Fundamentals of Soil Science (1+1)

Exercise: 4

Aim: Determination soil moisture content

• Soil water content is used to refer the water that may be evaporated from a soil by heating until there is no further loss of weight. Therefore, soil water content is either the ratio of the mass of water present in a soil sample to the mass of soil solid present in that soil or the volume of water present in unit volume of soil sample.
• Knowledge of soil water may be required to assess the extent of availability of soil water to plants.This determination provides a means for comparing the analytical values of samples with different moisture content.

Methods of determination

• There are numerous methods available for the determination of soil water which can be divided into two categories:
• Direct methods by which the amount of water present in a given soil is determined directly i.e. gravimetric method.
• Indirect methods by which a soil property related to the soil water content is measured.

The simplest and most widely used method for determining the soil moisture is gravimetric method.
Principle:

• Soil water content is determined by drying a known mass of moist soil sample in an oven at 105° C. The loss in weight of wet soil sample upon drying is equal to the water content retained by the soil.

Materials required:

• Soil sampling auger,moisture boxes, oven,desiccator and balance

Procedure:

• Weigh the empty moisture box with lid
• Take soil sample of about 100 g from the required depth with the help of auger.
• Put the soil sample immediately in the moisture box and close it to prevent loss of moisture by evaporation.
• Bring the boxes containing moist soil to the laboratory and weigh immediately.
• Remove the lids and place moisture boxes in an oven and dry to a constant weight at 105° C. This takes approximately 24-48 hours.
• Allow the sample to cool for some time in oven. Then close the boxes and put them in a desicator for further cooling. Now weigh the closed boxes with the oven dry soil.

Observations and calculations:

• Weight of empty moisture box with lid = X_g
• Weight of moisture box + lid + moist soil = Y_g
• Weight of moisture box + lid oven dry soil = Z_g
• Moisture content in soil(g) = (Y - Z)
• Weight of oven dry soil(g) = (Z - X)
• Per cent moisture in soil (w) = (Y - Z) / (Z - X) × 100

Precautions:

• Sample drawn should be representative of the site of sampling
• Put the soil sample into moisture box immediately following its collection and cover it with lid to avoid any moisture loss due to evaporation.
• Dry the sample to a constant weight at 105⁰C