Soil Type

Soil is composed of: 1) mineral solids; 2) organic solids (remains of once-living organisms); and 3) pore spaces filled with a combination of water and air. The “ideal” soil for growing plants would be composed of (by volume) 45% minerals, 25% water, 25% air, and 5% organic matter.

Organic soils: Very dark brown or nearly black soils formed in naturally wet, lowland areas. They contain at least 20% organic matter which provides the special chemical and physical characteristics of this soil type. Examples of organic soils include peat, muck, and Sphagnum peat.

Mineral Soils: Originate from rock that has undergone the process of weathering, the physical and chemical breakdown from exposure to water, wind, sun, glaciers, freezing/thawing, plant roots, and the activities of many living creatures. Over time, the rock is broken into a mixture of large, small and even microscopic mineral particles. In Michigan, glaciers were responsible for pulverizing rock into various particle sizes and mixing them together. The larger particles are termed sand and gravel, intermediate-sized particles are called silt, and the very small microscopic particles are clay minerals.

A soil's texture, how it looks and feels, refers to the percentages of sand, silt and clay particles that make up its mineral fraction. The relative proportions of sand, silt and clay particles have a major influence on a soil’s characteristics and usefulness for a multitude of purposes. Soils are grouped into textural classes according to the percent of sand, silt and clay.

A loam has approximately 40% sand, 40% silt and 20% clay. If the percent sand increases at the expense of silt and/clay, it may be termed a sandy loam, or a loamy sand or sand. If more silt is present, it may be a silt loam. When clay particles become more numerous, it becomes a clay loam and ultimately, clay.

A soil's texture, with its inherent particle and pore sizes, affects many characteristics of that soil, including:

  • Water-holding capacity -- soil texture affects pore size, and the water is best retained in the small pores.
  • Water drainage and aeration- air is stored in soil pore spaces after water drains away.
  • Infiltration/percolation rate- the pore size and type affects
  • Nutrient-holding capacity- particle size and type affects the surface area in a given volume of soil and the extent to which nutrients are held to those surfaces.
  • Soil temperature- affected by how much water is stored in the soil and how heat is transmitted within a soil.
  • Shrink/swell potential- Soils potential to contract when dry and expand when wet.
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