CHAPTER ONE

INTRODUCTION

Introduction

1 Introduction

Soil aggregation has been considered as an important factor not only for increasing

soil productivity and soil quality but also improving nutrient availability and water

use efficiency (Byung et al., 2007). Aggregate stability is often measured on a

specific aggregate size class which is not a measurement of whole soil structure. Soil

organic matter levels, soil biological activity and soil functions (such as water

infiltration, water holding-capacity, aeration and nutrient availability) are related to

soil aggregation (Pirmoradian et al., 2005; Six et al., 2004). A variety of time

consuming and complex methods exist for measuring soil aggregation, however these

methods frequently measure only a portion of the whole soil (i.e. one or more

aggregate size class) (Six et al., 2004) and seldom separate dry aggregate size

distribution (i.e. aggregate formation) from water-stable aggregation (WSA) (i.e.

aggregate stabilization) (van Steenbergen et al., 1991) and are not quantitatively

related to soil quality. Measurements of soil quality are dependent upon which

chemical, biological or physical indicators are measured and how those indicators are

interpreted (Andrews et al., 2004; Karlen et al., 2003). This makes soil quality

assessment vulnerable to subjective interpretation based on the perspective and the

level of scientific knowledge of the user.

Good-quality soil is important for crop production sustainability on agricultural lands.

Soil dry aggregate (DA) and water-stable aggregate (WSA) amounts and size

distributions affect the soil quality. The soil organic carbon (OC) concentration is an

indicator of soil quality (Doran and Parkin, 1994) and influences aggregate amounts

and sizes. Close linear relationships between OC concentrations and water-stable

aggregate variables (mean weight diameter or wet-sieve index) for various soils were

found (Angers, 1992 and Carter, 1992). In contrast, increases in water-stable

aggregates were not related to increases in total OC of a silt loam, which led Perfect

and Kay (1990) to suggest that some components of the soil OC pool are more

actively involved in soil aggregate stabilization than others. Soil OC also influences

many other soil physical, chemical and biological properties. Therefore, knowledge of

soil aggregate and OC interrelationships is useful for evaluating effects of

management practices such as tillage methods, cropping systems and crops grown on

soil regarding production sustainability. Soil OC generally declines when grasslands

in semiarid regions are brought under cultivation, with tillage methods, cropping

1

Introduction

systems and crops grown affecting the decline rate. The decline rate generally is

greater with intensive than with reduced tillage methods (Beare et al., 1994), with

cropping systems involving fallow than those without fallow (Johnson and Davis,

1972) and with row crops than with small grain crops (Hobbs and Brown, 1965). Dry

aggregates (including clods) at the surface of cultivated soils help control wind

erosion (Armbrust et al., 1982) with those >0.84 mm in diameter generally considered

nonerodible by wind (Chepil, 1943). Tillage methods, crops grown and cropping

systems used influence the DA size distribution (Layton et al., 1993; Singh et al.,

1994). When including clods, the mean weight diameter of DAs (MWD-DA) usually

is greater where tillage rather than no-tillage is used (Layton et al., 1993). As for

DAs, tillage methods, cropping systems and crops grown affect WSA size distribution

with tillage method generally having the greatest effect (Beare et al., 1994; La1 et al.,

1994). In contrast, Unger (1995) found the MWD-WSAs to be lower with no-tillage

than with other tillage methods in some cases, but water infiltration was greater with

no-tillage, possibly because the no-tillage aggregates were more stable. Besides

affecting the MWD-WSAs, no-tillage also resulted in greater surface or near-surface

soil OC (Singh et al., 1994).

There exits a closer inter-relation between SOC concentration and aggregation

(Hermawan and Bomke, 1997). Aggregate stability is significantly correlated with

SOC due to binding action of humic substances and other microbial by-products

(Haynes et al., 1997; Shepherd et al., 2001). In some cases, however, SOC may be

only moderately or weakly (Holeplass et al., 2004) correlated with aggregate stability.

The objective of this study was to establish relationship between soil aggregate

stability and organic carbon under different cropping patterns in Ganges river

floodplain soils.

2

Introduction

CHAPTER TWO

Literature

REVIEW

3

Introduction

4

Literature Review

2 Literature Review

2.1 Soil Aggregate

Soil is a dispersed system consisting of particles in a fine state of division or

dispersion. The primary particles or soil separates generally do not exist as completely

dispersed fractions, but frequently combined together to form secondary particles.

These secondary particles are generally referred to as secondary units, peds, clods,

aggregates or concretions. An aggregate is a group of primary particles that cohere

together more strongly than the surrounding soil particles (Ghidyal and Tripathi,

1987).

2.2 Aggregate Stability

Soil aggregate stability is the result of complex interactions among biological,

chemical and physical in the soil (Tisdall and Oades, 1982). Factors affecting

aggregate stability can be grouped as abiotic (clay minerals, sesquioxides,

exchangeable cations), biotic (soil organic matter, activities of plant roots, soil fauna

and microorganisms) and environmental (soil temperature and moisture) (Chen et al.,

1998). Plants contribute to water-stable aggregates by adding carbon materials to the

soil that are decomposed by soil microbes. Exudates from roots and soil microbes

contribute to the formation of microaggregates, while fine roots and mycorrhizal

hyphae contribute to the stabilization of macroaggregates.

The concept of aggregate stability depends on both the forces that bind particles

together and the nature and magnitude of the disruptive stress (Beare and Bruce,

1993). Several methods have been proposed to determine soil aggregate-size

distribution and stability (Kemper and Rosenau, 1986). The suitability of these

methods depends on the purpose of the study. The most widely used approaches are

based on the wet-sieving method (Kemper and Rosenau, 1986). In this method,

cyclically submerging and sieving soil in water emulates the stresses involved in the

entry of water into soil aggregates. The moisture content of the soil aggregates before

wet sieving controls the severity of the disruption (Kemper and Rosenau, 1986).

Several studies have used capillary-wetted and slaked pretreatments (Elliott, 1986;

Cambardella and Elliott, 1993; Six et al., 1998) as a means to study soil aggregates.

The capillary-wetted pretreatment involves slowly wetting the soil aggregates before

wet sieving. This pretreatment produces minimal disruption, because misted

3

Literature Review

aggregates do not buildup air pressure in the pores and the air escapes with minimal

aggregate disruption. In contrast, the slaked pretreatment causes considerable

disruption. When air-dry soil is submerged in water; the air that is trapped inside the

soil pores is rapidly displaced with water. Weak aggregates are disrupted as a

consequence of the sudden release of this large buildup of internal air pressure (Gale

et al., 2000). The combined use of the capillary-wetted and the slaked pretreatments

has been used for contrasting differences in aggregate-size distributions for soils with

different management histories and also for understanding the factors that influence

aggregate stability (Six et al., 1998). The combined use of the capillary-wetted and

the slaked pretreatments has been used for contrasting differences in aggregate-size

distributions for soils with different management histories and also for understanding

the factors that influence aggregate stability (Six et al., 1998). More recently, Gale et

al. (2000) used the comparison of slaked versus capillary-wetted pretreatments as a

means to differentiate stable macroaggregates from un-stable macroaggregates based

on their resistance to slaking. Although the conceptualization of Gale’s idea

represents an important contribution, more work is needed to clearly separate the

stable macroaggregates from the unstable macroaggregates and accurately specify

aggregate-size stability distributions. The aggregate-size stability distribution is the

quantity of stable and unstable soil aggregates categorized by their size and stability

to disruption.

2.3 Hierarchical organization of soil aggregates

The large aggregates (>1mm) so desirable for most soil uses are typically composed

of smaller aggregates, which in turn are composed of still smaller units, down to

clusters of clay and humus less than 0.001 mm in size. It may easily demonstrate the

existence of this hierarchy of aggregation by selecting a few of the largest aggregates

in a soil and gently crushing or picking them apart to separate them into many

smaller-sized aggregates. Then it can be tried rubbing the tiniest of these granules

between thumb and forefinger. It will be found that most of these break down into a

smear of still smaller aggregates composed of silt, clay and humus. The hierarchical

organization of aggregates seems to be characteristic of most soils, with the exception

of certain Oxisols and some very young Entisols. At each level in the hierarchy of

aggregate, different factors are responsible for binding together the subunits (Brady

and Weil, 2002).

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Literature Review

2.4 Importance of aggregate stability

A stable soil structure is important to maintain agricultural productivity and reduce

environmental pollution. Good soil structure is important for water infiltration and

internal drainage, aeration for roots and soil microorganisms, root growth and nutrient

uptake. The maintenance of aggregation is important for these soil functions.

Processes that reduce aggregation (reduce aggregate stability) are detrimental to crop

production and soil quality and can lead to soil degradation and erosion. Aggregation

affects erosion, movement of water and plant growth. Desirable aggregates are stable

against rainfall and water movement. Aggregate stability is one kind of soil quality

that is one of the most significant factors for high agricultural productivity and

sustainable agriculture (USDA, 1996).

2.5 Formation of aggregates

Soils containing >5% clay tend to form structural units known as aggregates by both

physical and biological processes (McKenzie, 1989). Aggregates may vary in size

from crumbs (<2 mm) to polyhedrons or subangular blocks (0.005-0.02mm) to prisms

and columns >0.1m (Horn et al., 1989). Aggregates have either sharp rectangular

edges or are defined by nonrectangular shear planes (Babel et al., 1995). Aggregated

soils are always stronger than homogenized material. Physical, chemical and

biological processes vary between the inter-aggregate and intra-aggregate volumes.

Thus, the dynamics of hydraulic, mechanical, biological and chemical processes

affect soil intensity properties strongly (Horn et al., 1994). Furthermore, when

strength is defined mechanically and/or hydraulically, one cannot extrapolate beyond

the imposed limits which must be specified.

Mechanical processes and their dynamic aspects will be described and compared in

homogenized and structured soils in order to highlight the effect of soil structure,

tillage and timber harvesting on properties and processes in an ecosystem. Both

capacity and intensity parameters and their measurement will be discussed in relation

to mechanical properties. Thereafter, the effects of structure on ion sorption and

desorption and hydraulics will be discussed and modeled.

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Literature Review

2.6 Factors involved in aggregation

Factors affecting soil aggregation and its formation have been widely investigated. It

is generally recognized that a relationship exists between the erodibility of the soil on

the one hand and the degree of aggregation and stability of the structural aggregates

on the other. In the instance of water erosion, resistance is attributed to the

functioning of water-stable aggregates. Several methods have been developed which

are designed to measure stability of the aggregates as an index of the resistance of the

soil to erosion. Middleton (n) was the first to propose such a method. His dispersion

ratio depends on the ratio of dispersed silt and clay to total silt and clay when a water

suspension of the soil is shaken in a cylinder. The number and size, and to a lesser

degree, the shape and arrangement of the soil aggregates will depend upon their

stability. Consequently, any study of the factors which influence stability must take

into consideration factors influencing or determining soil structure.

2.6.1 Effect of particle size distribution on aggregation

Generally which soil contains much clay percentage has much aggregation ratio. All

the experiments and results show that the clay content of the soils is related to the

amount of aggregation. Rowles found that the content of clay was positively

correlated with the aggregation ratio (Aggregation ratio × clay = + 0.8152.) (Rost and

Rowles, 1940).

2.6.2 Effect of wetting and drying and freezing and thawing on water-stable

aggregates

Changing weather conditions and the accompanying wetting and drying and freezing

and thawing have been found to change the amount of aggregation in soil. It is

obvious that many inherent qualities of the soil itself may and probably do influence

the amount of aggregation produced and the stability of the aggregates. Thus, the kind

and amount of colloids present, the amount of moisture in the soil, the rate of wetting

and the rate of freezing have been found to influence aggregate formation (Rost and

Rowles, 1940).

2.6.3 Organic matter and aggregation

Organic matter was calculated from organic carbon by multiplying by the factor

1.724. Cultivation has had a marked effect upon organic matter. Tillage operations

have mixed the soil so that organic matter content of the upper and lower layers of the

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Literature Review

A horizon is essentially the same, even in the forest soils. Cultivation and erosion

have also led to a net decrease in organic matter. The loss of organic matter from the

forest soils is more serious because they contained a smaller amount in their original

condition. The amounts of total nitrogen and alkali soluble humus usually increase or

decrease with the amounts of organic matter. It is to be noted that the discussion given

in connection with organic matter applies also to total nitrogen and alkali soluble

humus. In general, the soils which have the best aggregation contain the most organic

matter, and the greatest reduction in aggregation as a result of cultivation has been in

the soils which have suffered the greatest loss of organic matter. This relationship

between organic matter and aggregation was positively correlated with the

aggregation ratio (Aggregation ratio × organic matter = + 0.6752.) (Rost and Rowles,

1940).

2.7 Processes of aggregate formation

Both biological and physical-chemical (abiotic) processes are involved in the

formation of soil aggregates. The physical-chemical processes tend to most important

at the smaller end of the scale, biological processes at the larger end. Also, the

physical-chemical processes of aggregate formation are associated mainly with clays

and, hence, tend to be of greater importance in finer-textured soils. In sandy soils that

have little clay, aggregation is almost entirely dependent on biological processes

(Brady and Weil, 2002).

2.7.1 Physical-chemical processes

Most important among the physical-chemical processes are

1) The mutual attraction among clay particles and

2) The swelling and shrinking of clay masses.

2.7.1.1 Flocculation of clays and the role of adsorbed cations

Except in very sandy soils that are almost devoid of clay, aggregation begins with the

flocculation of clay particles into microscopic clumps or floccules. If two clay

platelets come close enough to attack the negative charges on the planar surfaces.

Clay floccules or domains, along with charged organic colloids (humus), form bridges

that bind to each other and to fine silt particles (mainly quartz), creating the smallest

size groupings in the hierarchy of soil aggregates. These domains, aided by the

flocculating influence of polyvalent cations (e.g. Ca2+ , Fe2+ , and Al3+ ) and humus,

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Literature Review

provide much of the long-term stability for the smaller (<0.03mm) microaggregates.

The cementing action of inorganic compounds, such as iron oxides, produces very

stable small aggregates sometimes called pseudosand in certain clayey soils (Ultisols

and Oxisols) of hot, humid regions (Brady and Weil, 2002).

2.7.1.2 Volume changes in clayey materials

As a soil dries out and water is withdrawn, the platelets in clay domains move closer

together, causing the domains and hence, the soil mass to shrink in so volume. As a

soil mass shrinks, cracks will open up along planes of weakness. Over the course of

many cycles the network of cracks becomes more extensive and the aggregates

between the cracks better defined. Plant roots also have a distinct drying effect as they

take up soil moisture in their immediate vicinity. Water uptake, especially by fibrous-

rooted perennial grasses, accentuates the physical aggregation process associated with

wetting and drying. This effect is but one of many examples of ways in which

physical and biological soil processes interact (Brady and Weil, 2002).

2.7.2 Biological processes

2.7.2.1 Activities of soil organisms

Among the biological processes of aggregation, the most prominent are

1) The burrowing and molding activities of earthworms,

2) The enmeshment of particles by sticky network of roots and fungal hyphae

and

3) The production of organic glues by microorganisms, especially bacteria and

fungi

In both cultivated and uncultivated soils, earthworms move soil particles around,

often ingesting them and forming them into pellets or casts. In some forested soils, the

surface horizon consists primarily of aggregates formed as earthworm castings. Plant

roots also move particles about as they push their way through the soil. This

movement forces soil particles to come into close contact with each other,

encouraging aggregation. At the same time, the channels created by plant roots and

soil animals serve as macropores, breaking up large clods and helping to define larger

soil structural units.

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Literature Review

Plant roots (particularly root hairs) and fungal hyphae exude sugarlike

polysaccharides and other organic compounds, forming sticky networks that bind

together individual soil particles and tiny microaggregates into larger agglomerations

called macroaggregates. The threadlike fungi that associate with plant roots are

especially effective in providing this type of relatively short-term stabilization of large

aggregates, because they secrete a gooey protein called glomalin, which is very

effective as a cementing agent (Brady and Weil, 2002).

2.7.2.2 Influence of tillage

Tillage can have both favorable and unfavorable effects on aggregation. If the soil is

not too wet or too dry when the tillage is performed, the short-term effect of tillage is

generally favorable. Tillage implements break up large clods, incorporate organic

matter, into the soil, kill weeds, and generally create a more favourable seedbed.

Immediately after plowing, the surface soil is loosened and total porosity is increased.

Over longer periods, however, tillage greatly hastens the oxidation of soil organic

matter, thus reducing the aggregating effects of this soil component. Tillage

operations, especially if carried out when the soil is wet, also tend to crush or smear

stable soil aggregates, resulting in loss of macroporosity and the creation of puddle

condition (Brady and Weil, 2002).

2.7.2.3 Influence of iron, aluminum oxides

Some well weathered soils of the tropics have more stable aggregation than soils with

comparable or even higher organic matter levels in temperate regions. This is thought

to be due primarily to the bindings effects of the irofn and aluminum sesquioxides

(particularly the amorphous forms) that are plentiful in many tropical soils. Films of

such compounds coat and cement soil aggregates, thereby preventing their ready

breakdown when the soil is tilled (Brady and Weil, 2002).

2.8 Determination of the aggregate size-stability distribution

The experimental procedure was used to determine the aggregate-size stability

distribution is shown in Fig. 2.1 (Márquez, 2004). This procedure involved the slaked

and capillary-wetted pretreatments; and a subsequent slaking treatment of aggregates

>250 µm in size. Theoretical considerations needed for the determination of the

aggregate-size stability distribution are given below. The determination of aggregate-

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Literature Review

size stability distribution involves the assumptions that soil aggregates can be

categorized in terms of their size and water stability. Therefore:

1. Soil aggregates with diameters >250 µm are labeled macro aggregates.

2. Macroaggregates are categorized as large macroaggrearegates when their diameters

are >2000 µm and small macroaggregates when their diameters range between 250

and 2000 µm.

3. Macroaggregates are also categorized in terms of their resistance to slaking.

Macroaggregates that survive slaking are labeled as stable and those that do not

survive are labeled as unstable.

4. Microaggregates have diameters ranging between 53 and 250 µm.

5. The mineral fraction (silt + clay) has diameters <53 µm (Márquez, 2004).

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Literature Review

Fig.2.1. Experimental procedure used to assess aggregate-size stability distribution (Márquez,

2004).

2.9 Stability indices

There are different stability indices for measuring soil aggregate stability. A number

of the indices have been proposed for assessing soil stability including mean weight

diameter (MWD), geometric mean diameter (GMD), water-stable aggregation (WSA)

(Kemper and Rosenau, 1986), aggregate stability index (ASI) (Niewczas and

Witkowska-Walczak, 2003), and normalized stability index (NSI) (Six et al., 2004).

Soil aggregate stability indices rarely express numerically the impacts of mechanical,

environmental, or biological factors.

2.9.1 Mean weight diameter

For wet sieving analysis mean weight diameter is an important index of aggregate

stability. For wet sieving 2mm, 1mm, 0.5mm, 0.25mm and <0.25 mm sieves are used.

The mean weight diameter (MWD) of the aggregates was calculated by the following

formula (Van Bavel, 1949).

MWD =

∑i=1

n

wixi

Where xi = The mean diameter of any particular size range of aggregates separated by

sieving and

wi = The weight of aggregates in that size range as a fraction of the total dry weight of

the sample analyzed.

2.9.2 Normalized stability index

A normalized stability index (NSI) is measured on selected soil samples (10–25 g)

according to the procedures described by Six et al. (2000a). Briefly, samples from

each treatment are separated into three groups of four. One group is rewetted

overnight prior to wet sieving (capillary). Another group is wet-sieved without

rewetting (slaked) and the final group is processed for a maximum disruption to

measure the coarse material. Maximum disruption used forced water to destroy

aggregates and wash them through stacked sieves. The capillary-rewetted and slaked

samples are wet-sieved for 2 min through three screens (2, 0.25, and 0.053 mm)

individually by transferring the material that went through the screen onto the next

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Literature Review

smaller screen. The material (i.e. water-stable aggregates) on screens in both the

capillary-rewetted and slaked treatments is collected, dried at 70°C, weighed and

corrected for the coarse material (i.e. sand and organic matter). The NSI is calculated

using the equations provided by Six et al. (2000a).

2.9.3 Whole soil stability index (WSSI)

Aggregate distribution and water-stable aggregation (WSA) are measured on dry-

sieved aggregates in four aggregate size classes (9.5-2, 2-1, 1-0.25 and 0.25 -0.05mm)

(Nichols and Toro, 2011). Dry sieving consisted of placing the soil atop a screen with

the size equal to the size of the largest aggregates in the size class, tapping the sides at

least 50 times with the palm of the hand to pass the soil through the screen, collecting

the soil passing through the screen on a piece of kraft paper and pouring it onto a

screen equal to the smallest aggregates in the size class followed by tapping. Each

aggregate size class is collected individually from largest to smallest. The weight of

aggregates in each size class is measured and used to calculate the proportion of

aggregates in each size class relative to the whole soil (Eq. (1)). Soil on top of the 9.5

mm screen and below the 0.053mm screen is collected and weighed as part of the

summed total weight (WT).

The proportion of dry-sieved aggregates in each size class is:

Pai=

[W A – [(W c

W o)×W A ]]

W T

- - - - - - - - - - - - - - - - - - - - - - - (1)

Where,

Pai = Proportion of dry-sieved aggregates for each size class i;

WA = Weight of total material in each size class i;

Wc = Weight ofcoarse material measured during wet sieving for size i;

Wo = Weight of aggregates placed on the sieve prior to wet sieving size i ;

WT = Summed total weights of all the aggregate size classes plus the soil from above

the 9.5 mm screen and below the 0.053 mm screen.

Water-stable aggregation (WSA) is measured on four subsamples from each

aggregate size class according to a modified Kemper and Rosenau (1986) method.

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Literature Review

Briefly, aggregates (4 g for the 9.5–2 and 2–1 mm aggregates, 2 g for the 1–0.25 mm

aggregates, and 1 g for the 0.25–0.053 mm aggregates) are placed onto screens 1/4 of

the smallest size and capillary-rewetted for 10 min. Stable aggregates are separated

via mechanical wet sieving for 5 min using an apparatus described by Kemper and

Rosenau (1986). Material collected on the sieve is washed gently into weigh boats,

dried at 70°C and weighed. The coarse material (sand, roots, and particulate organic

matter) is removed by dispersing the aggregates in 0.5% sodium hexametaphosphate,

shaking periodically over a 5 min period, and using forced water and rubber

policemen to push the disrupted aggregates through a screen matching the smallest

aggregates in the size class. The material remaining on the screen is collected, dried at

70°C, weighed and subtracted from the weight of aggregates collected after wet

sieving. The formula for calculating the WSA for each size class is:

WSAi = [(Wa – Wc) ÷ Wo] × 100 - - - - - - - - - - - - - - - - - - - - - - - - - (2)

Where,

WSAi = water-stable aggregation for each size class i;

Wa = weight of material on the sieve after wet sieving size i;

Wc = weight of coarse material in size i;

Wo = weight of aggregates placed on the sieve prior to wet sieving size i.

The dry aggregate size distribution and WSA calculated above are used in the formula

for the whole soil stability index (WSSI).

WSSI = ¿

Where,

WSSI = Whole soil stability index;

n = The number of the aggregate size classes;

i = n and decreases by an increment of 1 from the largest to the smallest aggregate

sizes class;

Pai = Proportion of aggregate weight for each size class i from Eq. (1);

WSAi = Water-stable aggregation for each size class i from Eq. (2).

2.10 Soil organic carbon

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Literature Review

Carbon is the fundamental building block of all life. Carbon is present in the

atmosphere, in plant and animal life, in nonliving organic matter, in fossil fuels, in

rocks, and dissolved in oceans. Movement of carbon molecules from one form to

another is known as the carbon cycle. Plants acquire carbon from the atmosphere

through photosynthesis. Using carbon dioxide (CO2) from the atmosphere and energy

from sunlight, plants convert CO2 to organic carbon as they produce stems, leaves,

and roots. The cycle of life and death of plants results in accumulation of

decomposing plant tissue, both aboveground and belowground (plant roots), and

produces a significant amount of soil organic carbon (McVay and Rice, 2002).

2.11 Factors influence organic carbon levels 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 cation 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 other factors.

2.11.1 Climate

Climate impacts on soil organic C content primarily through the effects of

temperature, moisture and solar radiation on the array and growth rate of plant

species, and on the rate of soil organic C mineralization. Post et al. (1982) found that

amounts of soil organic C were positively correlated with precipitation and, at a given

level of precipitation, negatively correlated with temperature. In the Great Plains of

North America, precipitation controls net primary productivity and temperature

controls rates of soil organic C mineralization (Parton et al., 1987; Sala et al., 1988;

Burke et al., 1989; USDA-SCS, 1994). Ladd et al. (1985) compared the mean loss of 14C-labeled plant residues from four soils in South Australia with those obtained by

Jenkinson and Ayanaba (1977) for soils in England and Nigeria and observed a

doubling of the rate of substrate C mineralization for an 8-9°C increase in mean

annual temperature. An influence of temperature on decomposition can also be

inferred from 14C signatures of soil organic C, which showed a latitudinal gradient in

the mean residence time of soil organic C (Bird et al., 1996).

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Literature Review

2.11.2 Soil mineral parent materials and products of pedogenesis

The mineral phase of soils can exert a strong influence on soil organic C contents as a

result of mechanisms capable of stabilizing organic materials against biological

attack. Each soil has a given capacity to protect soil organic C dictated by the

following soil characteristics:

1) The chemical nature of soil minerals

2) The presence of multivalent cations and their ability to form complexes with

organic molecules in soils

3) The adsorptive capacity of soil minerals for organic materials as governed by

particle size and surface area

4) Physical protection mechanisms which restrict access of organic materials to

biological attack.

2.11.2.1 Chemical nature of the soil mineral fraction

An analysis of different soil types indicates that soils with high contents of active

CaCO3 and amorphous Al arid Fe tend to have higher organic C contents (Sombroek

et al., 1993). In a study of influence of soil properties on soil organic C genesis,

Duchaufour (1976) suggested that the presence of CaCO3 in a Rendzina could

stabilize fresh and humified organic materials. Thin carbonate coatings visible under

stereoscan examination, and a precipitation of organic molecules induced by Ca2+

complexation were implicated in the stabilization of fresh and humified organic

residues, respectively, and helped to explain the observed impedance of

mineralization. Stabilization of organic C in high base status soils with less reactive or

low contents of CaCO3 results predominantly from the formation of Ca organic

linkages. In such soils, the initial decomposition of plant residues is rapid, but the

subsequent utilization of initial decomposition products is slow leading to higher soil

organic C contents, lower C/N ratios and longer retention times.

2.11.2.2 Impacts of multivalent cations

The presence of multivalent cations in soil has important implications on the behavior

of clays and organic materials and the biological availability of organic C. When

saturated with multivalent cations, clays remain flocculated, which reduces exposure

of organic materials adsorbed onto their surface and macromolecular organic

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Literature Review

materials bearing functional groups become more condensed, and thus, less

susceptible to biological attack. The dominant multivalent cations present in soils

include Ca2+ and Mg2+ in neutral and alkaline soils and hydroxypolycations of Fe3+

and Al3+ in acidic, ferrallitic and andic soils. A stabilizing effect of Ca2+, relative to

Na+, on organic C mineralization was effectively demonstrated by Sokoloff (1938),

where the extent of mineralization and solubility of organic C in two soils was

reduced by addition of Ca2+ salts and enhanced by addition of Na+ salts.

2.11.2.3 Adsorption of organic materials onto mineral surfaces

Clay particles provide a reactive surface onto which organic materials can be

adsorbed and it is generally accepted that such adsorption reactions provide a

mechanism of stabilizing soil organic C against microbial attack. Correlation between

soil organic C and clay contents have been observed (Schimel et al., 1985; Spain,

1990; Feller et al., 1991) and the various interactions- between soil clays and organic

materials have been summarized by Oades (1989). Such interactions are principally

defined by the chemical nature of organic materials (functional group content,

molecular size etc.) and the type of clay mineral (kaolinite, illite, smectite etc.).

Numerous studies utilizing isotopically labeled organic substrates have shown a

positive relationship between the contents of residual substrate C and soil clay content

(Amato and Ladd, 1992).

2.11.2.4 Physical protection within soil matrix offered by soil architecture

The architecture or structural condition of a soil can exert significance control over

processes of biological decomposition by limiting the accessibility of soil organic C to

decomposer microorganisms and of microorganisms to their faunal predators. This

limitation results from the ability of clays to encapsulate organic materials (Tisdall

and Oades, 1982), the burial of organic C within aggregates (Golchin et al., I994,

1997), and the entrapment of organic C within small pores (Elliott and Coleman,

1988). As outlined by van Veen and Kuikman (1990) and Hassink (1992), evidence of

the importance of these processes in the protection of organic C in soils can be

inferred from the following observations: (1) a faster turnover rate of organic

substrates in liquid microbial cultures relative to that of similar substrates in mineral

soils, (2) an enhanced mineralization of C and N when soils are disrupted prior to

incubation, and (3) a more rapid mineralization of organic C and plant residues in

16

Literature Review

sandy soils than clay soils. The physico-chemical protection mechanism is more

robust but monolayer or patchy adsorption of SOM onto clay surfaces requires further

detailed research. The adsorption of SOM and exo-enzymes on pore walls and clay

surfaces has been identified as a plausible concept of SOM stabilization (Rabbi et al.,

2010).

2.11.3 Biota

Vegetation and soil organisms are called together biota. These have a great effect on

aggregate formation.

2.11.3.1 Vegetative inputs: Variations across and within ecosystems

Vegetation can influence soil organic C levels as a result of the amount, placement

and biodegradability (chemical recalcitrance) of plant residues returned to the soil.

The greatest effects of vegetation on soil organic C contents are confined to the A

horizon. Concentrations of organic C detected below the A horizon result from

pedogenic processes, which occur over much longer time scales than the lifetime of

current vegetation. It was showed that for Brazilian soil profiles, current vegetative

cover was only in direct equilibrium with topsoil (A horizon) organic C, while that in

subsoils was largely unaffected by the nature of vegetative cover. Once the organic C

moves to depth (e.g. argillic or spodic horizons), it becomes less accessible to

decomposer organisms, as exemplified by the increased radiocarbon ages of soil with

depth (Pressenda et al., 1996).

2.11.3.2 Composition of plant materials: The parent material for soil organic C

Plant materials can he viewed as the parent material for soil organic C in much the

same manner as one views primary minerals as the parent materials of soil minerals

components. Plant materials are altered by soil fauna and microorganisms,

predominantly after deposition in or on the soil, resulting in changes in the original

chemical structure and in the synthesis of new compounds, just as some soil minerals

dissolve and others precipitate during pedogenesis. An understanding of the chemical

nature of plant materials is, therefore, important to studies of soil organic C genesis

and composition.

2.11.3.3 Relative impacts of soil fauna and microorganisms

17

Literature Review

The requirement of soil organisms for chemical energy and nutrients drives processes

of heterotrophic decomposition in soils, which account for the major pathways

through which soil organic C is mineralized. Abiotic chemical oxidation is unlikely to

account for >20% of total C mineralization (Moorehead and Reynolds, 1989) and

more often accounts for C <5% (Lavelle et al., 1993).

Microorganisms are the major contributors to soil respiration and are responsible for

80-95% of the mineralization of C. Hassink et al. (1994) calculated that the

contribution of the fauna to C mineralization in two sandy and two loam grassland

soils ranged from 5-13% of the total C mineralization.

2.11.4 Topography

Topography exerts its major control over soil organic C contents through a

modification of climate and soil textural factors and through its impacts on the

redistribution of water within a landscape. Soils in downslope positions are often

wetter, have warmer average temperatures, and have finer textures than soils in

upslope positions or at the top of knolls. Burke et al. (1995) examined the extent to

which soil organic C varied at a landscape scale at two sites differing in soil texture,

but having similar climatic characteristics. Burke et al. (1995) noted increased organic

C contents (and clay and silt contents) in downslope positions relative to the summits

at both sites. Such a finding has been attributed to the downslope movement of

organic C and organic rich clay (Reiners, 1983). However, additional gradients in

available water along slopes, especially in water limited systems, influence plant

production (Peterson et al., 1988) with greater biomass inputs and a greater potential

biological stabilization of organic C via higher clay contents at the base of slopes.

Where excessive water exists, drainage of depression in the landscape can be

restricted, leading to the development of anaerobic conditions and preservation of

organic C relative to the better drained higher landscape elements during wetter times

of the year.

2.11.5 Land management practices

Paustian et al. (1997) have comprehensively reviewed the influence of agricultural

management practices on soil organic C levels. The influence of forestry management

practices has been reviewed by Johnson (1992). The most dramatic influence of

agricultural practices occurs when soils are first brought into production. Typically,

18

Literature Review

soil organic C levels decrease for the first few years after cultivation and then stabilize

at a new equilibrium level which is dictated principally by the ability of the soil to

stabilize organic C and amount, quality and distribution of p1alnt residues inputs. For

example, 28-59% of the soil C was lost following 30 to 43 yrs of cropping at 11 sites

within the North American prairies (Haas et al., 1957). The following characteristics

of cereal production systems, in comparison to those of native grasslands, help to

explain the observed losses of soil organic C induced by cultivation: (1) 80% lower

allocation of organic C to soils (Buyanovsky et al., 1987), (2) reduced below ground

allocation of photosynthate (Anderson and Coleman, 1985), (3) enhanced aggregate

disruption and exposure of physically protected organic C due to cultivation and (4)

enhanced rates of decomposition of available organic C substrates due to more

favorable abiotic conditions (e.g. aeration, temperature and water content).

2.12 Relationship between aggregate stability and soil organic carbon

The uncertainty of aggregate-forming physical forces responsible for aggregate pore

space stability makes further investigations and management difficult. It is well

known that such cations as Ca2+, Fe2+ and Al3+ play the structure-forming role. The

mechanism of aggregate formation with cations mentioned above implies that these

cations are involved in crystalline-like linkages between soil mineral particles. Soil

organic matter (SOM) is found multiplying %SOC by 1.724. Soil organic matter

(SOM) is involved in aggregate formation, but the physical mechanism of this

influence is not uniquely determined. Previously researches experimentally

demonstrated that the organic matter of soils has amphiphilic properties and differs

with the relationship between hydrophobic and hydrophilic components (Milanovsky

and Shein, 2002). The aim of this research is to propose a new physical mechanism of

stable aggregate formation under influence of such specific SOM physicochemical

properties as relation of hydrophobic and hydrophilic components.

It is shown that hydrophobic components of SOM give more effective stability of soil

aggregates on different soils (chernozem, grey soils, podzolic, ferralitic and other

soils). These components generated and remaining stationary in soil profile form the

hydrophobic surfaces of soil mineral particles. The amphiphilic molecules of SOM as

usual have hydrophilic and hydrophobic components. In the presence of water

hydrophilic compounds are connected with soil mineral surfaces, which are also

hydrophilic. So, these polarized mineral and organic compounds form the stable

19

Literature Review

linkage. But another part of the same SOM molecule, the hydrophobic one, forms the

stable hydrophobic linkage to the same part of another molecule. This enables to

propose the hypothetic model of soil stable aggregate formation under influence of

amphiphilic (with hydrophilic and hydrophobic compounds) SOM. In the absence of

amphiphilic SOM soil mineral particles are repelled from each other. This process is

accounted for by the influence of exchangeable cations on soil mineral particles. An

elevated osmotic pressure between the particles produces the water movement into

interparticle space and mineral particles are repelled. The process of interparticle

repulsion takes place because of increased water pressure between the soil mineral

particles. So, this aggregate with two mineral particles is unstable. On the contrary, if

the amphiphilic molecules of SOM present in interparticle space so polarized parts

(hydrophilic) are combined readily with the hydrophilic surface of soil mineral

particles. But the hydrophobic parts of SOM molecules enter into chemical

hydrophobic combinations with each other. New energy connections, holding

particles together are formed.These connections of hydrophobic organic nature

provide either water or (supposedly) mechanical stability of soil aggregate.

Hence, the hydrophobic compounds of SOM make the formation of stable soil

aggregates possible. Mechanism of water stability of soil aggregates is governed by

amphiphilic fragments of SOM molecules, which form interparticle connections in the

system “mineral particle– (hydrophilic-hydrophobic components of SOM +

hydrophobic-hydrophilic components of SOM) – mineral particle”.

20

CHAPTER

THREE

METHODS AND

MATERIALS

Materials and

Methods

3 Materials and Methods

A study was conducted to evaluate the relation between aggregate stability and soil

organic carbon content of soils of Ganges river floodplain. General information about

the methods of analysis is described in this chapter.

3.1 General information about sampling sites

Top (0-15cm) soil samples were collected from different locations of Jessore and

Khulna district under different cropping patterns. General information of sampling

sites is presented in Table 3.1.

Table 3.1 General information about sampling sites

Sample No.

GPS Reading Address Soil Series Cropping Pattern

1N: 23º 07.669΄E: 89º 15.903΄

Village: ShabatiUnion: RamnagarThana: KotwaliDistrict: Jessore

MirpurPumpkin/Spinach-Red

Amaranth- T. aman

2N: 23º 11.065΄E: 89º 13.009΄

Village: BahadurpurUnion: NoaparaThana: KotwaliDistrict: Jessore

Sara

Lentil/Onion-Jute-T Aman

3N: 23º 10.085΄E: 89º 12.009΄

Brinjal/Mustard-Fallow-T. Aman

4N: 23º 12.095΄E: 89º 13.001΄

Mahogany garden

5N: 23º 10.084΄E: 89º 11.052΄

Mustard-Fallow-T. Aman

6N: 23º 10.085΄E: 89º 13.023΄

Wheat-Fallow-T. Aman

7N: 23º 13.323΄E: 89º 13.753΄

Village: BahadurpurUnion: NoaparaThana: KotwaliDistrict: Jessore

Ishurdi Wheat-Fallow-T. Aman

8N: 22º 59.420΄E: 89º 26.195΄

Village: JugnipasaUnion: Bejer danga

Thana: PhultalaDistrict: Khulna

Amjhupi Boro-Jute-T.aman

9N: 22º 59.413΄E: 89º 26.186΄

Village: GaithghatUnion: BondhobilaThana: Bagherpara

District: Jessore

Ishurdi Wheat-Dhaincha-T.aman

3.2 Preparation of Samples

The samples were collected from field in polythene bags. Then these were air dried

and crushed with wooden hammer. After crushing the samples were separated into

21

Materials and

Methods

three ranges by using three different size sieves. And the ranges were 8-2 mm, 2-0.25

mm and 0.25-0.05 mm.

3.3 Laboratory analyses

Chemical and physical analyses were done which are interrelated with one another.

3.3.1 Chemical analyses

3.3.1.1 Soil pH

Soil pH was determined electrochemically with the help of glass electrode pH meter

as suggested by Jackson (1973). The ratio of soil to water was 1:2.5 as suggested by

Jackson (1962).

3.3.1.2 Electrical conductivity (EC)

The electrical conductivity of the soil was measured at a soil: water ratio of 1:5 with

the help of EC meter (USDA, 2004).

3.3.1.3 Organic carbon (C)

Organic carbon of samples was determined by Walkley and Black’s wet oxidation

method as outlined by Jackson (1962). Organic matter was calculated by multiplying

the percent value of organic carbon with the conventional Van-Bemmelene’s factor of

1.724 (Piper, 1950).

3.3.2 Physical analyses

3.3.2.1 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.

3.3.2.2 Aggregate Stability (Normalized Stability Index)

The stability of aggregates was determined by the method as described by Six et al.

(2000b). For the determination of aggregate stability soil samples were air dried and

crushed by a wooden hammer. 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 hand

22

Materials and

Methods

sieving. For wet sieving with slaking pretreatment 10 grams air dried 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

3cm 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 dried 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 105°C 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 hexametaphosphate. The weight

of primary particles was recorded after oven drying at 105°C for 24 hours.

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

formula (Six et al., 2000b).

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 = ¿¿

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

23

Materials and

Methods

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

sand with size i in aggregates of size i in aggregates 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−Pp )|]

2×

1[P ¿¿ io−S io ]¿

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

complele disruption of the whole soil.

3.3.3 Statistical analysis

Statistical analysis was performed by using MINITAB (V. 13) statistical package.

24

CHAPTER FOUR

RESULTS AND

DISCUSSION

Results and

Discussion

4 Results and Discussion

Soils were analyzed in laboratory. The physical properties such as particle size

analysis and aggregate stability were determined. Chemical analysis including

percentage of organic carbon with pH and EC of the samples were determined.

4.1 Soil reaction or pH

The pH of the soils under study ranges from 7.32 to 7.85 which indicate that the soils

are neutral to mildly alkaline. The highest pH value was 7.85 in soils of Ishurdi series

located at Noapara in Jessore and the lowest was 7.32 in soils of Amjhupi series

located at Bejer danga in Khulna (Table 4.1). This might be due to the presence of

exchangeable bases within Ganges sediments in the surface soil.

4.2 Electrical conductivity (EC)

The EC value of studied soils varied from 0.52 to 1.29 dS m-1 with an average of 0.79

dS m-1 (Table 4.1). These results indicated that all the soils of the studied series were

nonsaline. This may be due to the land type of our investigated areas which were out

of tidal ingression or any other sort of inputs of salts. The highest value of EC was

obtained in soils of Ishurdi series developed in Bondhobila Union of Jessore and the

lowest was at soils of Sara series in Noapara Union of same district (Table 1 and

Table 2). The EC values were negatively correlated with pH, %Sand, %Silt and

positively with %Clay, %Organic carbon and Nomalized Stability Index.

4.3 Organic carbon

Organic carbon content was highest in Ishurdi Series of sample number 9. And it was

1.64%. The organic carbon content was greater than 1% in Ishurdi (Sample No. 7 and

9) and Amjhupi series (Sample No. 8). The soil organic carbon content of other

samples has been shown in 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% ii 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 that describe the interdependence through

Pearson correlation between the NSI one of the best aggregation indices that

25

Results and

Discussion

correlated with other variables (i.e. %sand, %clay and %OC). 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.

Table 4.1 Chemical properties of soil samples

4. 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.

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

were presented in Table 4.2.

The highest value of percentage of sand was determined 49.00 and the lowest value is

12.50 (Appendix I). The average of percentage sand is 24.44(Appendix I).

The highest value of percentage of silt was determined 57.50 and the lowest value is

34 (Appendix I). The average of percentage sand is 47.94(Appendix I).

The highest value of percentage of clay was determined 30 and the lowest value is 17

(Appendix I). The average of percentage sand is 27.61(Appendix I).

The textural class of Mirpur soil series (Sample no. 1) was detemined as clay loam.

The textural class of Sara soil series (Sample no. 2, 3, 4, 5 and 6) were determined as

clay loam, silt loam, silt loam, silt loam and clay loam. The texture of Mirpur and

26

Sample No.EC

dSm-1 pH %OC

1 0.67 7.80 0.35

2 0.66 7.79 0.50

3 0.81 7.71 0.67

4 0.62 7.53 0.84

5 0.52 7.45 0.54

6 0.62 7.62 0.67

7 0.90 7.85 1.43

8 1.07 7.32 1.49

9 1.29 7.39 1.64

Results and

Discussion

Sara soil series is Silt Loam. The causes of variation in texture may be the intensive

cultivation of field and manmade hinderances. The textural class of Ishurdi soil series

(Sample no. 7 and 9) were determined as clay loam and clay. The texture of this series

is Silty Clay. Loam The textural class of Amjhupi soil series (Sample no. 8) was

determined as silty clay loam. The texture of this series is Silty Clay. The texture of

soils is generally determined from subsoil because there may occur different types of

hinderances on top soil.

Table 4.2 Physical properties of soil samples

Sample No.

%Sand %Silt %Clay Textural Class NSI

1 23 47 30 Clay Loam 0.98

2 22.5 47.5 30 Clay Loam 0.89

3 30 55 15 Silt Loam 0.89

4 30 57.5 12.5 Silt Loam 0.47

5 27.5 57.5 15 Silt Loam 0.70

6 22.5 42.5 35 Clay Loam 0.40

7 27.5 42.5 30 Clay Loam 0.97

8 20 48 32 Silty Clay Loam 0.83

9 17 34 49 Clay 0.95

4.5 Normalized stability index (NSI)

The highest NSI value was estimated 0.98 (Appendix I) which texture was determined

as clay loam (Table 4.2) where the cropping pattern was Pumpkin/Spinach-Red

Amaranth-T. Aman (Table 3.1) and the lowest value was estimated 0.40 (Appendix I)

which texture was determined as also clay loam (Table 4.2) where the cropping

pattern was Wheat-Fallow-T. Aman (Table 3.1). The NSI values for Sara soil series

27

1 2 3 4 5 6 7 8 90

0.20.40.60.81

1.21.41.61.8

NSI

%OC

Soil Sample

NSI

and

%O

C

Results and

Discussion

varies from 0.40 to 0.89. The NSI values for Ishurdi soil series varies from 0.95 to

0.97. The NSI values for Amjhupi soil series varies from 0.40 to 0.89.

Fig. 4.1. Relation between NSI and %Organic carbon of nine soil samples.

4.6 Correlation among soil properties

Percent organic carbon is negatively correalated with soil pH, %Sand and %Silt. The

correlation value between NSI and percent organic carbon is 0.251 and the P-Value is

0.515. So, it is non-significant at 5% level. The value of NSI usually varies between 0

to 1 (Six et al., 2000b). The NSI of studied soil varied from 0.98 to 0.40 which goes

with Six et al. (2000b).

Table 4.3 Correlation among selected soil properties

Parameters EC pH %Sand %Silt %Clay %OC

pH -0.3960.292

%Sand -0.6070.083

0.3740.321

%Silt -0.6720.047

-0.0130.974

0.7800.013

%Clay 0.6840.042

-0.1370.726

-0.9090.001

-0.9700.000

%OC 0.8780.002

-0.4610.212

-0.4090.274

-0.5570.119

0.5310.142

NSI 0.5100.161

0.2860.455

-0.2560.505

-0.3330.381

0.3220.398

0.2510.515

Cell Contents: Pearson correlationP-Value

28

1 2 3 4 5 6 7 8 90

0.20.40.60.81

1.21.41.61.8

NSI

%OC

Soil Sample

NSI

and

%O

C

Results and

Discussion

In soils where SOM is the major binding agent an aggregate hierarchy has been

observed (Tisdall and Oades, 1982; Oades and Waters, 1991). SOM is expected to be

the primary binding agent in 2:1 clay-dominated soils because polyvalent-organic

matter complexes form bridges between the negatively charged clay platelets. In

contrast, SOM is not the only binding agent in oxide and 1:1 clay-dominated soils.

Electrostatic attractions occur between and among oxides and kaolinite platelets due

to simultaneous existence of positive and negative charges at field pH (El-Swaify,

1980). Thus in those soils aggregate formation is partly induced by electrostatic

interactions and aggregate hierarchy should be less pronounced (Oades and Waters,

1991).

The increased aggregate- and mineral-associated C content of small macroaggregates

vs. microaggregates (within treatment) at Sidney, Wooster, and KBS indicates that

both IPOM C and mineral-associated C are incorporated during formation of

macroaggregates. This also suggests that IPOM C is a major C source for microbial

activity and thereby induces the binding of clay- and silt-sized particles and

microaggregates into macroaggregates (Jastrow, 1996; Six et al., 1998, 1999) in these

2:1 clay-dominated soils. In addition, the similarity of aggregate-associated C

concentrations of slaked macroaggregates across management treatments indicates the

stability of slaked macroaggregates is correlated to their C content (Cambardella and

Elliott, 1993).

The stability of aggregates, in contrast, did not seem to be correlated to C content.

Where the organic carbon content was found high, there the NSI value was found low.

And on the other hand, where the organic carbon content was found low, there the

NSI value was found high. The possible causes may be the intensive cultivation of

land due to food production. For intensive cultivation the land should be undergone

with application of different organic and chemical fertilizer, application of pesticides

and fungicides and other cultivation related activities which were responsible for

these problems. These causes are enough to change the relationship between organic

carbon and soil aggregate stability.

29

Results and

Discussion

30

CHAPTER

FIVE

Summary AND

CONCLUSION

Summary and

Conclusion

5 Summary and Conclusion

Aggregate stability of soil closely related with the organic carbon content. Soil OC

concentration is recognized as an important indicator of soil quality which has

important implications regarding crop production sustainability. In most cases, soil

OC declines with the length of time that land is devoted to crop production. The

normalized stability index is an important index to determine the stability of

aggregate. The interrelation between percent organic carbon and normalized stability

index (NSI) has found very perfectly in sample 3 (Sara) and 5 (Sara) (Fig.1). In

sample number 1 (Mirpur) and 2 (Sara) the NSI value is much than organic carbon

concent (Fig.1). On the other hand, in sample number 4 (Sara), 6 (Sara), 7 (Ishurdi), 8

(Amjhupi) and 9 (Ishurdi) organic carbon content is much whereas the NSI is less

(Fig.1). So, the relationship between soil organic carbon and aggregate stability isn’t

perfectly related for the all soil samples.

30

CHAPTER SIX

REFERENCES

References

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APPE

NDICES

Appendices

Appendix: I

Statistical analysis

Descriptive statistics of the studied soils

Variable Mean MedianStandard Deviation

Minimum Maximum

EC 0.79 0.67 0.25 0.52 1.29

pH 7.6 7.62 0.19 7.32 7.85

%Sand 24.44 23.00 4.54 17.00 30.00

%Silt 47.94 47.50 7.80 34.00 57.50

%Clay 27.61 30.00 11.70 12.50 49.00

%OC 0.90 0.67 0.48 0.35 1.64

NSI 0.78 0.89 0.21 0.40 0.98

40