Written by Tom DeGomez, University of Arizona, Peter Kolb, Montana State University, and Sabrina Kleinman, University of Arizona

A soil is simply a porous medium consisting of minerals, water, gases, organic matter, and microorganisms. The traditional definition is: Soil is a dynamic natural body having properties derived from the combined effects of climate and biotic activities, as modified by topography, acting on parent materials over time.

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There are five basic components of soil that, when present in the proper amounts, are the backbone of all terrestrial plant ecosystems.


Figure 1. Soil is composed of a matrix of minerals, organic matter, air, and water. Each component is important for supporting plant growth, microbial communities, and chemical decomposition. Image courtesy of FAO.

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1. MineralThe largest component of soil is the mineral portion, which makes up approximately 45% to 49% of the volume. Soil minerals are derived from two principal mineral types. Primary minerals, such as those found in sand and silt, are those soil materials that are similar to the parent material from which they formed. They are often round or irregular in shape. Secondary minerals, on the other hand, result from the weathering of the primary minerals, which releases important ions and forms more stable mineral forms such as silicate clay. Clays have a large surface area, which is important for soil chemistry and water-holding capacity. Additionally, negative and neutral charges found around soil minerals influences the soil’s ability to retain important nutrients, such as cations, contributing to a soils cation exchange capacity (CEC).


The texture of a soil is based on the percentage of sand, silt, and clay found in that soil. The identification of sand, silt, and clay are made based on size. The following is used in the United States:

Sand 0.05 – 2.00 mm in diameterSilt 0.002 – 0.05 mm in diameterClay


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