Good garden soil has to have good texture and structure. Texture refers to the proportions of sand, silt, and clay in soil particles; structure is the way those soil particles are held together. Knowing how particles are arranged in different types of soil explains how easily water, air, and nutrients can circulate through them and how well the soil will hold moisture and nutrients.
Soil particles collect naturally into relatively stable granules called aggregates, or peds. These aggregates come in a variety of shapes, sizes, and cohesive strengths. They may be stuck together by organic matter or by a variety of other substances, such as carbonates, iron oxides, clays, and silica.
Some aggregates, such as clay, are columnar, with comparatively long vertical dimensions. Others are platelike and more horizontal. This latter type is associated with compacted and hardpan soils, both of which are very hostile to plant root growth.
Rounded and blocky aggregates, typical of desirable loamy soils, are heaped together like piles of children’s building blocks, with plenty of space between the granules for movement of air and water to foster plant root growth. Working organic matter into the soil helps improve its structure. As garden plants decompose, their dead leaves, stems, and roots also make valuable additions.
Structure is particularly important in clay soils. Unstructured or weakly structured clay drains poorly, is sticky and difficult to work, and contains little air. Well-structured clay can be one of the best garden soils. Its aggregates hold water while the spaces between them allow fast drainage and admit air.
Sodium destroys structure in clay soil. For help in correcting this problem, see Saline Soils. The structure of heavy clay soils that are not salty can be improved by adding organic matter. See Adding Organic Material to Soil and Organic Matter in the Soil.