Determination of Soil Moisture

Moisture content is an important physical property of soil. Soil moisture means the amount of water present in the pores of soil. Water in soil is necessary for plant growth because it dissolves nutrients and helps in transporting them to plant roots. It also supports many biological and chemical processes occurring in soil.

The quantity of moisture in soil depends on factors such as rainfall, irrigation, evaporation, soil texture and temperature. Sandy soils usually contain less water because water drains easily through them, while clay soils can retain larger amounts of water due to their fine particles and higher water holding capacity.

Determination of soil moisture is very useful in agriculture. It helps farmers understand the water available for crops and assists in planning irrigation. In analytical chemistry, soil moisture is commonly determined by the oven-drying method.

The principle of this method is based on the loss in weight of soil when it is heated. When moist soil is heated at about 105°C, the water present in it evaporates. The decrease in weight corresponds to the moisture originally present in the soil sample.

Procedure

1. A clean dry crucible or moisture dish is taken and weighed accurately.
2. A known amount of moist soil sample is placed in the crucible and weighed again.
3. The crucible containing the soil sample is kept in a hot air oven at about 105°C for several hours.
4. During heating, the water present in soil evaporates.
5. The crucible is removed and cooled in a desiccator to prevent absorption of moisture from air.
6. The crucible is weighed again to obtain the weight of dry soil.

Calculation

The percentage of soil moisture is calculated using the relation:

$$
\text{Moisture (%)} = \frac{W_2 – W_3}{W_2 – W_1} \times 100
$$

Where:

• $W_1$ = Weight of empty crucible
• $W_2$ = Weight of crucible + moist soil
• $W_3$ = Weight of crucible + dry soil

Importance of Soil Moisture Determination

• Helps in estimating water availability for plants.
• Useful for proper irrigation planning.
• Influences nutrient absorption by plant roots.
• Affects microbial activity in soil.
• Provides information about the water holding capacity of soil.