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SOIL AGGREGATE STABILITY OF ITAGUNMODI SERIES AND

APOMU SERIES

BY

ADEBAYO ADEBIMPE MOROLAKE

(S0S/2005/002)

A PROJECT SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENT

FOR THE AWARD OF BACHELOR OF AGRICULTURE (HONS) DEGREE IN THE

DEPARTMENT OF SOIL SCIENCE AND LAND RESOURCES MANAGEMENT.

TO THE DEPARTMENT OF SOIL SCIENCE AND LAND RESOURCES

MANAGEMENT, FACULTY OF AGRICULTURE, OBAFEMI AWOLOWO

UNIVERSITY, ILE-IFE, OSUN STATE,NIGERIA.

DECEMBER, 2011

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CERTIFICATION

This is to certify that this project was carried out by ADEBAYO ADEBIMPE MOROLAKE

under our supervision as part of the requirements for the award of bachelor of Agriculture

(B.Agric.) in the department of Soil Science and Land Resources Management, Faculty of

Agriculture, Obafemi Awolowo University, Ile-Ife, Osun state, Nigeria.

Professor D.O. Aina Professor A.A. Amusan

(Project supervisor) (Head of department)

Date Date

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DEDICATION

This project is dedicated to Almighty God, the author and the finisher of my faith, to whom all

glory belong to and to all who have contributed in one way or the other to the success of my

project.

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ACKNOWLEGDEMENT

My profound gratitude goes to God Almighty, the pillar that holds my life, my backbone and my

all in all. I owe a special debt to my Parents, Pastor Samuel Oluremi & Mrs. Deborah Funmilayo

Adebayo for their love, unfailing support, care and advise over the last few years, may God in his

infinite mercy spare your life to reap the good works of your labour, you are the best and l love

you.

I also give a big thank to my ever supportive supervisor in person of Professor Diipo Aina he is

not only a supervisor but a father for his advice, support and for all the times he spent directing

me towards the right path, l say thank you sir, and l also thank my head of department( Professor

A.A.Amusan)and l appreciate my other lecturers too, Professor Oyedele, Professor Olayinka,

Professor Okunsami, Professor Adepetu, Dr (Mrs)Idowu, Dr (Mrs) Adesanwo Dr Muda and l

would also like to express my gratitude to my siblings Adebayo Busuyi, Adebayo Odunayo,

Adebayo Abimbola, Adebayo Samuel, Adebayo Oluwadamilola and Adebayo Oluwapemilola

for their love and support and also my friends in and outside of the faculty Adeboye

Odunayo(OD), Aderogba Adedoyin, Durotoye Ifeoluwa, Oluremi Yemisi, Olaleye Abimfoluwa,

Dr.Opeyemi Idowu, Sanusi Temitope, Ajishe Tomilola, Anifowose Titilayo and I will not forget

to say thank you to my dear sister, Mrs Olufemi and her husband.I also appreciate my project

colleagues Olakayode Abiodun, Olukoju Oluwole, Amama Elizabeth and Owa Olalekan thank

you for cooperation.

Finally, to all my IMPERIAL 11(SLM) classmate, it was an immense pleasure to have

met all of you and lived through this program with unforgettable moments. Thanks to every one

of you and I hope that one day our path will cross again.

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TABLE OF CONTENT

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ABSTRACT

Aggregate stability measure structural stability of the soil and when soil aggregate stability is

reduced it means there is an increase in soil degradation. There are some factors that affect

aggregate stability which are important in determining the ease to which soil is eroded by water

or and wind. Two soils series whereas used, the Itagunmodi series and Apomu series. The

method used to determine or evaluate the aggregate stability is raindrop technique and the energy

of raindrop impact is the most important variable in describing time to breakdown the aggregate,

the same procedure was done on the two soils series and different results were gotten. From the

results, Itagunmodi series has higher energy mean than Apomu series and it required more

raindrop and time before it dispersed rather than Apomu series. Itagunmodi series is more stable

than Apomu series.

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CHAPTER ONE

INTRODUCTION

Soil aggregate stability is a balance established between disruptive forces and those that

favour aggregation. For the surface aggregate these forces are represented by the raindrop impact

together with dispersion and hydration. The soil surface response to these forces is shown by the

permeability values, which determine the quantity of rain water going into the soil profile, and

which will be available for the plants. Soil stability has been related to organic matter content,

structure, texture, aggregate size and presence of Fe and Al sesquioxides (Le Bissonnais ( 1997),

Stolte et al.,(1997) and Roth (1997)).Furthermore, the size distribution and aggregate stability

can be influenced by the conditions imposed during soil sampling, preparation and analysis.

Different methods have been applied to analyze the aggregate stability in order to predict the

field behaviour of soils: (Kemper a& Koch, 1966; Kemper & Rosenau, 1986; Beare et al, 1993;

Le Bissonnais, 1993, amongst others.

The resistance offered by the soil surface aggregate to the raindrop impact is determined by a

larger susceptibility to sealing formation. When the drop impacts on the aggregate surface, the

surface structure is disintegrated producing a variation in the water penetration rate and in the

accumulation of particles detached by the impact on the surface. These particles are in the

suspension and when sedimentation takes place, the finest particles rest on the upper part and

they are susceptible to transportation. The sealing which is formed is characterised by small

particles on the surface, sparse large pores, high bulk density and sometimes, with a laminar

structure due to the particles stratification and orientation ( Pla, 1995). The sealing degree during

a storm depends on many factors including rainfall intensity, energy, slope, aggregate stability,

texture and electrolytes.

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Soils are most important in many ecosystems as dynamic natural body and fundamental

resource. Human activities often influence the natural processes in soils. One of the most critical

natural resource management needs of the 21st century is information about the dynamic nature

of soils, or simply, soil changes (Tugel et al, 2005).

Soil aggregate stability is an important indicator of soil physical quality (Castro Filho et al.,

2002). Land use and management also influence soil aggregation and aggregate stability

(Bergkamp and Jongejans, 1988; Cerda, 2000).

Aggregate can be considered as quasi-permanent units of structure that exist in soil, despite

seasonal modification by weathering and the action of the short-term disruptive forces such as

swelling, impact of raindrops, mechanical overloading (compaction and shear) and creep (flow

of water under its own weight) such aggregate are often called peds and are distinguished by

their more permanent nature from fragment or clods formed at or near the soil surface by

cultivation and frost action (Hodgson,1976).Aggregate are formed mainly by physical forces

,whereas the particles within the aggregate are held together (stabilized) largely by linear organic

polymers that have many active groups that reacts with clay particles.Good stabilization of

aggregate necessitate that the forces outside of the aggregate be considerably weaker than those

inside.

Though here are macropores and micropores, Edward and Bremmer(1967) stated that the

only aggregate in the soil that have high stability are the micro aggregate(

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and they provide a store of materials which is organized by drop impacts into a surface crust

(Bryan,1973, Farres, 1978).

The impact of rain drop on the soil surface must include an analysis of the stress

produced which should correspond to the strength of the soil crumb needed to resist this impact.

There are many methods that have been used to analyse or measure aggregate stability. Direct

dry sieving of soils as they occur in the field has been used by Keen (1933),Cole(1939)and

others to evaluate the distribution of clods and aggregate. The wet sieving methods of

Tiulin(1928) and Elutration which may be used for separation aggregate with diameter between

1mm and 0.02mm.Several investigation (Baver and Rhoades,1932,Demolon and Henin 1932)

have successfully used the elutrator for making aggregate analysis. Sedimentation method have

been used to determine the aggregate distribution in the finer fraction that cannot be separated by

sieving. Cole and Edletson(1935) designed a large sedimentation tube in which the particles fall

in still water. Middleton(1930) has suggested the dispersion ratio as a measure of aggregate ,this

ratio represents the percentage of particles smaller than 0.05mm in the aggregate sample divided

by percentage of the same size in the dispersed sample.

EFFECT OF ORGANIC MATTER

Soil organic matter certainly improves the ability of the soil to resist erosion and enables the soil

to hold more water. Important is its effect in promoting soil aggregation in a granular soil and the

combination of increased water penetration (Stevenson and Cole, 1999).

The loss of organic matter and consequently soil fertility is often driven by unsustainable

practices such as deep plowing on fragile soils and cultivation of erosion-facilitating crops and

the continuous use of heavy machinery which destroys soil structure through compaction.

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Soil organic matter content has a direct relationship with soil erodibility. The stability of

soil aggregates is enhanced where organic material is combined with clay particles and where it

contributes to chemical bonding (Morgan, 1986). Generally, soils with a higher content of

organic matter and an improved soil structure have a greater resistance against soil erosion by

water and wind. Continuous soil erosion has a significant effect on soil erodibility. In the

research areas the material below a soil which was totally eroded is often more erodible because

of its low organic matter content, its lower clay content and its different structure. Low aggregate

stability enforces soil erosion.

RELEVANCE OF SOIL STRUCTURE IN CROP PRODUCTION

Soil structure is defined by the combination or arrangement of primary soil particles into

compound elements, which is separated from adjoining structural elements by surfaces of

weakness soil texture, soil structure, and the type of clay mineral, organic matter content and

type, cementing agents and cropping history influence the aggregate stability.

Among the mechanical destructive forces are soil tillage, impact of heavy machinery, treading

by animals and raindrop splash. Physicochemical forces are examples slaking, swelling and

shrinkage, dispersion and flocculation. Slaking is the process of structure breakdown under the

influence of wetting of soil aggregate, due to swelling of clay minerals, dissolving of cementing

agents, air explosion or reduction in pore water suction. Slaking may result in the formation of a

superficial crust, reducing water infiltration and enhancing sediment loss by downward

transportation with surface runoff water.

The most important function of soil is that it is the basic media of crop production

(Varallyay, 2002). Consequently, sustaining and ameliorating of soil structure is crucial.

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Decline of soil structure and degradation of its stability are frequently caused by tillage, crop

management and getting moist (Chan et al., 2003).

Soil structure is built up by groups of primary particles that cohere to each other, and which are

called aggregates (Kemper and Rosneau, 1986). (Di Gleria et al., 1957). Water stability is often

investigated by wet sieving. Earlier reported methodological problems were solved and

circumstances affecting soil structure were standardized with the new modified wet sieving

method proposed by Six (2000).

The soil structure depends namely on the grain size distribution, on soil formation processes and

the effects of plants, animals and humans. Freezing and thawing, water movement, the growth

and decay of plant roots and the activity of soil animals (e.g. Earth worms) as natural factors on

the one hand and human activities (namely management practices) on the other can cause in

rearranging of particles in soil aggregates. Therefore in many cases the structure of a soil directly

affects its properties (Marshall et al., 1996). A low status of organic matter (naturally low or due

to soil degradation) is an important reason for the instability of soil aggregates. Many

agricultural practices affect soil structure. Decrease in both the stability and the organic matter of

soils under annual tillage has been observed by several researchers (e.g. Low, 1972; Allen, 1985;

Gami et al., 2001; Caravaca et al., 2001).

SOIL STABILITY

The Normalized Stability Index (NSI) characterises aggregate stability by comparing the

aggregate distribution after two differently disrupting wetting methods. The two different wetting

methods are:

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1. Fast wetting, namely rapid immersion in water (Slaking (S)), which disrupts the

aggregates to the highest extent, therefore produce the lowest aggregate amount; and

2, Slow wetting, namely capillary wetting (Capillary (C)) to field capacity, which disrupts

the aggregates to the lowest extent and therefore achieves the highest aggregate amount.

The capillary wetting method was earlier tried to get different size fractions to evaluate

the distribution of soil organic matter in soils (Huisz et al., 2006).

FACTORS INFLUENCING SOIL INSTABILITY

Soil type

In sandy soils, soil particles are unable to form stable aggregates, but the soils are free draining.

As the clay content of the soil increases, the particles are held together more strongly and

structural strength increases. Soil slaking or dispersion is evident in soils with a high content of

fine sand and/or silt (loamy soil) and low organic matter levels, with crusting and hardsetting

most common in soils with 10 to 35 percent clay.

The soil has a major role in determining whether a soil is hardsetting. Soil types prone to

hardsetting are unable to develop water-stable aggregates and are a feature of many - particularly

those low in organic matter ( 6 indicates a sodic soil which is

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likely to suffer from problems of dispersion in the surface and/or subsoil layers when cultivated

and/or impacted by rain or surface run off.

Climate

The impact of raindrops interacts with the level of soil cover and the tendency of a soil to slake

(soil aggregates can collapse within minutes of rainfall starting) and disperse (separation of soil

particles may take a number of hours) to influence surface bonding (crusting, hardsetting). Soils

with low organic matter are more prone to surface crusting.

Organic matter retention

Increasing soil cover (organic residues) decreases the impact of raindrops on the soil surface,

whereas bare soil becomes compacted by successive rainfall events. Increased soil organic

matter promotes soil aggregation and soil stability. Organic matter, root exudates, soil fauna

(including fungi and bacteria) and organo-metallic complexes help bind particles and form stable

soil aggregates, and result in soils which are less likely to crust, have a faster rate of water

infiltration and are generally more fertile.

Soil disturbance

Although predominantly associated with natural processes, crusting can be caused by tilling or

excessive stocking on wet soils, and is exacerbated by a lack of cover and low organic matter

content. Zero tillage promotes the build-up of organic residues on and near the soil surface,

retains root biopores and improves soil structure compared to continuous cultivation.

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SOIL FORMATION

The formation of soil happens over a very long period of time. It can take 1000 years or more.

Soil is formed from the weathering of rocks and minerals. The surface rocks break down into

smaller pieces through a process of weathering and is then mixed with moss and organic matter.

Over time this creates a thin layer of soil. Plants help the development of the soil. How? The

plants attract animals, and when the animals die, their bodies decay. Decaying matter makes the

soil thick and rich. This continues until the soil is fully formed. The soil then supports many

different plants.

THE FIVE SOIL FORMING FACTORS ARE:

1. Parent material: The primary material from which the soil is formed. Soil parent material

could be bedrock, organic material, an old soil surface, or a deposit from water, wind, glaciers,

volcanoes, or material moving down a slope.

2. Climate: Weathering forces such as heat, rain, ice, snow, wind, sunshine, and other

environmental forces, break down parent material and affect how fast or slow soil formation

processes go.

3. Organisms: All plants and animals living in or on the soil (including micro-organisms and

humans!). The amount of water and nutrients, plants need affects the way soil forms. The way

humans use soils affects soil formation. Also, animals living in the soil affect decomposition of

waste materials and how soil materials will be moved around in the soil profile. On the soil

surface remains of dead plants and animals are worked by microorganisms and eventually

become organic matter that is incorporated into the soil and enriches the soil.

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4. Topography: The location of a soil on a landscape can affect how the climatic processes

impact it. Soils at the bottom of a hill will get more water than soils on the slopes, and soils on

the slopes that directly face the sun will be drier than soils on slopes that do not. Also, mineral

accumulations, plant nutrients, type of vegetation, vegetation growth, erosion, and water

drainage are dependent on topographic relief.

5. Time: All of the above factors assert themselves over time, often hundreds or thousands of

years. Soil profiles continually change from weakly developed to well developed over time.

SOIL STRUCTURAL DEGREDATION

Soil structural degradation produces condition less favourable for crop production has become a

global phenomenon in agricultural soils. The extent of the degradation varies from soil to soil

depending upon what causes it. The causes can be classified into four categories including:

i. Mechanical compaction as a result of the use of heavy machinery (Hakansson and

Voorhes. 1997; Brussard and Van Faassen. 1994; Hakansson et al.. 1988; Jakobsen and

Greacen. 1985).

ii. Surface crusting due to raindrop impact (Summer and Stewart. 1992; Le Bissonnias et al.

1989; Levy et al.. 1986; Kemper and Miller. 1974).

iii. Hardsetting because of soil structure collapse due to water unstable aggregate (Mullins et

al..1990; Gusli et at..1994a.b).

iv. And aggregate coalescence of relatively water stable aggregates (Cockroft and Olsson,

2000. Ghezzehei and Or, 2000)

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It is a soil condition in which aggregates become welded at their contact points

(Bresson and Moran. 1995; Ghezzehei and Or, 2000) and this welding is thought to increase soil

strength and restrict root growth in irrigated soils (Cockroft,2000).

OBJECTIVES.

The aim of this work is to assess those raindrops characteristics which determine when soil

aggregate breakdown and to consider some of the accumulated facts on issues of aggregate

formation and stability using two soil samples of different structural and textural classes(i.e.

Apomu series (sand) and Itagunmodi series

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CHAPTER TWO

LITERATURE REVIEW

Aggregate stability is a measure of the structural stability of soils. Factors that influence

aggregate stability are important in evaluating the ease with which soils erode by water and/or

wind, the potential of soils to crust and/or seal, soil permeability, quasi-steady state infiltration

rates and seedling emergence and in predicting the capacity of soils to sustain long-term crop

production. Aggregate stability of soils can be measured by the wet-sieving or raindrop

techniques. A reduction in soil aggregate stability implies an increase in soil degradation. Hence

aggregate stability and soil degradation are interwoven.

Definition of soil structures as mainly focused on the management of soil particles and

the spaces surrounding them. This was exposed by Marshal and Holmes (1978)as The

arrangement of the solid phase of the soil and of the pore space located between its constituents

particles this includes the size, shape and arrangement of aggregated formed where primary

particles are clustered together into larger separate unit (Marshall et al., 1996).

However, Dexter (1988)defines soil structure as the spatial heterogenecity of different

component or properties of soil. Thus, this definition encompasses all aspects of the soil

structure that affect root growth including soil strength.

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PHYSICAL AGENTS IN AGGREGATION

Tillage Impact in Aggregate

The overwhelming interest in agricultural sustainability is attributed to several changes facing

intensive agriculture, such as excessive fertilizer application, risks of environmental pollution

and degradation of soil and water resources. Conservation tillage systems, rather than plow-

based methods of seedbed preparation, have the potential to provide sustainable usage of soil

resources. Cultivation can alter soil physical, chemical, and biological properties, whereby plant

growth, development and yield could be influenced (Blevins and Thomas, 1983; Grant and

Lafond, 1993).

There are many examples of inappropriate agricultural management resulting in deterioration of

soil quality (Mullins et al., 1990). Several studies have reported the effects of management such

as tillage and rotation on soil structural characteristics, especially stability and size distribution of

aggregates (Angers, 1992; Ismail et al., 1994). Bear et al. (1994) reported that residue cover in

no-till method improved soil aggregation and organic carbon content. Carter and Rennie (1982)

also reported a higher soil organic carbon at the soil surface in no-till system. In a 28-year study

on Ohio soils, Lal et al. (1994) reported that no-till improved soil aggregate stability. Tisdall and

Oades(1982), Elliott (1986), and Kay (1990) reported that cultivation can cause a disruption of

soil aggregate sand loss of soil organic carbon. Hamblin (1980) found that no-till system could

result in a smaller aggregate mean weight diameter (MWD).

Tillage systems are location specific, so the degree of their success depends on soil, climate, and

management practices. Although little differences in soil structural characteristics have been

reported among tillage systems (Bauer and Black, 1981), low precipitation and high temperature

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in arid and semi-arid regions result in a lower potential for soil organic carbon accumulation.

After 11 years of study on a sandy-loam soil in a semi-arid region, Campbell and Souster (1982)

reported that due to equal residue production of both systems, neither tillage nor fallow elevated

soil organic carbon content. Measurement of aggregation characteristics as indicators of soil

structure has been reported extensively in the literature (Caron et al., 1996; Hajabbasi et al.,

1999).

Irrigation

Irrigation, applying water to assure sufficient soil moisture is available for good plant growth, as

practiced in North Dakota is called "supplemental irrigation" because it is used to augment the

rainfall that occurs during the growing season. Irrigation is used on full season agronomic crops

to provide a dependable yield every year. It is also used on crops where water stress affects the

quality of the yield, such as flowers, vegetables and fruits.

During most years it is not uncommon for some places in the state to receive sufficient rainfall

for good plant growth while other areas experience reduced yields or quality on non-irrigated

crops because of water stress from insufficient soil moisture. For irrigation planning purposes,

average precipitation during the growing season is not a good yardstick for determining a need

for irrigation. The timing and amounts of rainfall during the season, the soil's ability to hold

water, and the crop's water requirements are all factors which influence the need for irrigation.

Any location in the state can have what might be considered "wet" or "dry" weeks, months and

even years.

Under irrigation, soil and water compatibility is very important. If they are not compatible, the

applied irrigation water could have an adverse effect on the chemical and physical properties of

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the soil. Determining the suitability of land for irrigation requires a thorough evaluation of the

soil properties, the topography of the land within the field and the quality of water to be used for

irrigation. A basic understanding of soil/water/plant interactions will help irrigators efficiently

manage their crops, soils, irrigation systems and water supplies.

SOIL PROPERTIES

Soil Texture

Soil texture is determined by the size and type of solid particles that make up the soil. Soil

particles may be either mineral or organic. In most soils, the largest proportions of particles are

mineral and are referred to as "mineral soils." For mineral soils, the texture is based on the

relative proportion of the particles under 2 millimeters (mm) or 5/64th of an inch in size. The

largest particles are sand, the smallest are clay, and silt is in between. The soil texture is based on

the percentage of sand, silt and clay. Soil texture classes may be modified if greater than 15% of

the particles are organic (e.g. mucky silt loam). Soil particles greater than 2 mm in size are not

used to determine soil texture. However, when they make up more than 15% of the soil volume,

the textural class is modified (e.g. gravelly sand).

Soil Structure

Soil structure refers to the grouping of particles of sand, silt, and clay into larger aggregates of

various sizes and shapes. The processes of root penetration, wetting and drying cycles, freezing

and thawing, and animal activity combined with inorganic and organic cementing agents produce

soil structure. Structural aggregates that are resistant to physical stress are important to the

maintenance of soil tilth and productivity. Practices such as excessive cultivation or tillage of

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wet soils disrupt aggregates and accelerate the loss of organic matter, causing decreased

aggregate stability.

The movement of air, water, and plant roots through a soil is affected by soil structure. Stable

aggregates result in a network of soil pores that allow rapid exchange of air and water with plant

roots. Plant growth depends on rapid rates of exchange. Good soil structure can be maintained by

practicing beneficial soil management such as crop rotations, organic matter additions, and

timely tillage practices. In sandy soils, aggregate stability is often difficult to maintain due to low

organic matter, clay content and resistance of sand particles to cementing processes.

Soil Depth

Soil depth refers to the thickness of the soil materials which provide structural support, nutrients,

and water for plants. In North Dakota, soil series that have bedrock between 10 and 20 inches

from the surface are described as shallow. Bedrock between 20 and 40 inches is described as

moderately deep. Most soil series in North Dakota have bedrock at depths greater than 40 inches

and are described as deep. Depth to contrasting textures is given in the soil series descriptions in

the county soil survey report.

The depth to a contrasting soil layer of sand and gravel can affect irrigation management

decisions. If the depth to this layer is less than 3 feet, the rooting depth and available soil water

for plants is decreased. Soils with less available water for plants require more frequent

irrigations.

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Soil Permeability and Infiltration

A soil's permeability is a measure of the ability of air and water to move through it. Permeability

is influenced by the size, shape, and continuity of the pore spaces, which in turn are dependent

on the soil bulk density, structure and texture. Most soil series are assigned to a single

permeability class based on the most restrictive layer in the upper 5 feet of the soil profile (Table

1). However, soil series with contrasting textures in the soil profile are assigned to more than one

permeability class. In most cases, soils with a slow, very slow, rapid or very rapid permeability

classification are considered poor for irrigation.

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Table 2.1 Soil Permeability Classes.

Infiltration Rate

Classification (cm/hour)

Very Slow Less than 0.15

Slow 0.15 to 0.51

Moderately Slow 0.51 to 1.52

Moderate 1.52 to 5.08

Moderately Rapid 5.08 to 15.24

Rapid 15.24 to 50.8

Very Rapid Greater than 50.8

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Infiltration is the downward flow of water from the surface through the soil. The infiltration rate

(sometimes called intake rate) of a soil is a measure of its ability to absorb an amount of rain or

irrigation water over a given time period. It is commonly expressed in inches per hour. It is

dependent on the permeability of the surface soil, moisture content of the soil and surface

conditions such as roughness (tillage and plant residue), slope, and plant cover.

Coarse textured soils such as sands and gravel usually have high infiltration rates. The

infiltration rates of medium and fine textured soils such as loams, silts, and clays are lower than

those of coarse textured soils and more dependant on the stability of the soil aggregates. Water

and plant nutrient losses may be greater on coarse textured soils, so the timing and quantity of

chemical and water applications is particularly critical on these soils.

Water Holding Capacity of Soils

There are four important levels of soil moisture content that reflect the availability of water in the

soil. These levels are commonly referred to as: 1) saturation, 2) field capacity, 3) wilting point

and 4) oven dry.

When a soil is saturated, the soil pores are filled with water and nearly all of the air in the soil

has been displaced by water. The water held in the soil between saturation and field capacity is

gravitational water. Frequently, gravitational water will take a few days to drain through the soil

profile and some can be absorbed by roots of plants.

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Field capacity is defined as the level of soil moisture left in the soil after drainage of the

gravitational water. Water held between field capacity and the wilting point is available for plant

use.

The wilting point is defined as the soil moisture content where most plants cannot exert enough

force to remove water from small pores in the soil. Most crops will be permanently damaged if

the soil moisture content is allowed to reach the wilting point. In many cases, yield reductions

may occur long before this point is reached.

Capillary water held in the soil beyond the wilting point can only be removed by evaporation.

When soil is dried in an oven, nearly all water is removed. "Oven dry" moisture content is used

to provide a reference for measuring the other three soil moisture contents.

How Plants Get Water from Soil

Water is essential for plant growth. Without enough water, normal plant functions are disturbed,

and the plant gradually wilts, stops growing, and dies. Plants are most susceptible to damage

from water deficiency during the vegetative and reproductive stages of growth. Also, many

plants are most sensitive to salinity during the germination and seedling growth stages.

Most of the water that enters the plant roots does not stay in the plant. Less than 1% of the water

withdrawn by the plant is actually used in photosynthesis (i.e. assimilated by the plant). The rest

of the water moves to the leaf surfaces where it transpires (evaporates) to the atmosphere. The

rate at which a plant takes up water is controlled by its physical characteristics, the atmosphere

and soil environment.

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As water moves from the soil, into the roots, through the stem, into the leaves and through the

leaf stomata to the air, it moves from a low water tension to a high water tension. The water

tension in the air is related to its relative humidity and is always greater than the water tension in

the soil.

Plants can extract only the soil water that is in contact with their roots. For most agronomic

crops, the root distribution in a deep uniform soil is concentrated near the soil surface. Over the

course of a growing season, plants generally extract more water from the upper part of their root

zone than from the lower part.

Plants such as grasses, with a high root density per unit of soil volume, may be able to absorb all

available soil water. Other plants, such as vegetables, with a low root density, may not be able to

obtain as much water from an equal volume of the same soil. Vegetables are generally more

sensitive to water stress than high root density agronomic crops such as alfalfa, corn, wheat and

sunflower.

CHEMICAL AGENTS

Organic matter influence

The organic matter content of a soil is determined by climate, soil type and land use management

(Feller, 1994; Feller & Beare, 1997).

This relationship is probably best known for temperate agricultural soils where

aggregation depends on the quantity and mineralogy of clay (Feller & Beare, 1997). This fact is

shown clearly in the studies of Douglas and Goss (1982) where increasingly higher quantities of

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C were required to achieve the same level of aggregate stability in soils of increasing clay

content (16-49% clay).

By comparison, the agricultural systems of developed countries, where exogenous inputs

(fertilizers etc.) are high and the recycling of organic resources is often greater (e.g. crop

residues, manures, etc.) allow for better maintenance and/or restoration of the soil resource

(Sanchez et al. 1997). Successful tropical soil fertility recapitalization depends on adopting

measures that:

1. Improve the efficiency of soil resource utilisation and, thereby, minimize (slow) soil

degradation

2. Provide compensation for the resources removed in plant and animal products or lost to

the wider environment (e.g. leaching, gaseous emissions, erosion), and

3. Include restorative phases in the land use rotations.

Most tropical agricultural practices involve few external inputs and, therefore, rely heavily on the

mineral and organic properties of soils to sustain plant production. As a result, soil organic

matter management (SOM) is an important tool for soil fertility recapitalization. Furthermore,

the dynamics of SOM are in dissociable from soil biological activity, as SOM is the primary

source of energy and nutrients for soil biota and soil biota are responsible for the transformations

that regulate SOM storage. One of the important mechanisms by which soil biota influence SOM

storage is through the formation and stabilization of soil aggregates.

The size, quantity and stability of aggregates recovered from soil reflects an environmental

conditioning that includes factors which enhance the aggregation of soil (e.g. wet-dry cycles,

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organic matter amendments) and those that cause disaggregation ( eg. cultivation, bioturbation)

(Beare and Bruce, 1993). The measurement of soil aggregates depends on both the

forces that bind particles together and the nature and magnitude of the disruptive forces applied.

Soil aggregation influences the susceptibility of soil to erosion, organic matter storage, soil

aeration, water infiltration and mineral plant supply. Many studies have shown the effects of

organic constituents on the amount and stability of soil aggregates. However, understanding the

role that soil aggregation plays in fertility recapitalization also requires a knowledge of how

aggregation contributes to organic matter storage in soil. Both processes are mediated by soil

biological activity.

The nature of aggregate-associated SOM

Efforts to describe the quality and quantity of aggregate-associated organic matter stem from two

particular interests: 1) understanding the importance of organic matter constituents for

determining the structural stability of aggregates and 2) identifying the mechanisms by which the

aggregation of particles contributes to the physical protection and storage of SOM.

In each case there is a need to carefully define the size and stability of aggregates using methods

that are both quantitative and reproducible (Beare and Bruce, 1993).

In contrast to the relatively well-described relationship between bulk soil organic matter and

aggregate stability, there are conflicting results regarding the relationship between aggregate

size-classes and SOM constituents. Considering differences in the mineral and organic

components of aggregate size-classes may be critical to interpreting the results obtained. Elliott

et al. (1991) corrected for both the sand and light-fraction material in aggregate size-classes

collected from a chronosequence of tropical Peruvian Ultisols under cultivation.

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Bioavailability of aggregate-associated organic matter

The bioavailability and storage of aggregate-associated organic matter has important

implications for soil fertility recapitalisation. The influence of aggregation on the protection of

organic matter from microbial attack has been studied by comparing results obtained before and

after disaggregation of soil. A number of studies have demonstrated the importance that physical

protection mechanisms for organic matter storage in soils (Ladd et al., 1993; Elliott 1986; Feller

et al., 1996). Several studies (e.g. Elliott, 1986, Gupta and Germida, 1988) have shown that

from 15 to 45% of the N that is mineralisable from macro aggregates of native sods is protected

from microbial attack within the intact structure of macro aggregates. Other research has focused

on separating and characterising of free and occluded (aggregate associated) particulate

organic matter in soils (Golchin et al., 1994b; Puget et al., 1997).

BIOLOGICAL AGENTS

The maintenance of water-stability of soil aggregates bound by different chemical binding agents

produced in situ by pure cultures of microorganisms and by an indigenous soil micro flora was

assessed by wet-sieve analysis.

A teaspoonful of soil may contain billions of living organisms. On them crop growth, soil

fertility, and even soil development depend in many ways.

Among the soil's inhabitants are specialists that rot organic matter, transform nitrogen, build soil

filth, produce antibiotics, and otherwise affect plant welfare.

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Bacteria are the smallest and the most numerous of the free-living organisms in the soil. About

25 thousand of them measure an inch. Despite their minute size, their total weight in the top foot

of an acre of fertile soil may be as much as a thousand pounds, or 0.03 percent of the weight of

the soil. Poor soils and some sandy soils may harbor few bacteria.

The bacteria are little more than tiny blobs of jellylike protoplasm enclosed in a cell membrane.

Most of them subsist on waste organic materials.

Those that derive both their cell carbon and their energy from organic substances are called

heterotrophic bacteria. They use energy previously stored by other microbes or by higher plants.

Their metabolism or ability to carry on such life processes as oxidizing sugar to carbon dioxide

and water is like that of the higher animals, all of which depend on the energy stored in

carbohydrates, fats, and proteins.

A few bacteria possess pigments that enable them to trap the energy in light. They obtain their

cell carbon directly from the carbon dioxide of the atmosphere. Green plants similarly possess

this photosynthetic ability.

Still other bacteria, called autotrophic or chemosynthetic, draw upon the atmosphere for their

carbon supply and obtain their energy by oxidizing relatively simple chemical materials. In this

group are bacteria that oxidize carbon monoxide to carbon dioxide, sulfur to sulfates, hydrogen

to water, ammonia to nitrous acid, and nitrous acid to nitric acid.

Most soil bacteria require nitrogen that previously has been combined either into mineral forms

(such as ammonium and nitrate) or into organic nitrogen compounds (such as plant proteins and

animal proteins).

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Only a limited number of micro-organisms are able to make use of nitrogen gas as it commonly

occurs in the air. Among the soil bacteria that can do so are the legume-nodule bacteria, or

rhizobia, which use nitrogen from the air in partnership with leguminous host plants. The

nitrogen taken from the atmosphere is available to both partners. Consequently legumes can be

grown on soil that is poor in nitrogen but otherwise is favorable.

The amount of nitrogen fixed by nodulated legumes varies greatly, but the average is estimated

to be 50 to 150 pounds of nitrogen an acre each year.

The right kind of legume bacteria must be present for each legume plant. If the legume is native

to an area or has been grown for several years on the soil, the correct bacteria usually have

become established. But for newly introduced legumes, for soils in which the correct rhizobia do

not survive during the intervals between crops, and for soils in which there are only weak or

parasitic strains of nodule bacteria, inoculation of the legume seed at planting time with nitrogen-

fixing bacteria is desirable or necessary.

Packaged inoculants containing nitrogen-fixing bacteria are available at many seed stores.

Because the inoculants are prepared for specific legumes or groups of legumes, the names of

which are printed on the package, the purchaser needs only to specify to the dealer the legume

seed he wishes to treat. Inoculant labeled as effective on one legume, such as soybeans, is not at

all suitable for other legumes, such as clovers or garden peas.

Legume inoculant sufficient to treat a bushel of seed costs 15 to 55 cents, depending on the type

of seed and the amount of inoculant purchased.

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Directions on the package should be followed as closely as possible. The user should plant the

inoculated seed within a very few hours after he treats them. Meanwhile the seeds should not be

exposed to heat, drying, or sunlight. He should not purchase or use inoculant that is older than

the expiration date stamped on the label.

Gardeners or farmers who have highly fertile soils or who apply liberal quantities of nitrogenous

fertilizer can expect little or no additional benefit from legume inoculants. Commercial operators

or farmers who grow large acreages of legumes commonly consider seed inoculation to be a

desirable procedure and one that entails little extra cost.

The use of inoculant insures that the legume seedlings will be exposed to the right kind of

nitrogen-fixing bacteria early in the growing season.

There also exist in the soil free-living, or non symbiotic, forms of bacteria, such as Azotobacter,

that can use atmospheric nitrogen. These types occur in relatively small numbers. Their effect on

fertility has been questioned. Increases of 50 pounds of nitrogen an acre a year have been

attributed to Azotobacter. Some persons say non leguminous crops and even compost piles

should be inoculated with these bacteria, but we have little evidence that such inoculation is

economically sound.

Some of the pigmented bacteria that are capable of photosynthesis also can use atmospheric

nitrogen. They exist mostly in stagnant water and mud. They are believed to be of little

importance in the nitrogen fertility of ordinary field soil.

Soil bacteria are not distributed uniformly through the soil. They commonly occur in clumps or

colonies of few to thousands of individual cells.

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Because bacteria depend largely on organic matter for their food, they occur most abundantly

near organic residues. The upper layers of the soil profile are enriched almost continuously by

plant wastes, and they contain many more bacteria than the deeper layers do. Even within the

plow layer, islands of activity can be expected wherever food material exists.

Soil biota and soil aggregation

It is important to note that physical controls on the storage and loss of organic matter can not be

viewed in isolation from biological influences. Soil biota are clearly important in mediating

physical changes in soil structure that may alter the storage and transformations of SOM.

Biological constituents ranging from roots and fungi to micro arthropods and earthworms can

influence the formation and stabilization of soil aggregates. For example, several studies (e.g.

Martin, 1992; Lavelle and Martin, 1992) have shown that earthworm casts store and protect on

the order of 20% more organic C than non-ingested soils. This was attributed to the higher C

content and stability of the earthworm casts as compared to mineral soil (Blanchart, 1992;

Blanchart et al., 1993). Other organism may also contribute to the physical protection of organic

matter through their influence on soil aggregation. For example, Beare et al. (1997) indicated

that fungal hyphae were responsible for about 40% of the macro aggregation (>2000 um) and

significantly greater retention of soil organic matter in soils under no-tillage management but a

much lesser role in conventionally tilled soils. Some examples of biological influences (positives

or otherwise) on soil aggregation and the mechanisms involved are given in Table 1. Soil

microbial biomass appears to be a relatively poor indicator of aggregation. Indeed, soil

microorganisms may have a spatially heterogeneous influence on soil aggregation through the

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localized production and deposition of organic matter binding agents. For example, plants with a

ramified and fine root structure (Degens, 1997) and a high production of exudates, produce

aggregates in the rhizosphere, with consequences for structural stability in the root that may

influence the rooting zone of subsequent crops.

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CHAPTER THREE

MATERIAL AND METHOD

Material

Two soils of contracting properties were used that is, ltagunmodi series and Apomu series. The

two soils samples were collected from different locations within Obafemi Awolowo University

Campus in ile-ife, osun state, Nigeria.

Soil sample I which is the ltagunmodi series (clay) were collected near the school main gate

(road one before getting to the security post), under cultivated condition with the use of auger.

The soil is well-drained, very fine textured soil of uniform brownish red or dark chocolate brown

colour, the depth is of 0-15cm with a textural class of 45% or more clay,

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. Dispersion ratio method.

. Emerson test.

I used raindrop techniques and apparatus used are; weighing balance burette, retort stand, small

plastic cups and 10-mesh sieve (2mm in diameter).

METHOD

RAINDROP TECHNIQUES

The stability of the aggregate was determined by Raindrop Technique and single water-drop

simulator was designed, fabricated and calibrated for determining stability of natural soil

aggregate. The dry stable aggregate (from different land uses) of size 1-2, 2-4, 4-6, 6-8mm were

subjected to raindrop impact on a sand bath and the number of drops used to completely disperse

an aggregate were recorded. The total kinetic energy of the falling raindrop used for complete

dispersion of the aggregate was expressed on per gram basis of the soil to calculate the

stability.SISRT1/4=1/2MV.

Where N is the number of drops used to complete disrupt the aggregate on per gram of

soil basis, and M is the mass of the single water-drop of respective size and V is the terminal

velocity of the respective water-drop. The apparatus consisted of a 50.00cm burette mounted on

the retort stand such that it could be raised or lowered to control the height of water fall and

hence the velocity of drop impact. The height of fall ranges from 0.25m to 1.5m of water was

poured into the burette and the burette was constantly adjusted to maintain a constant drop rate.

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Each weighted aggregate was placed on a 10-mesh sieve at a required height below the tip of the

burette.

For each weighted of aggregate size, experiment are done repeatedly for the same height

which is one meter (1m) for four different sizes aggregate. The time taken for the water fall, the

number of drops and the total volume used (read on calibrated burette) require for the completion

of the aggregate were recorded, and three replicate was done.

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CALCULATION

20drops = 2ml

20 drops of water under the same condition give rise to 2ml

20 drops - 2ml

1 drop - 0.1ml

i. To calculate the mass of each drop

Density = mass/volume

Mass = density x volume

Density of water = 1g/cm3

Mass = 1g/cm3

x 0.1ml

= 0.1g

ii. To calculate the time taken for each drop of water at a height of 1m

It took 60s for 100 drops

1 drop = 60/100

= 0.6s iii. To calculate the work done by each stimulated rain drop

Work done = Kinetic energy of each drop

K.E = 1/2mv2

m = 0.1g

v = ?

Velocity = distance/time = 1m/0.6s

= 1.67m/s

Work done = 1/2mv2 = 1/2x0.1x1.672

= 0.139445J

Energy = Work done/time (for a single drop)

= 0.139445J/0.6s = 0.2324J/s

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CHAPTER FOUR

RESULT AND DISCUSSION

From this experiment, a large amount of data was generated and for each series of 3

replicates for each soil type experiments.

Volume of drops (cm3), Weight of each aggregate, Number of drops, Height of fall (Meter),

Time taken for drops to break aggregate.

The summary of the whole data obtained is shown in Table 1 for Apomu Series and

Table 2 for Itagunmodi Series, weights of aggregate, replicates and the energy

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Table 4.1 Apomu Series

Weight of Aggregate Replicate Energy

2g 1st 6.97J

4g 9.064J

6g 9.48J

8g 14.22J

2g 2nd 6.693J

4g 8.785J

6g 9.343J

8g 13.94J

2g 3rd

6.972J

4g 9.343J

6g 10.04J

8g 14.36J

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Table 4.2: Itagunmodi Series

Weight of Aggregate Replicate Energy

2g 1st 36.25J

4g 167.3J

6g 315.1J

8g 457.8J

2g 2nd 36.67J

4g 169.0J

6g 325.8J

8g 471.0J

2g 3rd

39.04J

4g 171.9J

6g 310.5J

8g 460.4J

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Table 4.3 Mean square values derived from an analysis of variance of two soils series for energy

level.

SOURCE Df Energy

Average weight 3 52411.66**

Replication/Rep 2 21.18

Soil 1 336433.60***

Error 17 8695.06

Total 23

CV% 72.66

Energy significant at

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The energy of weight is significant at < 0.01**and soil of both series are significant at

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be dispersed by raindrop and carried by runoff water thus resulting to soil erosion, while in that

of 8g which is significantly different which will require higher quantity of raindrop and time

taken for its dispersal.

Table 2, i.e. Itagunmodi series there is a wide range of time required for breakdown of aggregate

and volume, weight and number of drops needed for this. The aggregate size also ranges from 2g

to 8 g in increasing order. Each aggregate require huge volume of raindrop for its dispersal, most

especially in aggregate 4g, 6g and 8g.They are stable to raindrop impact and this may be due to

the chemical and physical binding properties within the aggregates.

Effect of Energy on soil type

The soil type bears dried relationship with the energy. The soil of high bulk density requires

more energy for total disintegration compared to the soil of low bulk density. For instance,

Itagunmodi series requires more energy than Apomu series because it is more compacted and

contained high bulk density.

Effect of Energy on the soil mass

This can be seen through the table that the energy increase as the mass of the soil increases. For

instance, 11.62 J/s Energy is required to breakdown 2g of Apomu series, also, 15.106 J/s Energy

is required to breakdown 4g of Apomu series. This increase as the mass increases. This is to

show that the energy required bears a dried relationship with the soil mass.

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CHAPTER FIVE

SUMMARY AND CONCLUSION

The study was to show the characteristics of raindrop impact that describes soil aggregate

breakdown under rainfall and to see the most important mechanisms involved in this breakdown

process. A single drop of rainfall simulator was constructed to produce the raindrops required to

breakdown any given size of soil aggregate from a predetermined height. Each characteristic of

raindrops were noted. The experiment shows that energy of raindrop impact is the most

important variable in describing time to breakdown the aggregate. The volume, number and

weight of drops involved in impact are important in aggregate breakdown and explained slaking

rather than chemical reduction in bonding forces as mechanism of breakdown. Though, they do

not prelude chemical process from having any effect.

The more the weight of aggregate, the more energy level expected. Whereby weight of

aggregate level eight (8) had the highest mean energy of 238.62J and the least weight of

aggregate(2) had the lowest mean energy level of 22.10J.From the mean values of the two soils

series relative to each other, it is observed that Apomu soil series had lowest mean energy level

of 9.93J compared to a far higher energy level of Itagunmodi soil series of 246.73J and this large

difference in energy level can be due to some factors which are soil type, climate, organic matter

retention ,soil disturbance and so on.The fall height of raindrops is thus an important factor in

aggregate breakdown. If the droplets are first intercepted by a cover crop before falling on the

soil, it will not have much impact on the aggregate like those falling from a greater height

without intercept will make the most impact. The time required in the breaking down of

aggregate of Itagunmodi series is greater than that of Apomu series.

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Tugel, A. J., Herrick, J. E., Brown, J. R., Mausbach, M. J., Puckett,W. and Hipple K. 2005.

Soil Change, Soil Survey,

Varallyay, Gy. (2002):A mezgazdasagi vizgazdalkodas talajtani alapjai. Budapest(egyetemi

jegyzet)

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APPENDIX

The SAS System 21:34 Monday, February 1, 20121

The ANOVA Procedure

Class Level Information

Class Levels Values

Agg_Wt 4 2 4 6 8

Replicates 3 1 2 3

soil 2 Apomu Itagunmodi

Number of observations 24

The SAS System 21:34 Monday, February 1, 20122

The ANOVA Procedure

Dependent Variable: Energy

Sum ofSource DF Squares Mean Square F Value Pr > F

Model 3 336656.0063 112218.6688 7.36 0.0016

Error 20 305085.5720 15254.2786

Corrected Total 23 641741.5782

R-Square Coeff Var Root MSE Energy Mean

0.524597 96.25204 123.5082 128.3175

Source DF Anova SS Mean Square F Value Pr > F

Replicates 2 40.0487 20.0243 0.00 0.9987soil 1 336615.9576 336615.9576 22.07 0.0001

The SAS System 21:34 Monday, February 1, 20123

The ANOVA Procedure

Duncan's Multiple Range Test for Energy

NOTE: This test controls the Type I comparison wise error rate, not the experiment wise errorrate.

Alpha 0.05Error Degrees of Freedom 20Error Mean Square 15254.28

Number of Means 2

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Critical Range 105.2

Means with the same letter are not significantly different.

Duncan Grouping Mean N soil

A 246.75 12 Itagunmodi

B 9.89 12 Apomu

The SAS System 21:34 Monday, February 1, 20124

------------------------------------------- Soil=Apomu ------------------------------------------

The ANOVA Procedure

Class Level Information

Class Levels Values

Agg_Wt 4 2 4 6 8

Replicates 3 1 2 3

Soil 1 Apomu

Number of observations 12

The SAS System 21:34 Monday, February 1, 20125

------------------------------------------- Soil=Apomu ------------------------------------------

The ANOVA Procedure

Dependent Variable: Energy

Sum of

Source DF Squares Mean Square F Value Pr > F

Model 5 85.74135833 17.14827167 360.17 F

Replicates 2 0.79340000 0.39670000 8.33 0.0186

Agg_Wt 3 84.94795833 28.31598611 594.73

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Alpha 0.05Error Degrees of Freedom 6Error Mean Square 0.047611

Number of Means 2 3 4Critical Range .4359 .4518 .4597

Means with the same letter are not significantly different.

Duncan Grouping Mean N Agg_Wt

A 14.1733 3 8

B 9.4367 3 6BB 9.0633 3 4

C 6.8767 3 2

The SAS System 21:34 Monday, February 1, 20127

----------------------------------------- Soil=Itagunmodi ---------------------------------------

The ANOVA Procedure

Class Level Information

Class Levels Values

Agg_Wt 4 2 4 6 8

Replicates 3 1 2 3

soil 1 Itagunmodi

Number of observations 12

The SAS System 21:34 Monday, February 1, 20128

----------------------------------------- Soil=Itagunmodi ---------------------------------------

The ANOVA Procedure

Dependent Variable: Energy

Sum ofSource DF Squares Mean Square F Value Pr > F

Model 5 304897.9913 60979.5983 2583.84 F

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Replicates 2 93.5335 46.7667 1.98 0.2184Agg_Wt 3 304804.4578 101601.4859 4305.08

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Bar chart showing the result of Table 4.4

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Bar chart showing the result of Table 4.5