The method and rate of soil formation differs throughout a body of soil. As a result, the soil develops layers. These layers are called soil horizons. Soil horizons may be thick or thin, and they may resemble or differ from the surrounding horizons. The boundaries between the layers can be distinct or barely noticeable.

Most soils include three major horizons. The upper two, called the A and B horizons, are the most highly developed layers. The A horizon is also known as topsoil. The lowest horizon, called the C horizon or the subsoil, is exposed to little weathering. Its composition resembles that of the parent material. Pedologists describe soils by the characteristics of the soil horizons, including (1) color, (2) texture, (3) structure, and (4) chemical conditions.

Color. Soils range in color from yellow and red to dark brown and black. The color of a soil helps pedologists estimate the amounts of air, water, organic matter, and certain elements in the soil. For example, a red color may indicate that iron compounds are present in the soil.

Texture of a soil depends on the size of its mineral particles. Sands are the largest particles. The individual grains can be seen and felt. Silts are just large enough to be seen, and clays are microscopic. Pedologists divide soils into textural classes according to the amounts of sand, silt, and clay in a soil. For example, the mineral portions of soils classified as loam contain from 7 to 27 per cent clay and less than 52 per cent sand. In silty clay, more than 40 per cent of the mineral particles are clay, and more than 40 per cent are silt. Texture helps determine how thoroughly water drains from a soil. Sands promote drainage better than clays.

Structure. When soil particles aggregate, they form clumps of soil that are called peds. Most peds range from less than 1/2 to 6 inches (1.3 to 15 centimeters) in diameter. Their shape and arrangement determine a soil's structure. The ability of peds and soil particles to stick together and hold their shape is called consistence.

Most soils contain two or more kinds of structures. Some soils have no definite structure. In some such soils, the peds lack a definite shape or arrangement. In others, the particles do not aggregate.

There are three main kinds of soil structures: (1) platelike, (2) prismlike, and (3) blocklike. Platelike peds are thin, horizontal plates that occur in any horizon. Prismlike peds are column-shaped subsoil structures. Blocklike peds look like blocks with flat or curved sides. Large, flat-sided, blocklike peds commonly occur in subsoils. Small, rounded, blocklike peds make up most topsoils. They contain more organic matter and hold water and nutrients better than do larger peds.

Chemical conditions. Soils can be acid, alkaline, or neutral. The amounts of acid and alkali in a soil influence the biological and chemical processes that take place there. Highly acid or alkaline soils can harm many plants. Neutral soils support most of the biological and chemical processes, including the process by which green plants obtain many nutrients. This process is called cation exchange. Many nutrients and other elements dissolve in the soil solution, forming positively charged particles called cations. The negatively charged clay and humus attract some cations and prevent them from being leached (washed away) from the topsoil by drainage waters. The solution that remains in the soil contains other cations. Nutrient cations on the clay and humus and those in the soil solution change places with nonnutrient cations that are on roots. The roots can then absorb the nutrients.
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