Soil infiltration is the process of water entering the soil; the infiltration rate is the velocity at which water penetrates the soil surface. Soils with low infiltration can be responsible for runoff and flooding and can become saturated during rain events. This, in turn, decreases soil strength and increases erosion potential. It can also cause nutrient deficiencies in plants and generate anaerobic conditions. There are several factors that affect a soil's infiltration rate, including the type of soil, which is determined by the portions of sand, silt and clay in a soil. Infiltration rates are generally measured in millimeters or inches per hour, meaning that the given depth of a water layer can enter the soil within one hour.
Clayey soils, generally included in the Natural Resource Conservation Service's hydrologic soil group D, tend to have a high potential for runoff and a very slow rate of infiltration when thoroughly wetted. Group D soils that have similar infiltration rates as clay include shallow soils over impervious materials and soils in areas with high water tables. The basic infiltration rate for clay, in millimeters per hour, is 1 to 5. Clay loam generally sees an infiltration rate between 5 and 10 millimeters per hour.
Loamy soils can encompass a variety of infiltration speeds. The NRCS soil hydrologic group B includes sandy loam soils with moderate textures. This group's infiltration rate ranges from about 12 millimeters to 25 millimeters per hour. Group C soils are typically silty loam, with moderately fine to fine texture. These soils have an infiltration rate ranging from 4 to 7 millimeters per hour.
Sandy or gravelly soils have a high infiltration rate and low runoff. The infiltration rate of especially sandy or gravelly soils can range from 25 to 200 millimeters per hour.
Other Factors That Influence Infiltration Rate
There are several other factors that work with soil type to affect a soil's infiltration rate. Soils with a crust have sealed pores that restrict water entry. Similarly, compacted soils will have lower infiltration. Soils with strong aggregates, like those with granular or blocky structures, have a higher infiltration rate than soils with weak structures. Infiltration rates are usually higher when soil is dry and decrease with wetter soil. Frozen soil surfaces can completely prevent infiltration. High organic matter content and continuous pores enhance the soil water infiltration.