What Are Soils?


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 — clay, silt, 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.

Soils are… Big

You may be surprised to hear “dirt” described as “big”. However, in the late 1800’s soil scientists began to recognize that soils are natural bodies with size, form, and history (Figure 4). Just like a water body has water, fish, plants, and other parts, a soil body is an integrated system containing soil, rocks, roots, animals, and other parts. And just like other bodies, soil systems provide integrated functions that are greater than the sum of their parts.

Soils are… Biological Bliss

If you like life, you’ll love soils. There are a host of small, medium, and large organisms that live in soils, including mammals, birds, insects, and protozoa. But the greatest biodiversity lies in the soil microbes — the bacteria, fungi, and archaea (Figure 10). A teaspoon of rich soil can contain one billion bacteria. We actually know very little about the diversity of soil microbes, partially because they are so diverse, but also because we have not been able to culture the vast majority of these organisms in the lab. Soil microbiologists are applying advanced molecular techniques to understand the diversity and function of soil microbes. It is an exciting field of exploration with new biological taxa frequently being discovered.

Soils are… Fertile

Soils are the primary provider of nutrients and water for much of the plant life on earth. There are 18 elements considered essential for plant growth, most of which are made available to plants through root uptake from soils (Brady & Weil 2007). Soils retain nutrients by several mechanisms. Most nutrients are dissolved in soil water as either positively or negatively charged ions; soil particles are also charged and thereby are able to electrically hold these ions. Soils also hold nutrients by retaining the soil water itself.

Arguably the greatest of all the ecosystem services provided by soils is the retention of water — without soils our land would be little but rocky deserts. Plants use much more water than one might think because they are constantly releasing water into the atmosphere as a result of transpiration, which is a component of the process of photosynthesis. Clay and silt particles are the primary mineral components in soils that retain water — these small particles slow the drainage of water and, like a sponge, physically hold water through capillary forces. Clay provides such strong force that plants can’t pull all the water away from it, which makes silt particles the ultimate ingredient for plant-available water storage — they hold large quantities of water but also release it to plant roots (Figure 3).

Soils are… Clay Factories

Among the most important functions performed by soils is to provide the ideal conditions for clay synthesis. Clays are important because they are often active, which is a general term soil scientists use to describe how chemically reactive a particle is with ions, water, and other particles. These reactions are critical for the provision of many ecosystem services. Clays are often the most active mineral particles because they have unique chemical characteristics and also because they have so much surface area — clays can have 10,000 times the surface area of sand of the same weight (Brady & Weil 2007). All this surface area makes clays a hot spot for chemical reactions.

Soils are… Service Providers

Soils are the among the great ecosystem service providers on earth (Haygarth & Ritz 2009) (Figure 11). They store and provide water for plants. They prevent floods by transferring water slowly to streams and groundwater. They filter and remediate pollutants. They cycle and recycle nutrients and wastes — transforming them into biologically available forms, storing them away for later use, and preventing their leaching to ground and surface waters. Soils provide habitat for a vast diversity of life. They take up and release important gases, including oxygen and greenhouse gases, a service called gas regulation. Many of these ecosystem services are being lost through the degradation and loss of soils. The conservation, restoration, and optimization of ecosystem services provided by soils is among the great challenges for humanity in the 21st century.

References and Recommended Reading

Ahrens, R. J. & Arnold, R. W. “Soil taxonomy,” in Handbook of Soil Science, ed. M. Summer (CRC Press, 2000) E117-E135.

Brady, N. C. & Weil, R. R. The Nature and Properties of Soils, 14th ed. Upper Saddle River, NJ: Prentice Hall, 2008.

Food and Agriculture Organization of the United Nations (FAO). Guidelines for Soil Description, 4th ed. FAO, Rome, 2006. ftp://ftp.fao.org/docrep/fao/009/a0541e/a0541e00.pdf

Haygarth P. M. & Ritz, K. The future of soils and land use in the UK: Soil systems for the provision of land-based ecosystem services. Land Use Policy 26S:S187-S197, 2009.

Jenny, H. The Factors of Soil Formation: A System of Quantitative Pedology. New York, NY: Dover Press, 1941.

Soil Survey Division Staff. Soil Survey Manual. Soil Conservation Service, United States Department of Agriculture, Handbook 18, 1993. http://soils.usda.gov/technical/manual/


“What Are Soils?”
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