In good loam soils, small soil particles adhere together to form larger particles or aggregates. This arrangement of soil particles into aggregates is termed the soil structure. Structure resulting from small porous aggregates is highly desirable since it blends the desirable qualities of looseness, drainage, and aeration with water and mineral retention. In the bare hand, good loam soil feels like short pastry dough. This may explain why such a structure is often referred to as a crumb structure or a granular structure.
Since the granular structure of soil is a desirable attribute, resulting in high-quality horticultural and field crops, it is important to understand how such soil structure develops and can be encouraged.
The most significant factor in the development of a granular structure is organic matter, including green manure, soil organisms, decomposing plant roots, and especially humus. The organic material and colloidal clay bind the small mineral particles together as crumb-like aggregates. Humus is highly significant as a binding agent.
The characteristics of organic matter have already been described briefly. When a field of grasses, weeds, and other herbaceous plants is turned under, the green manure represented by their plant parts is
rapidly acted on by organisms of the soil. Macrobial life, such as insects and earthworms, feed on the plant parts to obtain the chemical energy bound within them. Microbial life—principally algae, fungi, bacteria, and actinomycetes—may directly attack the decomposing plant parts or contribute indirectly through digesting the excretions of the mac – robes and the dead macrobes themselves.
The breakdown of dead plant tissue and other organic material by the microorganisms of the soil is accomplished with digestive enzymes in a manner similar to the way an animal’s stomach digests food. Some chemical compounds within the plant break down more quickly than others. The simple proteins, sugars, and starches decompose quickly; the more complex organic compounds, like lignin, a component of the cell wall, decompose more slowly. Eventually, though, all organic matter decomposes into either humus, energy, or a number of other end products including carbon dioxide, water, nitrates, phosphates, sulfates, and calcium compounds. The energy release explains why the temperature rises inside a compost pile.