Soil Water - Agricultural Field Officer (Hindi/English)

Soil Water
In nutrient management, a proper balance between soil water and soil air is critical since both water and air are required by most processes that release nutrients into the soil. Soil water is particularly important in nutrient management.
Soil acts as a sponge to take up and retain water. Movement of water into soil is called infiltration, and the downward movement of water within the soil is called percolation, permeability or hydraulic conductivity.

Types of water in soil:
1. Adhesion water
“HYGROSCOPIC WATER”

Remove by oven drying
Not available to plants

2. Cohesion water
“CAPILLARY WATER”

Remove by air drying
Most is available to plants
some unavailable to plants (especially in clay or high OM soils)
15 – 20 molecules thick

3. Gravitational water

Not available to plants
Drains through soil under influence of gravity
Through large pores
Small pores can hold water against pull of gravity through capillarity

Critical levels of water in soil:
Field capacity
Wilting point
Hygroscopic coefficient

Field Capacity
Amount of water in soil after free drainage has removed gravitational water (2 – 3 days)

Soil is holding maximum amount of water available to plants

Optimal aeration (micropores filled with water; macropores with air)

Wilting Point
Amount of water in soil when plants begin to wilt.

Plant available water is between field capacity and wilting point.

Hygroscopic coefficient
Amount of moisture in air dry soil

Difference between air dry and oven dry amounts

Not all capillary water is equally available to plants
Plants can extract water easily from soils that are near field capacity
Sponge example

Wilting point is not the same for all plants
Sunflowers can extract more water from soil than corn
Sponge example

Forces on soil water:
Adhesion
Attracts water to soil particles
Holds adhesion(hygroscopic) water and cohesion (capillary) water
Called “matric force”
Ions in solution
Attracts water to ions
Called “osmotic force”
Gravity
Pulls water downward
“gravitational force”


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