukla, 2004). It has a major influence on the ability of the

soil to receive, store, and transmit water and to favor C and nutrient cycles, and therefore, it

supports plant growth (kay, 1998).

2.6.1 Forces involved in aggregation

Two of the primary forces holding particles together in aggregates in moist soils are the surface

tension of the air and water interface and the cohesive tension (negative pressure) in the liquid

phase. Briggs (1950) has shown that the cohesive tension of water can have values up to 26MPa.

As a soil dries, the water phase recedes into capillary wedges surrounding particle-to-particle

contacts and films between closely adjacent platelets. The interfacial tension and internal

cohesive tension pull adjacent particles together with great force as soil dries. Soluble

compound such as silica, carbonates, and organic molecules are concentrated in the liquid phase

as soil dries. Many of these solute molecules and ions thus precipitate as inorganic

semicrystallaine compounds or amorphous organic compounds around these particle-to-

particle contacts, cementing them.

Soil aggregation may be determined by the mean weight diameter (MWD), the geometric mean

diameter (GMD) and the normalized stability index (NSI), which is obtained by breaking the soil

into aggregate classes by the wet sieving method (Kemper and Chepil, 1965).

2.6.2 Correlation of aggregate stability with potential causal factors

2.6.2.1 Soil texture

The more clay present in the soil, the more likely the soil is to form aggregates (clays carry an

electric charge and can stick together). However, the clay is also the part of the soil that

disperses if aggregate stability is poor. In generally considered that as the silt (0.002-0.05mm)

or silt + very fine sand (0.05-0.10 mm) fraction increases and clay decreases in consequences

aggregate stability decreases erodibility increases. This is because of

1) The aggregation and bonding effect of clay

2) The transportability of fine and non aggregated particles (i.e. silt)

3) The detachability of sand and silt

According to Edwards and Bremner (1967), the only highly stable aggregates are fine sand- and

silt-sized microaggregates (<250_m) consisting of clay–polyvalent metal–organic matter

complexes. Microaggregates are formed by bonding of C–P–OM clay sized units, where C clay

particle, P polyvalent metal (Fe, Al, Ca) and OM organo-metal complex, and are represented as

[(C–P–OM). It is evident that the C–P–OM units are equivalent to the clay domains of Emerson.

However, Edwards and Bremner (1967) envisioned C–P–C and OM–P–OM units too. They also

postulated that the organic matter complexed into the microaggregates would be inaccessible to

microorganisms and physically protected. In the aggregate hierarchy concept it is postulated

that the different binding agents (i.e. transient versus temporary versus persistent binding

agents) act at different hierarchical stages of aggregation. Free primary particles and silt-sized

aggregates (<20_m) are bound together into microaggregates (20–250_m) by persistent binding

agents (i.e. humified organic matter and polyvalent metal cation complexes), oxides and highly

disordered aluminosilicates.

2.6.2.2 Organic carbon

Aggregation generally increases with increasing soil organic matter, which is connected to clay

surfaces through positively charged polyvalent cat ions, thus overcoming the repulsion between

the negative charges of both clay and organic matter (Edwards and Bremner, 1967). In this

process, not only the amount of clay but also the clay type is important (Kay, 1998), because

mineralogical species differ in surface charge. Isomorphic substitutions, resulting in negative

permanent charges, prevail in layer silicates, whereas pH-dependent charges form on surface

hydroxyls and dominate in the case of oxides and hydroxides. The latter clays are present in the

soil with a wide range of crystallinity, and their effect on aggregation varies. Amorphous iron

oxides (Feo) are the most effective because they not only carry positive charges but also block

the negatively charged sites on layer silicates (Shao and Wang, 1991). The soil colloidal fraction

is thus characterized by a total charge, resulting from the complex interactions among all the

components. Igwe and Nwokocha (2006) investigated the role of SOC in the restoration of soil

fertility and stability of soil micro-aggregates, which is of special importance in soils that

degrade rapidly. They reported that soils were coarse-textured, deep and low in soil nutrients

and SOC, probably due to high mineralization rates. Micro-aggregate-associated SOC was also

low with most of the SOC protected by the <63 mm fractions. Principal component analysis

revealed that SOC fractions associated with 2000—63 mm aggregate sizes were the SOC

fractions that best explained the variance in aggregated silt + clay, indicating their contribution

to microaggregate stability. This was attributed to the production of polysaccharides and

materials released by microbial activities from this recently deposited or incompletely

decomposed SOC.

2.6.2.3 Chemical dispersing agent

The di-valent and trivalent cations, such as Ca2+ and Al3+, are tightly adsorbed and can effectively

neutralize the negative surface charge on clay particles; these cations can also form bridges that

bring clay particles close together. Monovalent ions, especially Na+, with relatively large

hydrated raddi, can cause clay particle to repel and each other and create a dispersed condition.

Two things contribute to the dispersion

1) The large hydrated sodium ion does not get close enough to the clay to effectively

neutralize the negative charges, and

2) The single charge on sodium is not effective in forming a bridge between clay particles.

Calcium ions associated with clay generally promote aggregation, whereas sodium ions promote

dispersion. Exchangeable sodium - can cause very poor aggregate stability. Soils with a high

percentage of exchangeable sodium are very likely to disperse and need to be managed

carefully.

Fe-oxide rich soils (e.g., many Oxisols) and allophanic soils are among the most stably micro-

aggregated (El-Swaify, 1980;). This is generally interpreted as evidence that oxides and

hydroxides of Al and Fe, as well as amorphous aluminosilicates, are important in aggregation.

Evidence for the importance of various materials as binding agents comes from studies of

disaggregation upon exposure to chemical extractants (e.g., Bartoli and Philippy, 1990;

Wierzchos et al., 1992) correlation of aggregate stability with soil properties (Molope et al.,

1985), and addition of binding agents to soil.

2.6.2.4 Clay mineralogy

Clay mineralogy influences aggregate stability but the effect is difficult to asses because soils

most often contain a mixture of clay minerals. Using pure clay minerals Emerson(1964) showed

that swelling clays like montmorillonite are less subject to slaking than kaolinite or illite because

the pressure which is developed by entrapped air is released by swelling ; however ,fissuring of

montmorillonite may occur, due to the combination of stress of entrapped air and swelling of

aggregates. As aggregating particles, the smectiteic clays should be more efficient than other

clays because of their large specific surface and high physiochemical interaction capacity .In

accordance with this statement, Young and Mutchler (1977) found that montmorillonite was

highly correlated with aggregate stability. The effects of clay and organic-matter content can be

seen in Fig.2. The soil highest in clay content had the highest aggregate stability at all water

contents and constrainment levels. Mostaghimi et al. (1988) predicted that aggregate stability

would increase with clay content. More clay implies more or stronger clay bridges between soil

particles. This suspected high degree of bridging was apparently little affected by water content

or constrainment in (Fig.2.4)

Fig. 2.4.Aggregate stability as a function of water content for both constrained and unconstrained samples of each soil.

2.6.2.5 Porosity

Elliott and Coleman (1988) adopted the concept of microaggregate formation within

macroaggregates from Oades (1984) and ascribed this microaggregate formation to the

anaerobic and resulting reducing conditions in the center of the macroaggregates. They also

described, as a mirror image of the aggregate hierarchy, four hierarchical pore categories in (Fig:

2.5)

(1) Macropores;

(2) Pore space between macroaggregates;

(3) Pores between microaggregates but within macroaggregates; and

(4) Pores within micro aggregates.

The concept of aggregation as a process involving different organic binding agents at different

scales was pioneered by Tisdall and Oades (1982) and based on their work, Oades and Waters

(1991) introduced the concept of aggregate hierarchy. Large aggregates (>2000µm) were

hypothesized to be held together by a fine network of roots and hyphae in soils with high SOC

content (>2%), while 20-250µm aggregates consist of 2-20µm particles, bonded together by

various organic and inorganic cements. Water stable aggregates of 2-20µm size, in turn, consist

of <2µm particles, which are an association of living and dead bacterial cells and clay particles.

The concept aggregate hierarchy degradation of large (relatively unstable) aggregates creates

smaller, more stable aggregates. Stabilization of macro-aggregates occurs mainly via binding by

fungal hyphae and roots.

Fig.2.5. The opposing chronology of the formation of the hierarchical aggregate orders

2.7 Different mechanisms of aggregate breakdown

Aggregate breakdown can result from several physico-chemical-physical mechanisms. Le

Bissonnais (1996a) reviewed four main mechanisms of breakdown (i) Slaking; (ii) Differential

swelling; (iii) Raindrop impact and (iv) Physico-chemical dispersion due to osmotic stress stress.

Swelling causes the volume of the aggregate to increase, and is often followed by the soil

slaking.

Slaking is when the air-dried aggregate breaks into smaller aggregates when immersed in

water. This indicates that the aggregates are not strong enough to withstand the pressures

involved in wetting. Some soils are strong enough to withstand this pressure, and increasing the

organic matter content of the soil may increase aggregate stability. Slaked soils can also

disperse.

Dispersion is caused by breakdown of the clay aggregate into individual clay particles.

Ashman et al. (2003) reviewed two of the most commonly used aggregate fractionation

methods: The slaking method is commonly used to simulate wetting stresses in the field and the

shaking method to simulate mechanical disruption followed by wet sieving. Slaking refers to the

disintegration of large aggregates (2-5mm diameter) into finer aggregates when placed in water.

Rapid disintegration is believed to be due to a lack of organic bonding between particles. They

found that slaking resulted in macro-aggregates being enriched in SOC and, after incubation to

measure microbiologically-available carbon, showed a faster respiration rate than in shaken

treatments. Here, micro-aggregates (<250µm) had more soil SOC and faster respiration rate.

While the general concept of aggregate hierarchy (depending on the size of aggregates, different

organic binding agents are active in aggregate stabilisation) (Oades, 1991) is generally accepted,

when reviewing the literature there are often different and conflicting results, depending on the

kind of fractionation scheme used (Ashman et al., 2003). The different results suggest that

chemical and biological properties of aggregates depend on the fractionation scheme used.

Accordingly, observed relationships can only be interpreted with respect to the specific

disruptive mechanism used and aggregate size can only be related to ‘energy inputs’. The results

from fractionation schemes therefore provide information with regard to the resistance of soil

to disruption, which is different from information about the “true” structure of the soil (Fig. 2.6).

Figure 2.6: Influence of fractionation procedures on biological and chemical properties of different aggregate sizes

(Ashman et al., 2003).

2.8 Soil aggregate stability and Organic matter

Soil aggregate stability was highly correlated with soil organic matter content but the addition of

crop residues and manure were not alone sufficient to restore soil physical quality. Organic

byproducts proceeding from industrial processes represent an important source of nutrients,

especially for organic fertilization. Aggregation generally increases with increasing soil organic

matter, which is connected to clay surfaces through positively charged polyvalent cations, thus

overcoming the repulsion between the negative charges of both clay and organic matter

(Edwards and Bremner, 1967). In this process, not only the amount of clay but also the clay type

is important (Kay, 1998), because mineralogical species differ in surface charge. Isomorphic

substitutions, resulting in negative permanent charges, prevail in layer silicates, whereas pH-

dependent charges form on surface hydroxyls and dominate in the case of oxides and

hydroxides.

Organic matter is known to stabilise aggregates by at least two different mechanisms:

(i) by increasing the inter-particle cohesion within aggregates and thus decreasing their

breakdown to the four above-mentioned breakdown mechanisms and

(ii) by increasing their hydrophobicity and thus decreasing their breakdown by slaking.

Kay and Angers (1999) and Greenland et al. (1975) observed relationships between SOC content

and aggregate stability. Using the Emerson crumb test, Greenland et al. (1975) found that at SOC

<2%, soil aggregates were considered unstable, moderately stable at 2-2.5% and very stable at

SOC contents >2.5%. Carter (1992) also found that maximum structural stability was obtained at

4.5% SOC.

2.8.1 Inorganic binding agents

2.8.1.1 Oxides

The aggregating and SOM stabilizing effect of oxides has been emphasized in many studies. The

aggregating effect of oxides is mainly at the microaggregate level (Oades et al., 1989) but also

macroaggregation has been related to oxide content (Imhoff et al., 2002). Shang and Tiessen

1998) reported that the stabilization of C in tropical soils is highest in stable microaggregates

consisting of oxides, soil organic matter and minerals. Oxides can act as binding agents in three

ways

(1) Organic materials adsorb on oxide surfaces (Oades et al., 1989);

(2) An electrostatic binding occurs between the positively charged oxides and negatively

charged clay minerals (El-Swaify and Emerson, 1975); and

(3) A coat of oxides on the surface of minerals forms bridges between primary and secondary

particles (Muggler et al., 1999).

In a kaolinitic soil, this binding of oxides to minerals will reduce the cation exchange capacity of

the kaolinite and increases the positive charge property of the kaolinite, further promoting the

aggregation through electrostatic binding (Dixon, 1989).

2.8.1.2. Calcium

It is generally accepted that calcium is a critical element for the stabilization of SOM and

aggregates through its role in the formation clay–polyvalent cation–organic matter complexes

(Clough and Skjemstad, 2000). Because calcium exerts its influence at the scale of the organo-

mineral complexation, its stabilization effect is mostly observed at the microaggregate level, but

it can also indirectly increase macroaggregation through a stimulation of microbial activity in

acidic soils (Chan and Heenan, 1999). Additions of calcium to field soils, in the form of lime or

gypsum, increased (∼10%) the aggregation level (Chan and Heenan, 1998, 1999). However, an

initial temporary decrease (1–3%) in aggregate stability has been observed upon the application

of lime to variable charged soils. This temporal decrease in aggregation has been related to an

increase in soil pH (Roth and Pavan, 1991) and microbial activity (Chan and Heenan, 1998,

1999) upon lime application to these acidic soils. An increase in pH of a variable charge soil

leads to an increase of negative charges (Roth and Pavan, 1991), resulting in a dominance of

repulsive forces over edge-to-face flocculation of kaolinite or oxide–kaolinite coagulation. The

dominance of repulsive forces causes dispersion. Nevertheless, this decrease in aggregation

seems to be reversed in the longer-term (Roth and Pavan, 1991; Chan and Heenan, 1998) and is

more pronounced if the calcium is added together with an organic matter source (such as wheat

straw) (Baldock et al., 1994). The latter suggests that the process of calcium bridging is the

dominant factor for the long-term positive effect of calcium addition on the structural stability of

a soil.

2.8.2 Soil texture

Soil organic matter tends to increase as the clay content increases. This increase depends on two

mechanisms. First, bonds between the surface of clay particles and organic matter retard the

decomposition process. Second, soils with higher clay content increase the potential for

aggregate formation. Macroaggregates physically protect organic matter molecules from further

mineralization caused by microbial attack (Rice, 2002). For example, when earthworm casts and

the large soil particles they contain are split by the joint action of several factors (climate, plant

growth and other organisms), nutrients are released and made available to other components of

soil micro-organisms. Under similar climate conditions, the organic matter content in fine

textured (clayey) soils is two to four times that of coarse textured (sandy) soils (Prasad and

Power, 1997). Kaolinite, the main clay mineral in many upland soils in the tropics, has a much

smaller specific surface and nutrient exchange capacity than most other clay minerals.

Therefore,

kaolinitic soils contain considerably fewer clay-humus complexes. In addition, the unprotected

labile humic substances are vulnerable to decomposition under appropriate soil moisture

conditions. Thus, high levels of organic matter are difficult to maintain in cultivated kaolinitic

soils in the wet-dry tropics, because climate and soil conditions favour rapid decomposition. In

contrast, organic matter can persist as organo-oxide complexes in soils rich in iron and

aluminium oxides. Such properties favour the formation of soil microaggregates, typical of many

fine-textured, oxide-rich, high base-status soils in the tropics (Uehara and Gilman, 1981). These

soils are known for their low bulk density, high microporosity, and high organic-matter

retention under natural vegetation, but also for their high phosphate fixation capacity on the

oxides when used for crop production. Current knowledge suggests that whereas organic matter

contributes to the dark colour of Vertisols, it is not considered important in determining either

the development, robustness or resilience of structure in these soils. Organic matter levels tend

to be low in Vertisols, even as low as 10 g/ kg (Coulombe et.al, 1996).

2.9 Effects of aggregation on OM stability

Aggregation can influence accessibility of substrate to microbes and fauna and rates of diffusion

of reactants and products of extracellular synthesis reactions. Theoretical calculations suggest

that aggregation should limit access to organic matter. Direct evidence for effects of aggregation

on accessibility is limited. Adu and Oades (1978) produced synthetic aggregates that were

labelled uniformly with 14C substrates. Aggregates of a sandy loam soil respired less starch than

did unaggregated soil, which they took as evidence of the presence of inaccessible micropores in

the aggregates. This pattern was not observed for a clayey soil, or when the substrate was

glucose. Bartlett and Doner (1988) incorporated lysine and leucine either homogeneously

throughout sterilized synthetic aggregates or only on their surfaces. After adding inoculum,

more of the amino acid was respired from aggregate surfaces than from within aggregates

indicating delay in microbial access to substrate within the aggregates.

2.10 Aggregate Stability-Time Relationship

Organic matter- soil aggregation relationships involve dynamic processes. Aggregate stability is

continually changing as organic matter is added and decomposed. The cementing agents that are

formed stabilize granules and are then decomposed to make the aggregates less stable. The

changes in aggregate stability with time after incorporation of organic matter are summarized

by the curves of Monnier (1965) in Figure (2.6).Stability has been based upon differences in

aggregate breakdown in water, alcohol, and benzene. The peak of the curve represents the

aggregation brought about by microbial bodies in the soil. The major impact during this period

of intense biological activity apparently is a mechanical binding action.

Fig 2.6: Monnier’s conceptual

model. Aggregate stability

dynamics are illustrated after

additions of three organic inputs (green manure buried straw and decomposed manure) and in relation to the

dynamics of aggregative factors (microbial corpses, prehumic substances and humic substances). Monnier

identified three periods (shown as zones A, B and C) during which the major aggregative processes occur.

These curves emphasize the necessity for replenishing organic matter in soils to maintain stable

granulation. Stabilization of the aggregates formed results from the transitory products of

microbial metabolism and the final stable humus that is produced. This soil quality, the

agronomical structure, i.e. the particle size distribution of soil, and the resistance of aggregates

against water and tillage; determines its fertility and productivity. This property is might be

characterized by the stability of these soil structural units against disrupting effects (Six et al.,

2000). Soil aggregates could be disrupted by several effects, for example slaking in water i.e.

because of too intense wetting during irrigation (Emerson, 1977).

Wetting, in general, disrupts aggregates because of the effect of entrapped air in the pores (i.e.,

slaking), as well as causing clay swelling and dispersion. During drying, clay particles may form

bridges and coatings on larger grains (Singer et al., 1992; Attou et al., 1998), leading to a closer

contact between particles and increasing the solid phase cohesion (Kemper and Rosenau, 1984).

The flocculation and cementation of mineral particles into secondary units with organic and

inorganic substances are the main soil aggregation processes (Duiker et al., 2003), producing

aggregates of a variety of sizes. On the other hand, rewetting may break large aggregates into

smaller ones, offsetting the stabilizing effect of drying.

Poor aggregate stability can result in:

Hard setting soils

Soil crusting impeding water movement and seedling emergence

Limited water holding capacity

Compaction due to structure collapse

Water logging

3. Materials and methods

A study was conducted to evaluate the physical and chemical properties of soil of different

characteristics and its stability in relation to soil organic carbon content. A general description

of the methods of analysis is described in this chapter.

3.1 Sampling sites

3.1.1 Short Description of sampling sites

Soil samples were collected from different location of Bangladesh (Bagherhat, Dumuria, Pirojpur

sadar, Kmalgonj sylhet, Tala and Batiaghata upazilla). Physiography, landuse and general

information of sampling sites as well as sample are presented (Table: 3.1):

Table 3.1: Description of sampling site

Sample no

Soil series Physiography Drainage Land use

1 Bajoa

Ganges Tidal floodplain

Well drained Fallow-Fallow-T.aman

2 Bajoa Well drained T. aman-Fallow

3 Jhalkathi Poor T. aus-T. aman-Fallow

4 Dumuria Imperfect T.aman-Fallow

5 Garuri Poor Jute-T.aman-Robi

6 Sara Ganges meander

floodplain

Imperfect Aus-Robi crops

7 Harta Peat basin Poor Mixed aus-B.aman-Fallow

8 Shrimangal

Late Pleistocene piedmont

Well drained Tea

9 Shrimangal Well drained Tea

3.2 Sample preparation

The collected soil samples were processed in the laboratory. All the samples were opened in

laboratory and air dried. Soil samples were dry sieved by hand to collect aggregate size class’s

one size class at a time rather than using a rotary sieve shaker with stacked sieves( Fig:3).

Collecting each size class individually allows for each size class to be wet sieved or analyzed

individually.

Fig 3:

The

material which passed through the screen is transferred from the brown paper onto the next

smallest screen (i), Aggregates collected on top of the screen are transferred into plastic bags,

tubes, or jars for storage (ii).

3.3 Laboratory analysis

3.3.1 Particle size analysis

The particle size analysis of the soils was carried out by combination of sieving and hydrometer

method as described by Gee and Bauder (1986). Textural classes were determined using

Marshall’s Triangular Coordinator system.

3.3.2 Microaggregate analysis

Soil structure was evaluated by microaggregate analysis of the soils following the method

Kachinskii (1965) with the exception that hydrometer was used to determine the particle size

distribution instead of pipette method. The state of aggregation and dispersion factor were

calculated by the using the following equations (Baver and Rhoades, 1932)

State of aggregation =

Dispersion factor =

Where a = percentage of aggregates larger than a specified size in micro aggregate analysis, b =

percentage of particles larger than a specified size in particle size analysis, x = percentage of clay

in micro aggregate analysis and y = percentage of clay in particle size analysis.

3.3.3 Aggregate stability

The stability of aggregates was determined by the method as described by Six et al., (2000a). For

the determination of aggregate stability, soil samples were air dried and crushed by a wooden

mallet. The crushed soils were then sieved through 8 mm sieve. The air dried soils that were

passed through 8 mm sieve but retained on 2 mm sieve divided into 8-2 mm, 2-0.25 mm and

0.25-0.05 mm size fractions by using mechanical shaker. For wet sieving with slaking

pretreatment 10 grams of air dry samples from each aggregate size fraction were submerged for

5 minutes on the top of smaller sieve of each size range prior to sieving. Soils were separated

manually by moving the sieve 3 cm up and down under water with 50 repetitions during a

period of 2 minutes. This manual separation technique was repeated for each size fractions. For

wet sieving with wetted pretreatment the air dry samples were adjusted to field capacity by

soaking with water for overnight before submerging in water. The soils were then sieved for 2

minutes by the method as stated before. The amount of aggregates retained after sieving was

oven dried at 1050C for 24 hours and then weighed. The amount of primary particles retained on

the sieves during wet sieving was determined by sieving after dispersing the soils with 5%

sodium hexametaphospahte. The weight of primary particles was recorded after oven drying at

1050C for 24 hours.

ba

100yx

The normalized stability index (NSI) of aggregates was calculated by the following formula (Six

et al., 2000a).

NSI = 1- [DL/DL (max)]

The whole soil disruption level (DL) was calculated as:

DL = 1/n

∑i

n

[(n+1)−i ]× DLSi

Where n = number of aggregate size classes. i = 1 for the smallest size class.

The disruption level of a size class upon slaking (DLSi) was calculated by the following formula:

DLSi =

[{(Pio−Sio )−(Pi−Si )}+ ¿ ]¿¿

¿¿¿¿¿ 1[P io−S io]

where DLSi = disruption level for each size class i; Pio = proportion of total sample weight in size

class i before disruption (i.e., rewetted); Pi = proportion of total sample weight in size class i

after disruption (i.e., slaked); Sio = proportion of sand with size i in aggregates of size i (=

aggregate-sized sand) before disruption; Si = proportion of sand with size i in aggregates of size i

after disruption.

The whole soil DL (max) was calculated by the following formula:

DL (max) = 1/n

∑i

n

[(n+1)−i ]× DLSi (max)

The maximum disruption [DLSi (max)] was calculated with the following formula:

DLSi (max) =

[(Pio−Pp )+|(P io−P p)|]2 ×

1[P io−S io]

Pp = primary sand particle content with the same size as the aggregates size class after complete

disruption of the whole soil.

3.3.4 Soil reaction (pH)

Soil pH (1:2.5) was determined electrochemically with the help of glass electrode pH meter as

suggested by Jackson (1973).

3.3.5 Electrical conductivity (EC)

The electrical conductivity of the soil was measured at a soil: water ratio of 1:5 by an EC meter

as described by USSL staff (1954).

3.3.6 Organic carbon

Soil organic carbon was determined by wet oxidation method of Walkley and Black’s method as

described by Piper (1950) and Jackson (1973).

3.3.7 Statistical analysis

Organic carbon content with respect to aggregate stability and soil properties were analyzed

using MINITAB basic statistics.

4. Result and discussion

4.1. General Analyses of Soil

4.1.1 Soil reaction (pH)

pH of Bajoa series varied from 6.79-7.64 (Table 4.1). Jhalkathi series was calcareous and pH

value was 7.89. Harta and Tea soils were non calcareous. pH of Harta series and Shrimangal soil

series was 5.18 and 5.19-5.72, respectively. Sara soil series demonstrate alkaline characteristics

(8.04).

4.1.2 Electrical conductivity (EC)

The EC values of soil sample were shown in (Table-4.1). Most soils shows slightly saline to

moderately saline behavior. Harta soil series was highly saline and EC ranged from 4.25-17.11

dS/m. EC of Sara, jhalakathi, and Bajoa soil series were 4.75 dS/m, 2.04-4.75dS/m and 1.45-2.11

dS/m respectively. The EC value of Tea soil was in the range of non- saline (0.78 to 1.45) ds/m.

Table-4.1: properties of soil series

Sample no

Soil Series %OC pH EC

(ds/m)

% Sand %Silt % Clay Texture

1 Bajoa 1.68 7.64 2.11 9.00 55.00 36.00 Silty Clay Loam

2 Bajoa 1.40 6.79 1.45 4.00 47.00 49.00 Silty Clay

3 Jhalkathi 1.42 7.89 4.75 4.00 58.00 38.00 Silty Clay loam

4Dumuria 2.11 7.9

77.40 5.00 43.00 52.00 Silty Clay

5Garuri 1.23 7.7

70.78 5.00 61.00 34.00 Silty Clay loam

6 Sara 0.96 8.04 4.75 30.00 31.00 39.00 Clay Loam

7 Harta 5.37 5.18 17.11 4.00 47.00 49.00 Silt loam

8 Shrimangal 0.88 5.19 0.78 60.00 18.00 22.00 Sandy Clay Loam

9 Shrimangal 1.03 5.72 1.45 60.00 19.00 21.00 Sandy Clay Loam

4.1.3 Organic carbon

Organic carbon content was highest in Dumuria and Harta soil series and it was 2.11% and

5.37% respectively. Harta soil series contain highest amount of organic carbon as it was peat

soil. The soil organic carbon content of Sara, Jhalakati and Bajoa soil series were 0.96%, 1.42%,

and 1.40-1.68%, respectively. Shrimangal soil series contain (1.03 to.88%). Results of organic

carbon content in soil sample shown on Table-4.1.

Rahman (1990) reported that organic matter content from 0.3 to 1.5% in upland soils, 1.5 to

2.0% in the medium low land areas and 2.0 to 3.5% in the low land areas in bill areas, this

fraction was about 4%.The correlation analysis has been used to relate the aggregate stability

with the organic matter. Organic matter content was higher in fine textured soil (Table-4.1).

Cook (1962) reported that fine textured soils contain roughly twice as much total organic

matter as do sandy soils in the same region.

4.1.4 Particle size distribution

The term particle size distribution of a soil refers to the percentage distribution of various sized

particles in a given volume of soils. Particle size distribution is one of the most stable soil

characteristics, being little modified cultivation or other practices. Although the usefulness of

particle size analysis in practical agriculture has sometimes questioned, its indirect benefits

have been extensive. It has been used to understand weathering and profile development and

determines the permeability, water retention, aeration, cation exchange capacity, workability

and erodibility (water and wind).

Soil texture refers to the relative proportion of sand, silt and clay. Soil texture detects the

physical, chemical and biological properties of soils. Textural classes of the soils are presented in

(Table-4.1).In the Sara soil series sand percentage varied from 30-62.00%. The silt and clay

percentage of Sara series varied from 3-31% and 35-39%, respectively (Table- 4.1). The

variation of primary particles along the depth was irregular, which is a characteristic of

floodplain soils and depended on the irregularity in time and kind of deposition. However, the

textural class was clay loam.

The Bajoa soil series exhibited similar textural variation as Sara series. The percentage of sand,

silt and clay in Bajoa series were 9.00-25.00%, 3.00-55.00% and 36.00-67.00%, respectively.

The textural class of Bajoa soil series was silty clay. In Jhalakati series the percentage of sand,

silt and clay along the depth were 4-33%, 3.00-58% and 38-64.00%, correspondingly. As the silt

fraction and clay is dominated so the textural classes was silty clay loam.

In Harta soil series the percentage of sand, silt and clay was 4.00-62.00%, 5-84.00% and 12.00-

33.00% respectively and define textural class was silt loam as because of silt fraction is

predominant.

SRDI (1977) reported that percentage sand, silt and clay in Bajoa, and Sara series were 0.5-4.2%,

51.2-66.6% and 32.6-47.9%; and 10.3-21.5%, 64.7-75.4% and 7.6-16.4%, respectively.

The percentage of sand,silt and clay of Dumuria and Garuri series was 5.00-18.00%,3-43.00%,

52.00-79% and 5.00-42.00%, 34.00%-54.00% respectively. Textural class of these two soil was

silty clay and silty clay loam. In Shrimangal soil series the percentage of sand, silt and clay was

60.00-67.00%, 5.00-18.00% and 22.00-28.00% respectively and the textural classes was sandy

clay loam.

4.1.5 Micro aggregate analysis

Micro aggregate composition of soils is one of the most important indexes of soil structure.

Results of micro aggregate analysis are regarded to be essential for agronomic characterization

of the soils. Micro aggregate analysis of a soil together with its particle size analysis gives an idea

about the degree of dispersion of the soil under natural condition and its potential capacity for

structural formulation. Kachinskii (1965) stated that micro aggregates having 0.25-0.05 mm and

0.05-0.01 mm diameter are most valuable from agronomic point of view. If micro aggregates

increase water holding capacity and improve air and water permeability of soils.

A plot of the summation percentage against the corresponding particle size for particle size

analysis and microaggregate analysis on a semi-logarithmic graph paper give an idea of the state

of micro aggregation of the soil (Fig: 4.2). The higher the area between the two curves the

higher the state of micro aggregation of the soil. The state of aggregation, degree of aggregation

and dispersion factor of soil varies comparatively. The highest state of aggregation found in

Harta soil series, at this sample degree of aggregation and dispersion factor were 0.60% and 275

Shrimongol soil series shown lower state of aggregation as well as lower dispersion factor

47.62%, respectively.

Table-4.2: Structural Indices of soil

From graph

value it being

Soil SeriesStructural Indices

State of Aggregation Degree of Aggregation Dispersion Factor

Bajoa 21 0.23 186.11

Jhalkathi 29 0.30 168.42

Dumuria 13 0.14 151.92

Garuri 37 0.39 158.82

Harta 58 0.60 275.00

Sara 32 0.46 89.74

Bajoa 21 0.22 146.94

Shrimanga

l7 0.18 127.27

Shrimanga

l15 0.38 47.62

interpreted for Bajoa soil series (Fig:4.2), state of aggregation is greater as because of the higher

the area between the two curves indicates the higher the state of micro aggregation of the soil.

Harta soil series shows higher state of aggregation than any other soil as because of it comprises

the maximum area deviate from the two curves. Harta soil also contain highest amount of

organic carbon (5.37%) which enhance the stabilization of soil. Sara and Dumuria soil series

shows lower state of aggregation and higher dispersion factor (Fig: 4.2).

Jhalakati soil series exhibit higher state of aggregation similar to Bajoa soil series (Fig: 4.2).

Lowest state of aggregation was found in Shrimangal soil series (Fig4.2). Distance between the

two curves was sparse in contrast to other soil series curves. Organic matter content of this soil

sample was also little (0.88%).

4.1.6 Indices of aggregation

4.1.6.1 Degree of aggregation (DA)

The highest value of DA was found 0.60% (Table-4.2) for Harta soil series and the lowest value

was 0.14% for dumuria soil series respectively. In sandy clay loam soil highest value was found

for shrimangal soil series and lowest value for Dumuria soil series that was silty clay in texture

(Table-4.1).Degree of aggregation for Sara, Bajoa and Jhalakati was 0.46%, 0.23% and 0.30%

respectively (Table-4.2) .Textural class of Sara was Clay loam, Bajoa and jhalakati was silty clay

loam correspondingly.

4.1.6.2 State of aggregation (SA)

The highest value of SA was found for Harta soil series 58% in silt loam soil, organic carbon %

was (5.37). In sandy clay loam texture the lowest value was 7% for shrimanmgal soil series. In

silty clay loam soil the highest value was 37% for Garuri soil series and lowest value was 21%

for Bajoa soil series (Table-4.2). State of aggregation for clay loam soil was 32% for sara soil

series. In silty clay soil the lowest value of SA was 13% for Dumuria soil series. State of

aggregation for jhalakati soil was 29% that was silty clay loam in texture (Table-4.1).

4.1.6.3 Dispersion factor

4.1.7 Normalized stability index (NSI)

The highest NSI value was estimated 1.01 for Garuri soil series and the lowest value was

estimated 0.38 for Bajoa soil series silty clay loam in texture (Fig: 4.1). In sandy clay loam soil

the highest value was 0.91 for shrimangal soil series. In silty clay soil NSI value was 0.82 for

Dumuria soil series and 0.79 for jhalakati soil series that was silty clay loam in textural class

(Table-4.1). In clay loam soil highest NSI value was 0.75 for sara soil series.

Fig4.1:

Relation between

aggregate stability

and %organic carbon of

nine soil series

4.2 Statistical analysis

Statistical analyses were performed using MINITAB basic statistics (Table-4.3). Correlations

were determined by the Pearson’s correlation factor (Table-4.4)

Table- 4.3: Descriptive Statistics

*St.Dev = standard deviation

4.3 Relationship among soil properties

Organic carbon associated with 8-2mm and 0.25-0.05mm was negatively correlated with % sand

and % clay (Table-4.4). Organic carbon associated with 8-2mm and 0.25-0.05mm was positively

correlated with % silt. Organic carbon content was positively correlated with state of

aggregation and degree of aggregation .State of aggregation was positively correlated with %

clay and was negatively correlated with % sand (Table-4.4). Degree of aggregation was

positively correlated with state of aggregation and state of aggregation was negatively

correlated with dispersion factor.

Table – 4.4: Correlations among selected soil properties

% OC pH EC ds/m NSI %Sand %Silt %Clay SA DF

Bajoa

Jhaloka

ti

DumuriaGaru

riHart

aSar

aBajo

a

Shrim

angal

Shrim

angal

0.001.002.003.004.005.006.00

NSI vs % OC

% OCNSISoil series

soil series and NSI

%O

C

Variable Mean Median St. Dev Minimum Maximum

% OC 1.51 1.40 0.66 0.88 2.96

pH 7.01 7.64 1.09 5.18 8.04

EC ds/m 4.51 2.11 5.24 0.78 17.11

NSI 0.73 0.79 0.21 0.38 1.01

%Sand 20.11 5.00 24.08 4.00 60.00

%Silt 46.22 47.00 21.34 18.00 84.00

%Clay 33.67 36.00 13.18 12.00 52.00

State of aggregation 25.89 21.00 15.37 7.00 58.00

Degree of aggregation 0.32 0.30 0.148 0.14 0.60

Dispersion factor 150.2 151.9 63.2 47.6 275.0

pH -0.283 0.461

EC ds/m 0.887 -0.356

0.001 0.347

NSI -0.511 0.119 -0.350 0.160 0.761 0.356

%Sand -0.599 -0.428 -0.383 0.449 0.088 0.250 0.309 0.225

%Silt 0.780 -0.007 0.644 -0.460 -0.838 0.013 0.986 0.061 0.213 0.005

%Clay -0.167 0.793 -0.342 -0.076* -0.470 -0.087 0.667 0.011 0.367 0.846 0.202 0.823

SA 0.600** -0.187 0.712 -0.268 -0.518 0.818 0.376 0.088 0.629 0.031 0.486 0.153 0.007 0.318

DA 0.327* -0.381 0.583 -0.059 -0.060 0.444 -0.609 0.864* 0.390 0.311 0.099 0.881 0.878 0.231 0.082 0.003

DF 0.839 -0.164 0.701 -0.540 -0.690 0.901 -0.197 -0.663 0.273** 0.005 0.673 0.035 0.133 0.039 0.001 0.612 0.051 0.477

*Correlation is significant at the 0.05 level**Correlation is significant at the 0.01 levelCell contents: Pearson correlation, P-ValueThe value of NSI can vary between 0 to 1 (Six et al. 2000a).The NSI of studied soil varied from

0.38 to 1.01 for different series of soil under different textural classes. The NSI was higher in

silty clay loam soil and lower in silty clay soil. The mineralogy of studied soils may play

important role in aggregate stability (Six et al., 2000a).

CONCLUSIONS/Summury

As soil aggregation strongly affected by soil organic carbon content and type of soil. It can be

conclude that yet soil aggregate is badly affected by soil organic carbon content rather than type

of soil also play a vital role in soil aggregate status. From my result, Harta soil series contain

large amount of % OC, but as it was peat basin soil so aggregate status was poor. Similarly Sara

soil contain little amount of % OC but aggregate status was rich as it was Ganges meander

floodplain soil and inherited texture clay loam.