Soils are classified according to particle sizes, reports the Michigan Department of Environmental Quality. Small soil particles are sand, silt and clay. Gravel particles are larger than sand, and high gravel soils are not especially fertile. Fertile soils are typically loam soils consisting of various mixtures of sand silt and clay, according to Purdue University consumer horticulture specialist Rosie B. Lerner. Fertile loam soils are generally evenly mixed soils with one dominant particle size.
Sandy loam soils may contain 30 percent silt to 30 percent clay to 40 percent sand mixture. This mixture will result in a fertile soil that drains well but may not retain nutrient-bearing water as well as hoped. Sandy soils need water more often than other soil types. Plants needing well-drained soil and frequent watering will survive well in sandy loam soils.
Silty loam soils may contain 30 percent of both sand and clay with 40 percent silt content. Silt particles are smaller than sand but larger than clay particles, which causes silt heavy soils to drain well and retain moisture well. Many types of plants do well in silty loam soils because of this ability.
Clay loam soils contain a larger percentage of clay particles. Clay particles are tiny and prone to chemical bonding that produces a sticky moist soil. Clay heavy soils retain water so well, it is at times difficult for plants to access water reserves but mixed in loams clay helps moisten soil. Many plants that need extra moisture do well in clay loam soils.
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