Next to soil texture, pore size is probably one of the most important physical features of a soil. It controls water and air movement and storage. Pores come in all sizes, although clays have predominantly small pores, and sands have large pores. Most soils are a mixture of sand, silt and clay particles, so there is a mixture of different sized soil pores (Figure 2.7).
An ideal soil condition is one with an equal number of large and small pores. Large pores allow for soil aeration. Aeration is needed for the exchange of oxygen from the atmosphere and carbon dioxide given off by plant roots and microorganisms. About 10 percent of the pores must be large enough for aeration so that root growth is not restricted.
Small pores are connected to large pores that are connected to medium-sized pores. This complex connection of pores can be compared to a maze with wide, medium and narrow passageways. In addition, some passageways, and soil pores, may be dead ends.
Within an aggregate, the pores are small. Between aggregates, pores are large. Small pores are usually called micropores, and large pores are called macropores. As organic matter is added, the number of macropores increases. These increases result from the increase in aggregation, decay of root channels and creation of earthworm channels. Macropores are crushed when a soil is compacted. Tillage tends to increase macropores in the short-term, but reduces the number of macropores in the long-term because of the loss of aggregation and severing of earthworm channels.