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Sustainable Agriculture: Programs

Soil Physical Characteristics

A soil’s physical properties have a lot to do with its ability to produce crops. The physical properties of soil include texture, structure, and bulk density. Texture is simply the amount of sand, silt, and clay in a soil. Most people would prefer farming a loamy soil, which is a mixture of about equal parts sand, silt, and clay.

Soil textural triangle showing the percent sand. silt, and clay in each soil textural class

Texture affects almost every other physical property, such as structure, bulk density, and water-holding capacity. It can also have an effect on chemical properties and nutrient availability in a soil. In Georgia, soil textures range from sand to loam to clay, with every combination in between. Management techniques will vary depending on soil type. For example, a sand or loamy sand will not hold water or nutrients as well as a clay soil, but a clay will be more subject to compaction if you work it when it’s wet.

Many people think, “You know a good soil when you see it.” When looking at a soil, you are largely observing its physical properties. The two most noticeable things are its color and structure. A good soil is typically darker than the surrounding soil due to presence of organic matter. The amount of organic matter in a soil has a strong effect on its structure as well as other properties.

Soil structure is the arrangement of soil particles into aggregates. Soil aggregates help create soil pores, which are the open spaces in the soil. Usually both air and water occupy soil pores and these pores allow air and water to move through the soil, so the amount, size, and distribution of pores is important. Good soil structure should allow for water to move into a soil (not puddle), but also hold onto some water for plant usage.

Structure is greatly affected by the amount of organic matter present in the soil. Organic matter serves as a binding agent for soil particles. This is especially important in soils with low clay content. The organic matter binds loose soil particles and prevents the rapid drainage of water through the upper soil profile where roots are present. Organic matter also reduces the impact of rainfall on soils, which prevents surface erosion and crusting. Erosion has been a major problem in Georgia soils and has caused a loss of soil structure and subsequent reduction in soil quality.

Tillage affects soil structure in two ways. It breaks up existing soil aggregates and it reduces the amount of organic matter by adding oxygen, thus exposing organic matter to decomposition. Tillage can also affect structure below the surface through compaction. Compaction of soil particles reduces the amount of pore space, limits ease of water movement, and prevents root growth. These areas are generally referred to as “hard pans” or “tillage pans” and require some type of subsoiling to prevent problems.