What Are Soils?

​http://www.nature.com/scitable/knowledge/library/what-are-soils-67647639
In this article readers are introduced to the many facets of soils – their unique characteristics and diversity, the ecosystem services that soils provide, and their use and misuse.
Soils are dynamic and diverse natural systems that lie at the interface between earth, air, water, and life. They are critical ecosystem service providers for the sustenance of humanity. The improved conservation and management of soils is among the great challenges and opportunities we face in the 21st century.

Soil is… a Recipe with Five Ingredients

Soil is a material composed of five ingredients — minerals, soil organic matter, living organisms, gas, and water. Soil minerals are divided into three size classes — claysilt, and sand (Figure 1); the percentages of particles in these size classes is called soil texture. The mineralogy of soils is diverse. For example, a clay mineral called smectite can shrink and swell so much upon wetting and drying (Figure 2) that it can knock over buildings. The most common mineral in soils is quartz; it makes beautiful crystals but it is not very reactive. Soil organic matter is plant, animal, and microbial residues in various states of decomposition; it is a critical ingredient — in fact the percentage of soil organic matter in a soil is among the best indicators of agricultural soil quality (http://soils.usda.gov/sqi/) (Figure 3). Soil colors range from the common browns, yellows, reds, grays, whites, and blacks to rare soil colors such as greens and blues.

Relative sizes of sand, silt, clay.

Figure 1

Relative sizes of sand, silt, clay.

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​http://www.nature.com/scitable/knowledge/library/what-are-soils-67647639

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