The Average Percolation Rate for Various Soil Types

Soil Grain

• All soils have a certain granular texture that can be measured. The soil's texture is determined by the relative amounts of different particle sizes. There are three particle sizes that are used to determine soil texture. These different particle sizes are classified into sand-sized minerals, silt-sized minerals and clay-sized minerals. If a soil has lots of sand, it is considered to be a coarse-textured soil and will have a high infiltration rate. Water will percolate very quickly into a sandy soil. The higher the percentage of finer particles, the lower the infiltration rate will be. The texture of your soil can be determined by a soil testing lab.

Infiltration Rates

• The infiltration rate for a soil is measured after the soil has been saturated. A dry soil absorbs water more quickly than a wet soil. Testing a saturated soil gives a more accurate account of the percolation rate of the soil. Soil textures are divided into five main types: sand, sandy loam, loam, clay loam and clay. The infiltration rate, or percolation rate, for a sand soil is approximately 1.2 inches per hour. The infiltration rate of a sandy loam soil is between 0.8 and 1.2 inches per hour. For a loam soil, it is between 0.4 and 0.8 inch per hour. For a clay loam soil, the percolation rate is between 0.2 and 0.4 inch per hour. Clay soils have the lowest infiltration rate, at less than 0.2 inch per hour.

Structure and Topography

• The way that sand, silt and clay particles stick together in the soil determines its texture. The aggregates or clods that form for the bonding of these different-sized mineral grains can help or hinder the entrance of water into the soil. A soil with a well-developed structure will have cracks and holes of different sizes for water to flow into and be absorbed. Soils with a well-developed soil structure typically have at least 30 percent clay and ample organic matter. If a soil is on a steep hill, water will run off it quickly. Flat ground will allow standing water time to seep into the soil. A soil on a hillside will have a lower infiltration rate than a soil with a more level topography.

Organic Matter

• Organic matter is made from the decayed matter left behind by insects, grass, trees and larger creatures. In the soil, organic matter acts like a glue that binds soil particles together. Organic matter is also highly absorbent. A dry soil with a lot of organic matter will initially have a very high infiltration rate. However, after the soil is saturated, the organic matter will swell, and the amount of water percolating into the soil will slow down a good deal.