This document discusses soil-plant-water relationships and irrigation. It covers the objectives of irrigation, which include adding water to supply moisture for plant growth and providing drought protection. It also discusses how soil physical properties like texture, structure, density and pore space influence water retention. Finer textured soils like clay have higher water holding capacity while coarser soils like sand facilitate drainage. The document defines concepts like field capacity, permanent wilting point, and available soil moisture. It describes how soil water is classified into gravitational, capillary and hygroscopic categories based on tension. Different types of water movement in soil like infiltration, percolation and seepage are also summarized.

1. Soil-plant-water relationship
Dr. Niraj Biswakarma
Assistant professor, SOAS
GDGU
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2. Irrigation
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Irrigation is the artificial application of water to the soil for the
purpose of crop growth or crop production in supplement to rainfall
and ground water contribution.
Objectives
1. To add water to soil for supplies the moisture essential for plant
growth.
2. To provide crop insurance against short period of drought.
3. To cool soil and atmosphere, thereby making more favorable
environment for
plant growth.
4. To washout salts from the root zone of plant.
5. To dissolve the plant nutrients present in soil.
6. To soften the tillage pan or facilitate tillage operation.

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Soil Physical Properties Influencing Soil Water
Relations
Soil – A three phase disperse system: solid, liquid and gas
Different soil physical properties are
• Soil depth, Soil texture, Soil structure, Density of soil and Pore spaces

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a) Soil depth
It is the thickness of soil cover over hard rock or hard substratum
below which roots cannot penetrate. A shallow soil - limited
moisture holding capacity, therefore would need frequent
irrigations
b) Soil texture
Relative proportion of mineral particles i.e. sand, silt and clay,
respectively
coarse textured soils (sandy soils) - low water holding capacity and
facilitate rapid drainage and air movement.
fine textured soils (clayey) - relatively high water holding capacity,

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C) Soil structure
The structure of a soil refers to the arrangement of the soil particles
and the adhesion of smaller particles to form large ones or
aggregates.

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D) Density of soil
1) Particle density (PD): ratio of a given mass of soil solids to the volume of
soil solid. It is usually expressed in terms of g/cm3. PD of most mineral soils
vary between - 2.6 to 2.75 g/cm3
2) Bulk density (BD): It refers to the ratio of a given mass of an oven dried soil
to volume of soil (i.e., solids + pore spaces). BD of clay soil is 1.2 and sandy
soil- 1.4 g/cm3
e) Pore space
volumes of pore space to total volume of soil. Total porosity of for arable
soils is between 30 to 60%

7. Water Retention in Soil
Adhesion: Attraction between water molecules and solid
surfaces
Cohesion: Attraction of water molecules for each other.
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8. Soil moisture tension
it is a measure of the tenacity with which water is retained in the soil and
reflects the force per unit area that must be exerted by plants to remove
water from the soil
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Soil moisture characteristic curves
The graphical representation of the relationship between soil moisture tension
and soil water content is known as soil moisture characteristic curve or water
retention curve

9. Classification of soil water
Physical Classification of Water
• Gravitational water: Water that moves freely in response to
gravity and drains out of the soil. Soil moisture tension at
gravitational water is > than - 1/3 bars
• Capillary water: Water held by forces of surface tension and
continuous films around soil particles and in the capillary spaces.
it is held between the tension of - 1/3 and - 31 bars.
• Hygroscopic water: Water held tightly to the surface of soil
particles by adsorption forces and is present in the vapour form.
The tension is about > -31 bars
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Terminology
Infiltration: The process of water entry into the soil
Seepage: The lateral movement of water through soil pores
under unsaturated condition is known as seepage.
Percolation: Percolation is the downward movement of water
through saturated or nearly saturated soil in response to the
force of gravity. it occurs at the tension smaller than about ½
atmosphere.
Permeability: It is the characteristic of a pervious medium
relating to the readiness with which it transmits fluids

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Types of water movement

13. Soil moisture constant
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The water contents expressed under certain standard conditions are
commonly referred to as soil moisture constants.
Maximum water holding capacity: When all the pores of the soil are
filled with water, the soil is said to be under saturation capacity of
maximum water holding capacity.
Field capacity: is “the amount of water held in soil after excess
water has been drained away. The soil moisture tension - 0.10 to -
0.33 bars. it is the upper limit of available soil moisture
Permanent wilting point: the soil moisture content at which plants
can no longer obtain enough moisture to meet transpiration
requirements; and remain wilted unless water is added to the soil.
The soil moisture tension – 15 bars. It is the lower limit of available
soil moisture

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Hygroscopic coefficient: is the percentage of water
remaining in an air-dry soil. At this point, water in the soil is
adsorbed in vapour form and soil moisture tension is about –
31 bars
Available soil moisture: The amount of soil moisture held
between the field capacity (- 0.33 bars) and permanent
wilting point (-15 bars) as available soil moisture

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Any Doubts ?????
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