S S R A N AS R S C I E N T I S T

SOIL IMPROVEMENT AND AMENDMENTS: SOIL

COMPONENTS

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Soil is foundation to every field/garden. Every healthy,productive field, yard and garden starts with healthy,productive soil. Preparing the soil properly makesmore difference than any other thing you can do. Youcannot put on enough nutrients and water to make upfor poor soil. Since soils are so different in differentareas, it is necessary to know what soil is, what yoursoil is like and what to add to improve it.

NPOP standards for soil and water conservation

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3.2.7 Soil and Water ConservationGeneral PrinciplesSoil and water resources should be handled in a sustainable manner.RecommendationsRelevant measures should be taken to prevent erosion, salination of soil, excessive andimproper use of water and the pollution of ground and surface water.Standards3.2.7.1. Clearing of land through the means of burning organic matter, e.g. slash-and burn,straw burning shall be restricted to the minimum.3.2.7.2 The clearing of primary forest is prohibited.3.2.7.3. Relevant measures shall be taken to prevent erosion.3.2.7.4. Excessive exploitation and depletion of water resources shall not be allowed.3.2.7.5. The certification programme shall require appropriate stocking rates which do notlead to land degradation and pollution of ground and surface water.3.2.7.6. Relevant measures shall be taken to prevent salination of soil and water.

SOIL COMPONENTS

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� Soil is made up of four main components: mineral, water, air and organic.

mineral Solidorganicwater Liquidair Gas

Soil-the three phase system

� Solid phase – minerals and organic matter

� Liquid phase - moisture� Gaseous phase - soil air

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Physical properties

� Soil texture� Soil structure

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Soil texture

Soil separate Particle diameter (mm)Very coarse sand 2.0-1.0Coarse sand 1.0-0.5Medium sand 0.5-0.25Fine sand 0.25-0.1Very fine sand 0.1-0.05Silt 0.05-0.002Clay <0.002

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General terms for basic soil textural classesGeneral terms Class

Sandy soils Coarse textured soils Sands

Loamy sands

Loamy soils Moderately coarse textured soils Sandy loam

Fine sandy loam

Medium textured soils Very fine sandy loam

Loam

Silt loam

Silt

Moderately fine textured soils Clay loam

Sandy clay loam

Silty clay loam

Clayey soils Fine textured soils Sandy clay

Silty clay

Clay

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� Sand has large spaces between the particles, which allow airand water to move easily, so sand has good aeration anddrainage. Clay packs down with only tiny spaces betweenparticles so there is poor aeration and drainage. However, clayhas about 100 times the surface area as the same volume ofsand. More surface area means that clay will hold more waterand more nutrients. Silt has some of the qualities of both sandand clay. Loam combines the best features of all three:aeration, drainage and storage capacity for water andnutrients. Often, soil particles are clumped together intocrumbs, which create large spaces between the crumbs foraeration and drainage. Pebbles and rocks also increasedrainage; sometimes so much that plants wilt from lack ofwater. Rocks also may interfere with root growth.

Triangle to find out textural class of soil10

Soil improvement

Soil textural classification chart

The dotted lines indicate the method of determining the textural class of a soil containing 18% clay, 34% silt and 46% sand. The three lines join at point A which lies within the class loam.

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Soil improvement

Soil structure

� Influences the rate at which water and air enter andmove through the soil.� It affects root penetration and the soil’s nutrients

supply.� In many soils kinds of structure differs in different

horizons.� Single grained (loose sands) and massive soils are

structureless.� In single grained soils water percolates very rapidly.

Water moves very slowly through massive soils.

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Types of soil structure and their effect on downward movement of water

The more favourable water relations are usually in soils that have prismatic, blocky and granular structure. Platy structure impedes the downward movement of water.

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Soil water

� Kinds of water in the soil� Movement of water in the soil� How water is held in the soil� Available water

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Kinds of water in the soil

� Gravitational water or free water: The water in thelarge pores that moves freely under the influence ofgravity.

� Capillary water: Available form of water that heldagainst gravity. Held in fluid state. It moves more slowlythan free water. It moves in any direction but always indirection of greatest tension.

� Hygroscopic water: It is in non liquid state and isimmobile in nature. Non-available water to the plants.

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Soil improvement

Movement of water in the soil

� Gravity lets water move downward. It depends on the size and continuity of the pore spaces. Percolation is retarded by ploughpan or claypan (next slide).

� Adhesive and cohesive forces lets water moves in small pores by capillarity. Affected by soil texture. Movement is from thick films to thin films i.e. low tension to high tension.

� Heat vapourizes and diffuse through the air. As water is evaporated from the surface, it is replaced by rise in the capillary water. This continues until few inches of surface soil become dry and capillary is broken. Water then leaves the soil only by vaporising.

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Examples of water penetration from an irrigation furrow

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How water is held in the soil

� Adhesive forces� Cohesive forces

� Not much work or energy is required to remove water from a soil near saturation.� As more and more water is removed, more and more

energy is required.� The tenacity with which water is retained in the soil

and the force that must be exerted to remove water from the soil is referred to as soil moisture tension.

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Soil improvement

Moisture characteristic curve

In clay the moisture is released in fairly even increments as tension increases. In sand there is proportionately greater release of moisture at low tension than the curve for clay. The curve for loam is intermediate in shape between the curve for the clay and the sand.

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Typical moisture characteristic curves of clay, loam and sandy soils

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Available water

� Total amount of water between field capacity and permanent wilting point.� Soil moisture tension, osmotic pressure and

temperature are important determinants of available water.� Low soil temperature decreases availability.

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Relation between soil moisture content and soil moisture tension in a sandy loam and silt loam

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Kinds of water in the soil

Kinds of water in the soil and difference in available moisture content between a sandy loam and a silt loam. The silt loam contains more than twice as much readily available water.

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Water intake

� The movement of irrigation water from the surface into and through the soil is called water intake. It is the expression of several factors.

� Infiltration� Percolation� Factors affecting intake rate� Variation in intake rate by irrigation method

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Infiltration and percolation

� The downward flow of water from the surface intothe soil is known as infiltration.� The movement of water through the soil profile is

known as percolation.� Permeability is the quality of the soil that enable it to

transmit air and water.� Hydraulic conductivity is the coefficient ‘k’ in Darcy’s

Law (v = ki) in which ‘v’ is the effective flow velocityand ‘i’ is the hydraulic gradient. It depends on theproperties of the fluid as well as on those of the soil.

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Factors Affecting Intake Rate

� Surface sealing� Soil compaction� Soil cracking� Tillage� Crop rotation

� Soil and water salts� Sediments in irrigation

water� Soil erosion� Land laveling� Temperature

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Variation in intake rate by irrigation method

� The water intake rate varies with the methods of water application.

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Soil tilth

� Good� Fair� Poor

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AIR

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� Air in the soil is made of the same gases as the air in the atmosphere.

� Air doesn’t move as freely in the soil so there may be too much or too little of certain gases in parts of the soil.

� Plant roots need oxygen to absorb water and nutrients. The lack of oxygen limits how deep roots can grow.

� Roots may grow thirty feet deep in well-drained sandy soil, but most roots are in the top foot of clay soil.

� Oxygen is replenished in the soil when water forces the air out of the soil, then fresh air is pulled back into the soil as the water drains away.

� Large spaces between soil particles and crumbs allow soil to breathe better.

� Deep watering helps the soil breathe much better than frequent, shallow watering.

ORGANIC

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� The organic component of the soil is made up of living and dead plants and animals.Living: Bacteria, Fungi, Insects, Worms, and Roots.Dead: All of the above as they decay.Humus: Dead organic matter that has decomposed until it is very fine, black and sticky.

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� Bacteria and fungi extract nutrients from the soil minerals and make them available to plants. Mycorhizaeare fungi that are partly in the soil and partly inside plant roots. They can transport a very large amount of nutrients into plant roots. Insects and worms create air passages deep into the soil. The carbon dioxide produced by roots becomes carbonic acid, which breaks down minerals to make nutrients available. Dead organic material provides rich nutrients for the living. It also holds the nutrients from applied fertilizers until the plants can use them. Humus sticks the soil particles into larger crumbs so there are bigger spaces for air and water.

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� Dead organic matter is decaying continually, so it needs to be replenished every year. Excessive nitrogen fertilizer will cause the dead organic matter to decay even faster; so more organic matter will have to be applied. Also, careless use of pesticides may harm or kill the living organisms and damage the soil.

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