Soil is the weathered part of the earth that supports the growth of plants. Soil supplies mineral elements, water and is a medium of root anchorage for many plants, both on land and aquatic. Gardeners often amend soils with compost to increase beneficial microorganisms, improve texture and improve its ability to hold water and nutrients. Soils are usually made of a mixture of particle types with the relative proportion of types determining the overall texture. There are five main types of soil.
Clay soil particles are small--about the size of bacteria and viruses, according to Washington State University extension. Typically, clay particles are flat in shape and feel hard when dry, but mold easily when moist. Clay soils hold moisture longer than sandy soils. Clay soils, also referred to as heavy soils, compact easily if they are low in organic matter. Large amounts of organic matter improves clay soils significantly by encouraging it to form granules and crumbs rather than being cloddy and hard-packed. Adding equal amounts of sand with compost and manure improve soil with high clay content.
Sandy soils are much lighter than clay soils and dry out quickly. It breaks up easily when moist and is rough and gritty. Sandy soils are easy to dig, and warm up quickly in spring. To improve sandy soil, the University of Minnesota extension recommends incorporating 2 inches or more of well rotted compost and manure into the top 6 to 12 inches.
Silt particles are smaller than sand and have a smooth floury feel when dry, but are slippery when wet. Silt soils do not dry out as quickly as sandy soils and retain more nutrients.
Most soils contain a mixture of types. Loam soils are roughly equal in sand, silt and clay particles. It is often considered ideal garden soil because of its texture--a balance of small and large pores creates good water-holding capacity and permeability. This soil is the well-drained, moisture retentive medium often preferred by plants and gardeners. The ideal loam contains 40-percent sand, 40-percent silt, 20-percent clay and organic matter.
Peat soil is rare and endangered, according to the University of Minnesota extension. It is heavy, dark, extremely moisture retentive and difficult to use. It is said that ecologically conscious gardeners no longer use peat because it is so rare.