Diversity

Diversity is defined as the state of being diverse, having a variety. Diversity is apparent in many aspects of soil. Two examples of soil diversity are the different soil orders and the soil biodiversity.

Soil Orders in the United States

The United States exhibits a vast array of soil orders, as shown in the image above. The soil orders included in this map are: Alfisols, Andisols, Aridsols, Entisols, Histosols, Inceptisols, Mollisols, Oxisols, Spodosols, Ultisols, and Vertisols. 

Alfisols have a base saturation over 35 percent and have subsoil horizons enriched with clay. This type of soil is typically found under forest and savanna vegetation. Alfisoils are generally fertile, with production levels similar to that of Mollisols and Ultisols. They are typically abundant in nutrient cations of Ca, Mg, K, and Na. They make up about 10 percent of the world's ice free land areas.

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Andisols form in or near areas of recent volcanism and are made from volcanic parent materials unique chemical properties. They have limited geographic distribution, but have potentially high productivity due to their tendency to accumulate organic material, and are easily cultivated. They lack development of soil horizons and are not typically extensively weathered. They make up about 1 percent of the worlds ice-free land areas.

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Aridsols are soils in arid climates that have visible chemical/weathering alteration. They often have accumulation of CaCo3(lime) and NaCl(salt), but normally have low amounts of organic matter content. Aridsols normally are water deficient, and do not allow for the growth of crops without irrigation. Without irrigation and fertilization, the productivity of Aridsols are generally low. They make up 12 percent of the world's ice free land areas.

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Entisols lack development of Soil Horizons. This type of soil can be formed from a wide variety of parent material, making its properties and productivity varied. These soils are often found in dry or cool environments, commonly being found with Aridsols. Very productive Entisols can be found on floodplains, while low productivity in this soil can be found on steep slopes or sandy areas. They make up 16 percent of the world's ice free land areas.

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Histosols are soils that are mainly composed of organic materials. The organic matter accumulates because the soil is usually very saturated, which creates anerobic conditions that cause the accumulation of organic matter to be faster than decompostion. These soils are typically found in wetland environments and make up 1 percent of the world's ice free land areas.

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Inceptisols are freely draining soils soils that exhibit beginning stages of soil horizon development. This type of soil is typically found in mountain areas and have a varied productivity. They make up 10 percent of the world's ice free land areas.

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Mollisols typically develop in grasslands and have a thick organic-rich A horizon. This type of soil is extremely productive due to its saturation of the cations Ca2+, Mg2+, Na+, and K+. All of those cations are essential plant nutrients. This type of soil makes up 7 percent of the worlds ice free land areas.

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Oxisols are typically found in the tropics and are highly chemically altered. These soils are naturally low in fertility and high in acidity. They are leeched and require fertilizers and an input of nutrients in order to be productive for agriculture. This type of soils makes up 8 percent of the world's ice free land area.

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Spodosols are coarse-textured soils with high leeching potential. They are typically located in northern latitude forests, forming from sandy parent material. The Fe and Al compounds in this soil have a strong geochemical separation. This soil is naturally low in fertility and high in acidity, like Oxisols. This soil requires nutrient input and fertilizers in order to be productive for agriculture. This type of soil makes up 4 percent of the world's ice free land area.

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Ultisols occur in warm, humid climates and have clay-enriched B horizons. They form from weathered parent material in older regions. Ultisols are low in natural fertility and high in acidity, so they require nutrient input and fertilizer to be productive for agriculture. They make up 8 percent of the world's ice free land area.

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Vertisols exhibit a shrink-swell behavior with changing water content due to their high concentration of silicate clay. When the soil shrinks it forms deep cracks, which allow material to fall into it. This material will then be incorporated into the soil when the soil swells again. These typically form in warm climates in limestone, basalt, or topographic depressions. Vertisols make up 2 percent of the world's ice free land area.

Soil Biodiversity

Soil biodiversity refers to the diversity of living organisms in the soil. The most biologically diverse part of the earth is the soil. The soil has a vast biological web of interactions between microorganisms, plants, and macroorganisms. Bacteria, fungi, worms, spiders, springtails, ants, and countless other organisms make up the diversity of the soil. These organisms are important to the flow of nutrients through the soil, which helps productivity. They help plants that grow on top of the soil by providing services like: retaining nutrients, preventing nutrient leeching, decomposing dead matter, returning nutrients to their mineral form, and improving water filtration by forming soil aggregates. The soil may not seem like it is alive because many of the organisms are very small, but that does not mean it is not diverse in life. A handful of soil can contain a billion different organisms.

Citations

1 - Amundson, R., Guo, Y. & Gong, P. Ecosystems (2003) 6: 470.

2 - Bailey, Robert G. Description of the ecoregions of the United States. U.S. Dept. of Agriculture, Forest Service, 1995

3 - Guo, Yinyan, et al. “Pedodiversity in the United States of America.” Geoderma, vol. 117, no. 1-2, 2003, pp. 99–115.

4 - Brussaard, Lijbert. “Biodiversity and Ecosystem Functioning in Soil.” Ambio, vol. 26, no. 8, 1997, pp. 563–570.

5 - Wardle, D. A. (2006), The influence of biotic interactions on soil biodiversity. Ecology Letters, 9: 870–886.

6 - Diana H. Wall, John C. Moore; Interactions Underground: Soil biodiversity, mutualism, and ecosystem processes, BioScience, Volume 49, Issue 2, 1 February 1999, Pages 109–117