Water Requirements of Crops - Soil Water and Soil Water Constants

Crop Period and Base Period

Water requirements of Crops

Each crop has a different crop period and base period. The time interval from the soil preparation to the harvesting of the crop is called a crop period. The time interval from the sprouting to the final watering of the crop is called a base period.

Saturation Capacity and Field Capacity of Soil

When water reaches the soil, a state when complete soil voids are filled with water, the soil is said to be in complete saturation. The moisture content of the soil at this state is called the saturation capacity of the soil. This is called as maximum water holding capacity of the soil.
At this state the soil moisture tension = 0.

Gravity Water

The water present in the soil won't stay in the soil as such. Some water moves down due to the action of gravitational force. This water is called Gravity water.

The moisture content present in the soil after the movement of gravity water gives the field capacity of the soil.

Capillary water and Hygroscopic Water

Basically, the field capacity of soil has two parts:
  1. Capillary Water
  2. Hygroscopic Water
The capillary water is the water content present around the soil particles by surface tension in the capillary pores. These water molecules are easily utilized by the plants for their growth. This is the only water that is available for plant growth. This is the water that is available after the movement of gravity water in the soil.

The hygroscopic water is the water molecules that are adhered around the soil particles by some sort of chemical bond and cannot be utilized for plant growth.

Note: The Upper Limit of capillary water is the field capacity

Permanent Wilting Point

The permanent wilting point is the water content at which the plants started to wilt. At this point, the plants can no more consume water for their growth. If we artificially supply water, the plants will survive. The permanent wilting point is 2 % for sandy soil and 30% clayey soil
The permanent wilting point has two parts:
  1. Ultimate Wilting Point
  2. Temporary Wilting Point
Ultimate wilting point is the soil moisture content at which the plants are completely wilted. The plants won't survive even if we apply artificial water.

Available water and Readily Available Water

The difference between water content at field capacity (FC) and Permanent wilting point (PWP) gives the available water for crops.

Readily available water is 75% of the available water. 

All the definitions mentioned above are explained in figure-1 and figure - 2 below.

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