Estimation Of Soil Moisture Constants

Soil Moisture

1. Water holding capacity – Water holding capacity is estimated with Keen and Razowaski cup. The soil is paced in this cup after fitting a filter paper at the bottom. Soil is soaked by capillary action. Weight is taken immediately after wiping the water on the sides of the cup and moisture is computed on oven dry basis.

2. Field capacity – Field capacity is estimated directly in the field by ponding water in the plot covered all round by a bund. The test area may be 2 m2. After a copious rain or heavy irrigation, estimation may be taken up. Soil is allowed to drain the excess water. Surface is covered to prevent evaporation. This may be accomplished by spreading a polythene sheet or thick straw mulch on the ground surface. Soil sampling is to be done at 24, 36, 48 and 72 hours. Soil moisture content is estimated by gravimetric method after drying in an oven at 105°C for 6–9 hours till concordant weights are obtained.

Moisture curves are to be drawn to locate the relatively stable values against time. For all soils except heavy clay soil, the sampling time may be 48 hours from irrigation. For experimental fields, field capacity may be estimated for few layers (0–15, 16–30, 30–45 and 45–60 cm) depending upon the rooting depth and information to be generated. Field capacity is also estimated with pressure plate apparatus by maintaining 1/3 atmosphere in disturbed soil sample.

3. Moisture equivalent – Moisture equivalent is estimated in the disturbed and air- dried soil sample. The soil is passed through 2.0 mm sieve. A porcelain buchner funnel of 5 × 2 cm is taken. A filter paper is slightly wet to enable to stick on the bottom. Air-dry soil is added to the funnel with gentle tapping against a smooth surface to ensure uniform packing. Soil is added to the full capacity of the buckner funnel and cut of the surface with the spatula.

Soil sample in the funnel is left into water to enable the water to move by capillary action through the stem of the funnel. Soil in the funnel is left for 24 hours to be in equilibrium with water through capillary movement. After 24 hours, the funnel is removed from water column and fitted to a filter flask. The filter flask is connected to a vacuum pump (550 rpm) and subjected to suction for 15 minutes. During the process of suction, the soil is put into an aluminum cup without filter paper and moisture content is estimated by oven dry method.

4. Wilting point – Estimation of wilting point moisture involves growing of sunflower as indicator plant in tin can. The tin can is closed with lid and the plant is allowed to grow through an opening in the lid. The plant is watered to grow for three to four weeks till three to four leaves develop. The plant is watered last and the space in the lid around the stem of the plant is plugged with cotton to control evaporation. The plant in the tin is allowed to wilt gradually. When the plant shows signs of loss of turgor, the can with plant is transferred to a dark humid cabinet to create high humidity.

To reduce transpiration the humid cabinet is covered with a black polythene sheet. Inner sides of the cabinet are lined with gunny to retain moisture. The plant is allowed to extract moisture form the soil. If the plant is gaining turgidity, it is exposed to atmosphere for two hours and then transferred to humid cabinet. This process is repeated till the plant does not recover in the humid cabinet. At the stage the moisture content of the soil in the can is estimated to find out the wilting point of the soil.

5. Use of pressure plate apparatus for estimating soil moisture constants – Soil moisture content values may be obtained by the use of pressure plate apparatus or pressure membrane apparatus. The soil in test is placed in this layer in the cups of the pressure membrane apparatus. Desired pressure is applied to obtain the required soil moisture constant (FC 1/3 atm) and the moisture content of the soil sample is estimated.

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