Soil Composition

Global map of soils showing the fraction of sand contained in the top two centimeters

Soil may be composed of one or more types of soil textures. When soil contains a large percentage of sand, infiltration and drainage of water occurs more quickly because of the larger pore space. Such soil can absorb relatively high rates of rainfall or snowmelt.

Global map of soils showing the fraction of clay contained in the top two centimeters of soil

Soils with a relatively high percentage of clay will have smaller pore space and correspondingly lower infiltration rates. Therefore, soils dominated by clay are less capable of absorbing high rainfall rates.

Global map of soils showing the fraction of silt contained in the top two centimeters of soil

Silt has a particle size between sand and clay. Soils with a high percentage of silt will have higher infiltration and drainage rates than clay, but not as high as sand.

Consequently, clay soils may result in greater surface runoff than sandy or silty soil during intense rainfall or rapid snowmelt. Sandy soil will generally produce the least surface runoff.

Soil texture information can assist with anticipating potential water storage and runoff. Keep in mind, of course, that it is still important to consider specifics of the situation such as soil moisture content and the rate of rainfall or snowmelt.

For example, if the soil is saturated as shown here, surface runoff will occur regardless of the soil texture.

Comparison of pore space in sandy soil versus clay soil.  A zoomed image shows larger spaces existing between large, irregularly shaped particles of sandy soil.  The zoomed image of clay soil shows small spaces between many small, round clay particles.

Although it may seem counter-intuitive, the small pore spaces of clay add up to more total void space than the fewer number of large pore spaces in sand. Consequently, in light rain or slow snowmelt, clay may be able to hold more water than sand.

However, water drains from clay soil more slowly than from sandy soils. So in successive rain events, clay soils may remain saturated between storms and therefore produce more runoff in the later rain events.