SOIL-WATER PLANT RELATIONSHIP

Both soil and water are essential for plant growth. The soil provides a structural base for the plants and allows the root system (The foundation of the plant) to spread and get a stronghold. The pores of the soil within the 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 useful nutrients dissolved in water, essential for the growth of the plants.

The roots of most plants also require oxygen for respiration. Hence, complete saturation of the soil pores leads to restricted root growth for these plants. (There are exceptions, though like the rice plant, in which the supply of oxygen to the roots is made from the leaves through aerenchyma cells which are continuous from the leaves to the roots.

Since irrigation practice is essential, an adequate and timely supply of water to the plant's root zone for optimum crop yield, the study of the interrelationship between soil pores, its water–holding capacity, and plant water absorption rate is fundamentally essential.

Soil moisture constants

For a particular soil, certain soil water proportions are defined which dictate whether the water is available or not for plant growth. These are called the soil water constants, which are described below:

1)      Saturation capacity

2)      Filed capacity

3)      Permanent wilting point

a)   Temporary wilting point

b)   Ultimate wilting point

Computation of Crop Water Requirement

A plot of land growing a crop has to be applied with water from time to time for its healthy growth. The water may come naturally from rainfall or may be supplemented by artificially applying water through irrigation. A crop should be irrigated before it receives a setback in its growth and development. Hence the interval between two irrigations depends primarily on the rate of soil moisture depletion.

Normally, a crop has to be irrigated before soil moisture is depleted below a certain portion of its availability in the root zone depending on the type of plant.

When the water supply is very limited, then the interval may be prolonged which means that the soil moisture is allowed to deplete below 50 percent of available moisture before the next irrigation is applied.

The optimum rates of soil moisture for a few typical crops are given below:

Maize: field capacity to 60% of availability

Wheat: field capacity to 50% of availability

Sugarcane: Field capacity to 50% of availability

Barley: Field capacity to 40% of availability

Cotton: Field capacity to 20% of availability

As for rice, the water requirement is slightly different than the rest. This is because it requires a constant standing depth of water of about 5cm throughout its growing period.
This means that there is a constant percolation of water during this time and it has been estimated that about 50 to 70 percent of water applied to the crop is lost in this way.
For most of the crops, except rice, the amount of water applied after each interval should be such that the moisture content of the soil is raised to its field capacity. The soil moisture depletes gradually due to the water lost through evaporation from the soil surface and due to the absorption of water from the plant roots called transpiration. The combined effect of evaporation and transpiration, called Evapotranspiration (ET) decides the soil water depletion rate for a known value of ET (which depends on various factors, mainly climate.

Some of the operational soil moisture ranges of some common crops are given below:
1) Rice: 

For low-land rice, the soil saturated or up submergence of about 50mm. throughout the growing period. When water resources are limited, the land must be submerged at least during critical stages of growth. The major portion of the water applied to the rice crop, about 50-75% is lost through deep percolation which varies with the texture of the soil.
Since the soil is kept constantly submerged for rice growth; all the pores are completely filled with water and it is in a state of continuous downward movement. The total water required by the rice plant is about 1.0 to 1.5m for heavy soils and soils with high water 2.0m for medium soils and 2.0 to 2.5 for light soils with deep water table.

2) Wheat: 

The optimum soil moisture range for tall wheat is from the field capacity to 50% of availability. The optimum moisture range is from 100 to 60 percent availability. The active root zone of the crops varies from 0.5 to 0.75m depending upon the soil type.

3) Barley: 

The active root zone of Barley extends between 0.6m. to 0.75m on different soil types. The optimum soil moisture ranges from the field capacity to 40% of availability.
4) Maize: The optimum soil moisture range is from 100 to 60% of availability in the maximum root zone depth which extends from 0.4 to 0.6 on different soil types. The actual irrigation requirement of the crop varies with the amount of rainfall. In north India, 0.1m and 0.15m are required to establish the crop before the onset of monsoon. In the south, it is found that normal rainfall is sufficient to grow the crop in the monsoon season whereas 0.3m of water is required during water.

5) Cotton:

 The optimum range of soil moisture for cotton crops is from the field capacity to 20% of available water. The root zone varies up to about 0.75m. The Total water requirement is about 0.4 to 0.5 m.

6) Sugarcane :

 The optimum soil moisture for sugarcane is about 100 to 50 % percent of water availability in the maximum root zone, which extends to about 0.5 to 0.75 m in depth.