Topsoil is the most productive layer of the earth's soil. It is the uppermost eight to 12 inches of soil and natural processes can take over 500 years to create an inch of it. The tiny living organisms in topsoil create the nutritional content of vegetables and fruit grown there. The health of topsoil is the primary focus of sustainable agriculture and home organic gardening.
Soils are classified as one of four major textural classes, according to a Cornell University Extension report. These are sand, silt, loam and clay. The combined portions of sand, silt and clay in a soil determine its textural class. Clay soil is a fine textured soil with very small particles. Other soil, such as sand, has large mineral particles.
Clay soil forms large, hard clods and cracks on the surface. It feels sticky to the touch and bends easily when moist. A ribbon can be formed by pinching the soil between the thumb and forefinger. Longer ribbons form when the soil has high clay content. Soil samples are sent to a soil testing laboratory to determine the exact percentage of clay, sand and silt. Average soil is 45 percent minerals, 25 percent each water and oxygen and 2 to 5 percent organic matter.
Importance of Texture
Soil texture determines the rate at which water moves through saturated soil. Other characteristics also determine a soil's texture. These are organic matter content, aeration, tilth and susceptibility to erosion. Erosion of topsoil is a major focus of sustainable agriculture practices. Soil is eroding faster than it is being created, according to a report on topsoil loss by the Center for Earth Leadership.
Texture and Topsoil Loss
Soil becomes degraded and agriculturally unproductive when it does not have adequate organic matter content, such as dense clay soils. When land is overgrazed, over tilled or deforested, there is not enough plant litter (vegetation) that remains on the ground to nourish the soil. The tiny living organisms die and nutrient production stops. Rain easily washes fine clay particles away.
Ideal soil for growing plants is dark brown or black and crumbly. Dense clay soil holds together in a firm ball when held in the hand. It can be easily amended into well-draining productive soil by adding compost. Organic matter creates space between soil particles for roots to hold and spread. Organic matter in soil also allows water to drain easily.