How to Measure Soil Moisture Content? | Soil Science

In this article we will discuss about how to measure soil moisture content.

Direct Measurement of Soil Moisture Content in the Soil:

(a) Gravimetric Method:

Soil is dried at 105-110 °C and before and after the drying, weight of the soil is taken. And from the differ­ence, moisture percentage is calculated.

(b) Infra-Red Moisture Balance Method:

This method is used either for individual tests or series of test, in practically all organic and inorganic materials directly. Infrared lamp is used for drying with moving air.

Indirect Method:

(a) Look at the soil and feel it.

(b) Neutron Scattering Method:

Rapid tests, in situ measurement of soil moisture, direct measurement of moisture content by volume.

Principle:

Measurement of the no. of H⁺ nuclei those are present in a unit volume of soil.

(c) Tensiometer:

Two types – (i) Tensiometer with mercury mano­meter, used for research, (ii) With vacuum gauge for practical field use. Tensiometer (or Irrometer) measures Soil moisture tension directly. It is used in coarse soil and the practical limit is about 0.8 bars. Hence it cannot be useful under all soil moisture conditions.

Tensiometer consists of porous ceramic cup which may be chocked in clay soils and this cup is sensitive to salts to a certain extent. Therefore it is not suitable for clay soils & salty soils. Soil moisture tension in a salt free soil at field capacity ranges from less than 0.1 to 0.7 atm depending on soil texture.

(d) Electrical Resistance Method:

Osmotic pressure is determined by using vapour pressure psychrometer. Resistance blocks are made up of various materials like gypsum, nylon, fibre glass, plaster of Paris or a combination of these. Nylon and fibre glass units are more sensitive in the higher ranges of soil moisture.

Plaster of paris blocks function most effectively between 1 to 15 atm, and Nylon block upto 2 atm and fibre glass units over entire range of available water. A combination of fibre glass + plaster of paris provides sen­sitivity in both wet and dry range and provides good contact be­tween soil and the unit.

Gypsum block or plaster of Paris resistance units first prepared by Bouyoucos and Mick (1940) hence elec­trical resistance instrument is also called Bouyoucos moisture meter. It is especially designed on Wheat-stone bridge principle.

Resistance blocks like tensiometer can be used to schedule irrigation, to irrigate greenhouse beds, to determine rooting zone and soil properties and to evaluate fluctuation in water table. Unlike tensiometers resistance blocks do not work properly at higher moisture levels. Their efficiency is better at lower moisture levels upto wilting point.

Resistance blocks do not give precise result in saline conditions because the soluble salts interfere with conductance of the blocks. Thus nylon, plaster of Paris and fibre-glass blocks should never be used while gypsum blocks may be used in saline conditions upto some extent. Resistance units are used in fine textured soils.