Soil, Chemistry of

The chemistry and fertility of soils have been of concern to humans since ancient times. One of the earliest books to correctly identify the soil as the source of plant mineral nutrients is Organic Chemistry in its Application to Agriculture and Physiology, authored by the German chemist Justus von Liebig (1803-1873) and published in 1840. Liebig's book was based, in part, on research conducted and reported in the 1820s and 1830s by German agronomist Carl Sprengel (1787-1859). Although this field still includes study of plant nutrients, modern research is also focused on the reactions and chemistry of pollutants such as mercury, arsenic, and organic pesticides in soils.

Soil Components

The mineral fraction of soils is derived from rocks and minerals and composed largely of oxygen, silicon, and aluminum. After these elements, the most abundant in soil are iron, carbon, calcium, potassium, sodium, and magnesium. The organic fraction of soils is usually about 1 to 5 percent by weight; it forms during microbial decomposition of dead plant and animal material. Carbon, oxygen, and hydrogen are the major constituents of soil organic matter, which also contains nitrogen, phosphorus, and sulfur. Although plants do not directly absorb organic forms of nutrients, microbial processes can transform the nitrogen, phosphorus, and sulfur in soil organic matter into plant-available (inorganic) forms.

The chemical structure of clay minerals gives them charge; most have a net negative charge or a very low net charge close to zero. Negatively charged soils retain positively charged ions called cations (e.g., Mg²⁺, Ca²⁺). The total amount of negative charge in a soil is called cation exchange capacity (CEC). Some plant nutrients, such as calcium, magnesium, and potassium, are cations, and, therefore, soils with higher CEC values are able to retain more plant nutrients than those with lower CEC values. Organic matter has a higher CEC value than clay minerals and increases a soil's fertility.

Essential Elements for Plant Growth

An element is considered essential for plant growth when plants are unable to complete their life cycles without it. Sixteen to eighteen elements are recognized as essential including carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, sulfur, calcium, magnesium, iron, manganese, copper, zinc, boron, molybdenum, chlorine, and, for some plants, cobalt and nickel. Plant carbon comes from carbon dioxide in the atmosphere, and plant hydrogen and oxygen from water in soil. The other elements come primarily from the inorganic and organic fractions of soil. Macronutrients (nitrogen, phosphorus, potassium, sulfur, calcium, and magnesium) are needed by plants in relatively large quantities. Micronutrients, also called trace elements, are those elements usually contained in concentrations less than 100 milligrams/kilogram plant tissue (iron, manganese, copper, zinc, boron, molybdenum, chlorine, cobalt, and nickel).

Importance of Soil pH

Soil pH is an important property that influences many chemical and biological processes occurring in soils. Acidification of soils is a natural geologic process. Rainwater contains carbonic acid produced when atmospheric carbon dioxide dissolves in the rain. In addition, plants may acidify the soil around their roots by releasing hydrogen ions. Human processes, such as combustion of fossil fuels, may add acidity to the atmosphere; this acidity eventually reaches soils via precipitation (acid rain) and deposition of dry particles. Although many soils have a large capacity to neutralize incoming acidity without changes in their pH values, over geologic time soil pH values decrease.

Soils in arid environments tend to have pH values above 7. The presence of soluble carbonates in these alkaline soils maintains high pH values. Soils containing high amounts of sodium carbonate can have pH values in the range of 8.5 to 10. These soils are called sodic and generally present severe limitations to plant growth. Alkaline and sodic soils may become neutral over time if exposed to enough precipitation to remove all the carbonates by dissolution and leaching. Neutral soils, which contain no carbonate minerals, tend to have pH values between 6.6 and 7.3 and are generally suitable for the growth of a wide range of plant species. Acid soils, which have pH values of 6.5 and below, tend to be found in regions with abundant rainfall and moderate to high temperatures.

One of the most important consequences of soil acidity is the dissolution of aluminum (Al³⁺), which is toxic to plants, from soil minerals. Aluminum in soil solution interferes with both cell division and cell elongation and produces short, stubby root systems. Because of its strong positive charge, aluminum is strongly held on negative exchange sites, partially displacing calcium, potassium, and magnesium and reducing their availability. It may be difficult for plants to take up sufficient phosphorus when growing in acid soils because of chemical reactions between aluminum and phosphorus. Many micronutrients become more soluble at lower pH values, including manganese. Manganese is abundant enough to be toxic to plants in some low-pH soils. Lime (ground calcium and magnesium carbonate) is often added to acid soils to correct these problems and improve the soil environment for plant growth.

Effects of Excess Nutrients on Ecosystems

The nutrients most commonly limiting for plant growth in both terrestrial and aquatic systems are nitrogen and phosphorus. Both are often added as fertilizer to agricultural ecosystems . Nitrogen is generally readily soluble and leaches from soils to surface and ground waters. Phosphorus is strongly absorbed in most soils and typically reaches surface waters attached to particles eroded from agricultural fields. Both nutrients may promote excess algal growth in lakes and the ocean. When large quantities of algae grow, die, and are decomposed in the water, dissolved oxygen is depleted and aquatic organisms may die. Scientists have known since the 1960s that nitrogen and phosphorus were negatively affecting some lakes and rivers. During the 1980s and 1990s dissolved oxygen levels declined in the Gulf of Mexico. By the late 1990s, a large area of the Gulf was almost devoid of aquatic life apparently due to nutrients transported by the Mississippi River.

see also Agricultural Ecosystems; Atmosphere and Plants; Biochemical Cycles; Decomposers; Fertilizer; Nitrogen Fixation; Nutrients; Soil, Physical Characteristics of.

M. Susan Erich

Bibliography

Brady, Nyle C., and Ray R. Weil. The Nature and Properties of Soils, 12th ed. Upper Saddle River, NJ: Prentice-Hall, 1999.

Foth, Henry D., and Boyd G. Ellis. Soil Fertility, 2nd ed. Boca Raton, FL: Lewis Publishers, 1997.

Moffat, Anne Simon. "Global Nitrogen Overload Problem Grows Critical." Science 279 (1998): 988-89.

Pierzynski, Gary, M., J. Thomas Sims, and George F. Vance. Soils and Environmental Quality. Boca Raton, FL: Lewis Publishers, 1994.

PH LEVELS

pH is a measure of acidity (determined by hydrogen ions [H⁺]) or alkalinity (determined by hydroxyl ions [OH⁺]). In a pure water solution the concentration of hydrogen ions equals the concentration of hydroxyl ions at a value of 10^-7 M (moles per liter). Such a solution of equal amounts of hydrogen and hydroxyl ions is said to be neutral and have a pH of 7. Acid solutions contain a higher concentration of hydrogen ions than of hydroxyl ions. Solution acidity is usually reported as the negative logarithm of the hydrogen ion concentration. So a solution with a hydrogen ion concentration of 10^-4 moles per liter has a pH value of 4. In an aqueous solution, pH values less than 7 indicate an acid solution while pH values greater than 7 indicate an alkaline solution. In soils pH values generally range between about 4 and 10.