Cornell Soil Health Assessment Training Manual
Briefly, soil is composed of four basic components: mineral solids, water, air and organic matter (including living biota). The mineral solids are stone fragments, sand, silt, and clay. It is the proportion of the latter three that determines the soil’s texture. For example, a soil that is composed of 70% silt, 20% sand and 10% clay can be classified as a silt loam using the soil texture triangle (Figure 1). Soil texture contributes to the inherent soil quality, the characteristics of the soil that result from soil forming processes. These characteristics are difficult to change through soil management. Water is essential for soil life. Water is the medium that facilitates nutrient transport through the soil and enables plant nutrient uptake. Water also enables/facilitates the movement of microbes such as nematodes and bacteria through the soil. Air is constantly moving in and out of the soil. Air provides the oxygen required for cell functioning in aerobic organisms including plant roots. Both air and water occupy the pore spaces (Figure 2) created within and between soil aggregates (clusters of sand, silt and clay particles bound together by particle surface chemistry and microbial and plant exudates).
Organic matter is any material that is part of or originated from living organisms. Organic matter may be divided into three fractions, the living, the dead (active fraction) and the very dead (stable fraction). The living soil organic matter fraction includes microorganisms, soil-dwelling insects, microarthropods, animals and plants. The dead fraction consists primarily of fresh residues from crops, recently dead microorganisms and insects, sloughed-off root cells, leaf litter, and manure, etc. This fraction is considered active. The sugars, proteins, cellulose and other simple compounds are quickly broken
Source : ASDMA