The speed that water is absorbed into the soil is called the infiltration rate. The infiltration rate measures the amount of time it takes for the soil to absorb a certain volume of water. A typical infiltration rate is measured in millimetres per hour or inches per hour. There are many different factors that influence the infiltration rate. The amount of clay and the type of clay are important factors in determining the infiltration rate of a soil.
Clay soils can be expected to have an infiltration rate of 1 to 5 millimetres per hour or about 0 to 0.5 inches per hour. Compared to the infiltration rate of a loam soil (10 to 20 millimetres per hour 0.4 to 0.8 inches per hour) this is very slow. With an infiltration rate this low it is very difficult to grow most agricultural plants and grasses. This is because the infiltration rate is slow enough that much of the water will evaporate before it is absorbed. There are several different types of clay. Certain clays expand when wet. An expansive clay can reduce the infiltration rate of a soil because as it swells it fills soil pores and blocks the water from being absorbed.
The percentage of sand, silt and clay in a soil determines its texture. A soil with equal parts of these three mineral particles size classes is called a loam soil. A soil with mostly sand is called a sandy soil. A soil with more than 40 per cent clay is considered to have a clay soil texture. Wet clay soil feels sticky in your hand. The higher the percentage of clay in a soil the lower the infiltration rate will be.
Clay particles act as a joining material that bonds soil particles together. A soil with lots of clay typically has a well developed soil structure. Clay binds with silt and sand particles to form tiny soil aggregates. These aggregates are then joined with other aggregates until larger soil structures are formed. A well developed soil structure will increase the infiltration rate of the soil. Soil structure is very delicate. Vehicle traffic can destroy the soil structure of a clay soil because of compaction. Restoring soil structure and the pre-compaction infiltration rate on a clay soil is very difficult.
The drier a soil is the higher the infiltration rate. The initial infiltration rate on dry clay soil is much higher than when the soil is wet. The dry clay particles are extremely absorbent because they are so small. After the surface of a clay soil becomes wet the infiltration rate quickly goes down. This is why it is important to allow at least an hour for the water to infiltrate before recording a rate on a clay soil.