| Themes > Science > Earth Sciences > Geology > Soils > Soil Composition > Components and Structure > Anion Nutrients |
Reserves of anion nutrients are held in the organic portion of the soil and are released to plants and crops through the decay of organic matter or through air and water. Soil anions, which form acids in solution, continually change in form and quantity. As the major building blocks of proteins and carbohydrates, anions are required in larger quantities than are cation nutrients. Elements that form soil anions include nitrogen, carbon, phosphorus, and sulfur. Nitrogen. Nitrogen tends naturally towards the gaseous state as its most stable and plentiful form. Although plants and crops cannot use atmospheric nitrogen directly, certain soil microbes such as the rhizobium bacteria are able to capture it from the air and transform it into a biologically useful form: the nitrate anion. The nitrate form, which is present in the soil solution, is extremely transitory and will fluctuate significantly from day to day and even at different times of the day. Carbon. Carbon is the major constituent of plant (and animal) tissue. It is the food consumed by plants and crops more than any other mineral. Although abundant in the organic fraction of the soil, carbon is taken in by plants and crops almost entirely from the atmosphere as carbon dioxide. Some of the carbon dioxide reacts in the soil to produce carbonate and bicarbonate anions Phosphorus. Although more mineral-like than other anion nutrients, phosphorus is easily immobilized ill the soil through its tendency to form insoluble compounds with calcium and other minerals. It is most readily available to plants and crops when released gradually through the decomposition of organic matter. Its relative mobility means that distribution of phosphorus throughout the soil is only accomplished through the movement of earthworms and other soil organisms. Sulfur. Sulfur, an essential component of protein and fats, acts a lot like nitrogen in the soil ecosystem and is particularly important for nitrogen-fixing microorganisms. Sulfur deficiency is rarely a problem, especially where adequate soil organic matter levels are maintained. Acid rains containing sulfur compounds released by coal-burning industrial plants also adds sulfur to the soil |
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