granular and cloddy are among those used to describe structure. Soil structure can be modified by factors such as tillage, moisture level, freezing and thawing, root growth, earthworms and other soil inhabiting animals, and driving or walking on the surface.
Very sandy soils nearly always have a loose structure because they don't form aggregates or become hard-packed or cloddy. Fine-textured soils can become hard-packed. This condition interferes with root growth and inhibits movement of water into (infiltration) and through (percolation) the soil. The micro-pores in fine textured soils can easily be filled with too much water to the exclusion of air, which limits the exchange of gases (oxygen and carbon dioxide). The macro-pores of coarse-textured soils facilitate infiltration and percolation of water and the exchange of gases, but they retain little water for crop use. By loosening and granulating a fine textured soil, we can improve water infiltration, percolation and gas exchange and still maintain the ability to retain water for plant growth. A granulated soil consists of granules that resemble crumbs. A granule consists of millions of clay or silt particles clumped together as aggregates. A well-granulated soil has micro-pores within the granules and macro-pores between the granules.
Natural activities, including freezing and thawing and the movement of roots, contribute to granulation of soils. Tillage at proper levels of soil moisture is an effective way to cause granulation. Excessive tillage in an effort to prepare a fine seed bed, especially when soils are dry, will destroy soil aggregates. It is very easy to overwork a soil with a rototiller. Rain or irrigation can also destroy soil aggregates. We must therefore be aware of factors that influence the stability of soil aggregates.
It might seem as though granulation is only a physical process, but biological processes are just as important. Earthworms pass soil through their digestive systems, adding viscous juices which bind particles together. Snails and other organisms leave a trail of slime behind them which acts as a glue. Organic matter is an important factor in the formation of soil aggregates and it adds greatly to their stability. Soil organic matter, particularly humus, is a binding agent which holds clay particles together. It is often said that organic matter is the sticky stuff that holds soil particles together. There is a lot we don't know about these processes, but it appears that chemical unions occur between humus and clay particles. It seems clear that soil organic matter plays a major role in granulation. Increasing the stability of soil aggregates makes the soil easier to work and more resistant to compaction.
Organic matter not only improves the structure of fine-textured soils; it is equally beneficial for coarse textured soils, but in a different way. These soils have a high proportion of macro-pores, facilitating gas exchange and water movement. However, due to a low proportion of micro-pores, these soils are not moisture retentive. This makes frequent irrigation a necessity during dry periods. Organic matter substantially increases the proportion of micro-pores, greatly improving the water holding capacity of a coarse-textured soil.