It drains, dries out and warms up slowly, but is very fertile and once wet, holds water well. The characteristics of silty soil are similar to those of clay, but moderated by its larger particle size. The way sand, silt and clay particles are grouped together in aggregates is called the soil structure. The size and arrangement of these aggregates influence the nutrient availability and drainage of the soil. Whether you have dry, sandy soil, silty soil or wet, heavy clay soil, adding organic matter in its many forms will improve the soil structure.
Organic matter helps any soil become more like the ideal loamy soil.
Microorganisms feed on organic matter and produce polysaccharides. Polysaccharides help form humus, which enables small clay or silt particles to stick together to form larger aggregates. Larger aggregates create more pores for water and air to flow. The soil drains better, the plants grow better because of the increased pore space and more nutrients are available. In sandy soils, humus acts like a sponge to catch and hold moisture.