Soil is composed of a matrix of minerals, organic matter, air, and water. Each component is important for supporting plant growth, microbial communities, and chemical decomposition. Soil biota are key players in providing nutrients through soil organic matter decomposition, nutrient cycling, promoting soil structure, and maintenance and play an important role in promoting plant growth.
The largest component of soil is the mineral portion, which makes up approximately 45% to 49% of the volume. Soil minerals are derived from two principal mineral types. Primary minerals, such as those found in sand and silt, are those soil materials that are similar to the parent material from which they formed. They are often round or irregular in shape. Secondary minerals, on the other hand, result from the weathering of the primary minerals, which releases important ions and forms more stable mineral forms such as silicate clay. Clays have a large surface area, which is important for soil chemistry and water-holding capacity. Additionally, negative and neutral charges found around soil minerals influences the soil’s ability to retain important nutrients, such as cations, contributing to a soils cation exchange capacity .
Water is the second basic component of soil. Water can make up approximately 2% to 50% of the soil volume. 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.
The capacity of a soil to hold water is largely dependent on soil texture. The smaller particles in soils, the more water the soil can retain. Thus, clay soils having the greatest water-holding capacity and sands the least. Additionally, organic matter also influences the water-holding capacity of soils because of organic matter’s high affinity for water. The higher the percentage of organic material in soil, the higher the soil’s water-holding capacity.
3. Organic matter
Organic matter is the next basic component that is found in soils at levels of approximately 1% to 5%. Organic matter is derived from dead plants and animals and as such has a high capacity to hold onto and/or provide the essential elements and water for plant growth. Soils that are high in organic matter also have a high CEC and are, therefore, generally some of the most productive for plant growth. Organic matter also has a very high “plant available” water-holding capacity, which can enhance the growth potential of soils with poor water-holding capacity such as sand. Thus, the percent of decomposed organic matter in or on soils is often used as an indicator of a productive and fertile soil. Over time, however, prolonged decomposition of organic materials can lead it to become unavailable for plant use, creating what are known as recalcitrant carbon stores in soils.
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% 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 belowground plant functions such as for nitrogen-fixing bacteria. If soils remain waterlogged (where gas is displaced by excess water), it can prevent root gas exchange leading to plant death, which is a common concern after floods.
Micro-organisms 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% 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. Decomposers consume organic matter, water, and air to recycle raw organic matter into humus, which is rich in readily available plant nutrients.