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Unit 6. SOIL-WATER-CROP RELATIONSHIP. INTRODUCTION. Both soil and water are essential for plant growth. The soil provides a structural base to the plants and allows the root system (the foundation of the plant) to spread and get a strong hold.

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unit 6

Unit 6


  • Both soil and water are essential for plant growth.
  • The soil provides a structural base to the plants and allows the root system (the foundation of the plant) to spread and get a strong hold.
  • Root zone hold moisture which clings to the soil particles by surface tension in the driest state or may fill up the pores partially or fully saturating with it useful nutrients dissolved in water, essential for the growth of the plants.
  • For optimum crop yield, the study of the inter relation ship between soil pores, its water-holding capacity and plant water absorption rate is fundamentally important.
soil profile
Soil Profile
  • Various layers different in colour and composition: Horizon
  • The succession of horizons is called profile of the soil.

The three zones, beginning from the surface downward are:

1.Zone A

2.Zone B

3.Zone C

Zone A :
  • Zone of leaching and decomposition
  • Minerals & organic matter are decomposed & converted into colloids.
  • Extending from a few centimeters to 60cm below
  • Grass roots are embedded
  • Maximum biological activity
  • Plough layer is generally 20-30cm thick
Zone B :
  • Lies immediately below zone A
  • Zone of deposition
  • It extend up to 100cm thick
  • It varies from zone A in colour ,structure, clay content and organic matter (low)
  • Main size is rather coarse

Zone C :

  • Layer of unconsolidated material(decomposed rock or freshly deposited alluvium.
  • Coarse grained, pebbly texture
  • Roots penetrate into this zone
  • Extends up to 2m in thickness
  • Soil is a heterogeneous mass consisting of a three phase system of solid, liquid and gas.
  • The void space within the solid particles is called the soil pore space.
  • Decayed organic matter derived from the plant and animal remains are dispersed within the pore space.
  • The soil air is totally expelled from soil when water is present in excess amount than can be stored.
Soil properties
  • Colour
  • Soil texture
  • Soil structure
  • Porosity
  • Permeability
  • pH value of soil
  • Depth of soil
  • Infiltration capacity
  • Important characteristics for soil identification
  • Depends on composition, drainage condition, porosity and age
  • Concentration of organic matter imparts grey, black, dark brown colour to the soil
  • Iron oxide : red & yellow
  • Manganese oxide & hydrated iron oxide: red colour
Soil texture:
  • This refers to the relative sizes of soil particles in a given soil.
  • According to their sizes, soil particles are grouped into gravel, sand, silt and clay.
  • The relative proportions of sand, silt and clay is a soil mass determines the soil texture.
According to textural gradations a soil may be broadly classified as:
  • Open or light textural soils: these are mainly coarse or sandy with low content of silt and clay.
  • Medium textured soils: these contain sand, silt and clay in sizeable proportions, like loamy soil.
  • Tight or heavy textured soils: these contain high proportion of clay.
Soil structure:
  • This refers to the arrangement of soil particles and aggregates with respect to each other.
  • Aggregates are groups of individual soil particles adhering together.
  • Soil structure influences aeration, permeability, water holding capacity, etc. of a soil.
The classification of soil structure is done according to three indicators as:-
  • Type: there are four types of primary structures-platy, prism-like, block like and spheroidal.

• Class: there are five recognized classes in each of the primary types. These are very fine, fine, medium, coarse and very coarse.

• Grade:Grades are termed as structure less, weak, moderate, strong and very strong depending on the stability of the aggregates when disturbed.


Ratio of volume of voids to total volume of soil mass.

It varies with:

  • Structure
  • Size ,shape & gradation of soil particles
  • Presence of organic matter.


  • Important quality which enables the soil to transmit water or air
  • It is defined as velocity of flow under a unit hydraulic gradient.
  • Sandy soils have large pore spaces ,high permeability
  • Clayey soils ,low permeability
pH value of soils:
  • It’s a measure of the intensity of acidity or alkalinity of a soil.
  • Value ranges from 0 to 14, 7 is neutral in the sense of chemical reaction.
  • Below 7 , soil is acidic.
  • Above 7 , soil is alkaline.
  • Irrigation soils have pH ranging between 6 and 8.5.

Depth of soil:

  • Sufficient depth of soil for the storage of irrigation water & penetration of roots is essential.
  • Shallow soils require frequent irrigation
  • Deep soils of medium texture & granular structure provide

(i) Adequate storage of water (ii) root penetration & sustain relatively long periods between irrigation.

infiltration capacity
Infiltration capacity
  • Infiltration capacity of a soil is the maximum rate at which soil is capable of absorbing water in a given condition.
  • It is the downward flow of water from the surface into the soil.
  • It is expressed as cm per hour over an area.
  • Larger the voids grater is the infiltration capacity
  • Vegetated soils have more infiltration capacity
Soil Classification:

Alluvial soils:

These soils are formed by successive deposition of silt transported by rivers during floods, in the flood plains and along the coastal belts.

  • They are found in the Indo- Gangetic plains.
  • Alluvial soils area very fertile.
  • They absorb fair percentage of rainfall and act as a good source of ground water reservoir.
  • These soils are formed by the weathering of igneous and metamorphic rock comprising gneisses and schist’s.
  • Red soils are light textured ,porous and friable.
  • They have varying fertility & have low content of soluble salts.


  • Residual soil
  • Porous and well drained,
  • but lack common nutrients
  • Poor in crop production
Black Soil
  • Texture varies from clay to loam.
  • They develop cracks on account of drying by heat or sun.
  • Quite fertile if adequate rainfall is available.

Clayey Soil:

  • Potentially rich soils but lacks drainage
Sandy Soil
  • These are found in deserts of some parts of Punjab and greater part of Rajasthan.
  • High pH value & low organic content
  • Free drainage
functions of irrigation soil
Functions of irrigation soil
  • Adequate moisture holding capacity to meet water requirements of crops.
  • Provide proper circulation of air to a suitable depth for the development of root system of plants.
  • Free from harmful concentration of soluble salts and parasites
  • Promote growth of bacteria which make the soil richer in organic matter for crop growth.
  • Supply nutrients for crop yield.
  • Resistant to soil erosion or soil depletion under the cropping system
  • Suitable for agricultural implements
  • Suitable to providing anchor for plant roots.
maintaining soil fertility
Maintaining soil fertility:
  • Rest to the soil
  • Additions of manures & fertilizers
  • Crop rotation:

Changing the crops to be grown in the land every year for the 3 reasons,

  • To avoid use of same nutrient.
  • Keeping the soil free from certain soil diseases.
  • Different crops have different depths of root zone.

Usual crop rotation:

i) Wheat – great millet gram, ii)Rice-gram iii)Cotton-wheat-gram iv) cotton-wheat-sugarcane v)cotton-great millet-gram

soil water plant relationship
Soil-Water-Plant relationship

Soil Factors:

a) Infiltration


c) Drainability : drain off excess water

d) Leachability :removal of soluble salts by passing good quality of water.

Plant Factors:
  • Rooting characteristics
  • Evapotranspiration
  • Effect of soil water level on crop growth and yield
Water Factors:
  • When to irrigate
  • How much water to apply
  • Water application method
soil water
Soil Water

It is defined as suspended water in the uppermost belt of soil of zone of aeration lying near enough to surface to be discharged into atmosphere by transpiration of plants or by evaporation from the soil.

It includes :

  • Gravitational water
  • Capillary water
  • Hygroscopic water
Gravitational water:
  • The volume of water that could easily drain off is termed as the gravitational water. This water is not available for plants use as it drains off rapidly from the root zone.
  • Capillary water:
  • The water content retained in the soil after the gravitational water has drained off from the soil is known as the capillary water.
  • This water is held in the soil by surface tension.
  • Plant roots gradually absorb the capillary water and thus constitute the principle source of water for plant growth.
Hygroscopic water:
  • The water that an oven dry sample of soil absorbs when exposed to moist air is termed as hygroscopic water.
  • It is held as a very thin film over the surface of the soil particles and is under tremendous negative (gauge) pressure.
  • This water is not available to plants.
  • Water is also classified as :
  • Unavailable water
  • Available water
  • Superfluous water
Un available Water:
  • It refers to hygroscopic water which is not available to crops due to its inability to move by gravity or capillary forces.
  • Soil moisture is held so firmly by molecular forces that it cannot ordinarily be absorbed by plant roots

Available Water:

  • It refers to the capillary water which readily contributes to the plant roots.
  • Plants wilt if the capillary water is used up
  • Forms the line of demarcation between the capillary water and hygroscopic water

Superfluous Water:

  • It refers to the gravitational water which drains down so deeper that plant roots cannot draw it
soil moisture irrigation relationship
Soil-moisture-Irrigation Relationship

Saturation Capacity:

  • The soil whose all soil pores are filled with water and there is no air left in it is termed as saturated soil.
  • Saturation capacity ,termed as saturation point or maximum moisture holding capacity.
  • It is the amount of water required to fill all the pore spaces between the soil particles by replacing the entire air held in pore spaces.
  • Expressesed as cm of water per meter of soil
field capacity
Field Capacity
  • Defined as the amount of water held in the soil after excess gravitational water has been drained.
  • After the irrigation is stopped, part of water held in the larger pores moves downward called drainage or percolation. As the drainage stops the large soil pores are filled with both air and water whereas smaller zones are still full of water. The soil, at this stage, is said to be at field capacity.
  • The water and air contents of the soil at field capacity are highly conductive for crop growth.
Soil moisture content:

The amount of water present in a soil is termed as soil moisture content, expressed as mm of water depth present in 1 m depth of soil. It is not constant but varies with time.

Permanent wilting point.

  • When the water is rendered insufficient to meet the requirement of the plant, its green leaves turn yellow and the plant wilts and finally dies out.
  • The soil water content at the stage when the plant dries is called permanent wilting point.
  • Alternatively moisture content in the soil at which permanent wilting of plants occurs is termed as permanent wilting point.
wilting coefficient.
  • In any given soil, all forms of vegetation wilt when the moisture content is reduced to a certain percentage known as wilting coefficient percentage or wilting coefficient.
  • It is defined as the moisture content of soil expressed at a percentage of the dry weight, at the time when the leaves of a plant growing in the soil first undergo a permanent reduction in their moisture content as a result of the deficiency in the soil moisture supply.
  • Permanent wilting percentage of soils varies greatly for different soils. It may vary from 3 to 13 per cent for a coarse sandy soil to fine clay respectively, corresponding to field capacity range o 40 per cent.
Root Zone:
  • The soil root zone is the area of the soil around the plant that comes in contact with the plant root.