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When rain or irrigation water is supplied to a field, it seeps into the soil. This process is called infiltration.

Infiltration can be visualized by pouring water into a glass filled with dry powdered soil, slightly tamped. The water seeps into the soil; the colour of the soil becomes darker as it is wetted (see Fig. 31).

Fig. 31. Infiltration of water into the soil

2.2.2 Infiltration rate

Repeat the previous test, this time with two glasses. One is filled with dry sand and the other is filled with dry clay (see Fig. 32a and b).

The infiltration of water into the sand is faster than into the clay. The sand is said to have a higher infiltration rate.

Fig. 32a. The same amount of water is supplied to each glass

Fig. 32b. After one hour the water has infiltrated in the sand, while some water is still ponding on the clay

The infiltration rate of a soil is the velocity at which water can seep into it. It is commonly measured by the depth (in mm) of the water layer that the soil can absorb in an hour.

An infiltration rate of 15 mm/hour means that a water layer of 15 mm on the surface of the soil, will take one hour to infiltrate (see fig. 33).

Fig. 33. Soil with an infiltration rate of 15 mm/hour

A range of values for infiltration rates is given below:

2.2.3 Factors influencing the infiltration rate

The infiltration rate of a soil depends on factors that are constant, such as the soil texture. It also depends on factors that vary, such as the soil moisture content.

i. Soil Texture

Coarse textured soils have mainly large particles in between which there are large pores.

On the other hand, fine textured soils have mainly small particles in between which there are small pores (see Fig. 34).

Fig. 34. Infiltration rate and soil texture

In coarse soils, the rain or irrigation water enters and moves more easily into larger pores; it takes less time for the water to infiltrate into the soil. In other words, infiltration rate is higher for coarse textured soils than for fine textured soils.

ii. The soil moisture content

The water infiltrates faster (higher infiltration rate) when the soil is dry, than when it is wet (see Fig. 35). As a consequence, when irrigation water is applied to a field, the water at first infiltrates easily, but as the soil becomes wet, the infiltration rate decreases.

Fig. 35. Infiltration rate and soil moisture content

iii. The soil structure

Generally speaking, water infiltrates quickly (high infiltration rate) into granular soils but very slowly (low infiltration rate) into massive and compact soils.

Because the farmer can influence the soil structure (by means of cultural practices), he can also change the infiltration rate of his soil.