The layer of unconsolidated particles derived from weathered rock, organic material (humus), water, and air that forms the upper surface over much of the earth and supports plant growth. The formation of soil depends on the parent material (i.e. the original material from which the soil is derived), the climate and topography of the area, the organisms present in the soil, and the time over which the soil has been developing. Soils are often classified in terms of their structure and texture. The structure of a soil is the way in which the individual soil particles are bound together to form aggregates or peds. The structure types include platy, blocky, granular, and crumbs. The texture of a soil denotes the proportion of the various particle sizes that it contains. The four main texture classes are sand, silt, clay, and loam, of which loams are generally the best agricultural soils as they contain a mixture of all particle sizes. A number of distinct horizontal layers can often be distinguished in a vertical section (profile) of soil—these are known as soil horizons. Four basic horizons are common to most soils: an uppermost A horizon (or topsoil) containing the organic matter and living organisms forming a complex ecosystem; an underlying B horizon (or subsoil), which contains little organic material and is strongly leached; a C horizon consisting of weathered rock; and a D horizon comprising the bedrock. Topsoil contains a vast assemblage of organisms, including bacteria, protists, fungi, algae, earthworms, nematodes, and insects. These decompose and recycle dead organic matter, forming the humus and ultimately making nutrients available in the form of inorganic minerals for plant roots to absorb. They also contribute to the physical and chemical nature of the soil; for example, earthworms clump particles together, thereby improving aeration.