How to Measure Moisture Content of Soil ? (2 Ways)

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Way # 1. Measuring Soil Moisture Content in Laboratory:

1. Gravimetric method:

This consists of obtaining a moist sample, drying it in an oven at 105-C unit it loses no more water, and then determining the percentage of moisture as follows:

Thus, if 100 grams of moist soil (soil and water) when dried loses 20 grams of water, the 80 grams of dry matter are used as a basis for the percentage calculation.

Therefore, 20/80 x 100 = 25%

The gravimetric method is time consuming and involves, laborious processes of sampling, weighing and drying in laboratory.

2. Electrical Conductivity methods:

This method is based upon the changes in electrical conductivity with changes in soil moisture. A gypsum block inside of which are two electrodes at a definite distance apart are used in this method. These blocks require previous calibration for uniformity. The blocks are buried in the soil at desired depths and the conductivity across the electrodes measured with a modified Wheatstone Bridge.

With proper calibrations, the percentage of moisture from the field capacity to wilting percentage can be readily determined. These electrical measurements are affected by the salt concentration in the soil solution and are not very helpful in soils with high salt contents.

Way # 2. Measuring Soil Moisture Potential in Situ (Field):

Suction methods or Equilibrium Tension methods. Field tensiometers measure the tension with which water is held in soils. They are used successfully in determining the need for irrigation. In tensiometer, a porous cup is attached to a glass tube which is connected to a mercury manometer. The tube and cup are filled with water and cup inserted in the soil (Fig 4.6).

The water flows through the porous cup into the soil until equilibrium is established. These tension readings in the manometer, expressed in terms of cm or atmosphere, measures the tension or suction of the soil. If the soil is dried, water moves through the porous cup, setting up a negative tension (or greater is the suction).

The tensiometers are more useful for measuring moisture in sandy soils that in fine textured soils. Once air gets entrapped into the tensiometer, the reliability of readings is questionable.