Soil Moisture Constant


Soil Moisture Constant

Soil Moisture Constant:

Water contents under certain standard conditions are referred as soil moisture constants.

Under field conditions, water content of soil is always changing constantly with time and depth of soil and is not static or constant. However, the concept of soil moisture constants greatly facilitates in taking decision in irrigation.

Important Soil Moisture Constant:

While studying soil water and discussing its availability or other wise to plant, some specific terms called as soil moisture constants are used. A brief explanation of some important and commonly used terms is given below and the methods of expressing them are indicated in the table below.


of soil

Type of Soil

Soil Moisture Constant

Moisture Tension

in Atmosphere

Wet soil

Gravitational water

Maximum water


Moist soil

Available water

Field capacity

0.33 (1/3)


Water held in micro pores

Wilting point


Dry soil

Unavailable water tightly held

Hygroscopic coefficient




Air dry




Oven dry


Important soil moisture constants:

1. Oven dry weight: This is the basis for all soil moisture calculations. The soil is heated in an oven at 105 degree Celsius until it looses no more water and final weight is recorded as oven dry weight. Equivalent moisture tension at this stage is 10,000 atmospheres.

2. Air-dry weight:  Unlike oven dry weight, this is a variable constant. Soil exposed in humid atmosphere will have a higher weight than the same soil, if put in dry atmosphere. Under average conditions, moisture at air dryness is held with a force of about 1000 atmosphere.

3. Hygroscopic coefficient: It is the maximum quantity of water absorbed by any soil in a saturated atmosphere (i.e. at 99 percent relative humidity) at 25 degree Celsius temperature. The hygroscopic coefficient varies with the type of soil, its texture and organic matter content. This constant is equal to a force of about 31 atmospheres and determined by placing the soil in a saturated atmosphere at 25oC temperature. Water held by the soil at this constant is not available to plants because it is mostly in vapour form but it is useful to certain bacteria.

4. Permanent Witling Point (PWP): The wilting point is also known as a wilting coefficient or permanent wilting point or permanent wilting percentage.
After using the water from outer capillary portion, the plant roots begin to utilize although with difficultly the inner capillary water. Thus, as the moisture film becomes thinner, it is held more and more tightly and it is difficult for plant roots to remove each successive portion of the water film. But later on, a stage is reached at which plants cannot obtain enough water to meet transpiration requirement and remain wilted even under saturated atmosphere, unless water is added to soil. The soil moisture constant at this stage (wilting is called as wilting co-efficient or permanent wilting percentage. Water at this constant is with force of a 15 atmosphere. The wilting co-efficient differs in different soils. It is as low as 4 to 6 percentage in sandy soils and as high as about 16 to 20 percent in clayey soils which are rich in organic matter. The wilting point is a lower limit of available soil moisture.

5. Field Capacity (F.C.): Field capacity is the moisture content in percentage of a soil on oven dry basis, when it has been completely saturated and down ward movement of has practically ceased.

With 2 to 3 days after a heavy rains or irrigation, the gravitational or free water is drained. The moisture content at this stage in the soil is said to be at field capacity. The field capacity is the upper limit of available soil moisture range in the soil moisture and plant relations. The moisture tension at this stage is about 1/3 atmosphere. The fine textured granular soil with high organic matter content more soil moisture than sandy soil at field capacity.

6. Moisture equivalent: According to the modified technique, moisture equivalents is the amount of moisture in percentage on oven dry weight basis held by 30 grams of dry soil when subjected to 1000 times the gravitational force in a centrifuge for 30 minutes.

For practical purpose, field capacity may be considered as equal to the moisture equivalent. The value (moisture content may be considered as equal to the moisture equivalent are nearly equal in loamy soil but for sandy soils, the moisture equivalent is slightly higher than filed capacity.

7. Maximum capillary capacity: When water is added to the soil whose field capacity is already reached, that water goes on thickening the moisture film. A stage is then reached after which any further additional of water will get percolated down by the force of gravity. This is the point of maximum capillary capacity.

8Maximum water holding capacity: Any further addition of water to the soil after its maximum capillary capacity is reached will start moving down by force of gravity, if it is a well drained soil but when drainage is restricted, maximum amount of water can be held until all micro and macro pores are filled with water. This stage is called the maximum water holding capacity. It is only in case of poorly drained soils or soils having hard pan near the surface that maximum water is retained in the soil for a long period.

The values of different soil moisture constant (moisture percent) differ according to soil type. The values for these moisture constant for some the soils are given below.     

Table: Moisture constants for few typical Indian soils (in percent of oven dry soil)

Soil type

Air dry









water holding capacity

Heavy black






Medium black
























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