Soil: The Foundation of Agriculture
Soil Composition
Soil composition is an important aspect of nutrient management. While soil minerals and organic matter hold and store nutrients, soil water is what readily provides nutrients for plant uptake. Soil air, too, plays an integral role since many of the microorganisms that live in the soil need air to undergo the biological processes that release additional nutrients into the soil. Soil composition is a mix of soil ingredients that varies from place to place. The difference in soils is the reason why there is such a wide variety of crops grown in the United States. The basic components of soil are minerals, water, organic matter, microorganisms, and gases.
Minerals
The largest component of soil is the mineral portion and are derived from two principal mineral types. Primary minerals are those soil materials that are similar to the parent material from which they formed. For example, quartz, a mineral composed of silicon dioxide (SiO2), is the principal constituent ordinary sand. Calcite (CaCO3) is the primary mineral in limestone and chalk and is abundant is many soils. Orthoclase-feldspar (KAISi3O8) is a very common soil mineral, which contains potassium.
Soil Organic Matter
Soil organic matter plays an important role in integrating many aspects of soil health. Soil organic matter is the fraction of the soil that consists of plant or animal tissue in various stages of breakdown (decomposition). Most of our productive agricultural soils have between 1 and 6 percent organic matter. Organic matter inputs can be influenced by crop management, such as the use of cover crops, crop rotations, and residue management, as well as soil management, such as using organic forms of nutrients like compost and manure or conservation tillage (Section 17.3).
Soil Microorganisms
Microorganisms are the final basic element of soils, and they are found in the soil in very high numbers but make up much less than 1 percent of the soil volume. A common estimate is that one thimble full of topsoil may hold more than 20,000 microbial organisms. The largest of these organisms are earthworms and nematodes and the smallest are bacteria, actinomycetes, algae, and fungi. Microorganisms are the primary decomposers of raw organic matter. These microbial communities perform a wide array of “ecosystem services” essential for the health of soil and the organisms it supports. For example, Saprophytic bacteria and fungi decompose the complex substrates of dead plant and animal matter to generate humus (Section 6.1).
Water
Water is the second basic component of soil. Water is important for transporting nutrients to growing plants and soil organisms and for facilitating both biological and chemical decomposition. Soil water availability is the capacity of a particular soil to hold water that is available for plant use (Section 7.2). The capacity of a soil to hold water is largely dependent on soil texture.
Gases
Gases or air is the next basic component of soil. Because air can occupy the same spaces as water, it can make up approximately 2 to 50 percent of the soil volume. Oxygen is essential for root and microbe respiration, which helps support plant growth. Carbon dioxide and nitrogen also are important for below ground plant functions such as for nitrogen-fixing bacteria.
Click on the following topics for more information on soil, the foundation of agriculture.
