Macropores also are common in very coarse-textured soils and in soils that crack extensively upon drying. Macropores are especially important where they are connected to the soil surface. Heavy rainfall or irrigation events may create temporarily saturated surface soil, which can lead to rapid flow through macropores. If soluble pesticides also are present, they can be carried deep into the soil in a short time. If pesticides are bound tightly to soil particles, however, macropore flow may reduce groundwater vulnerability because water moving through macropores does not have a chance to react with the pesticides and remove them from the soil.
Tillage generally reduces the number of macropores that are open to the soil surface.
Dense, compact, or cemented soil layers have very slow rates of permeability. Permeability of soil in its natural setting is highly variable and extremely difficult to measure. Soil permeability can be determined in a laboratory by measuring the rate of flow through a column of soil under a constant head of water. Permeability rates are given in inches per hour. Typical rates are 0.01 inches per hour for compact clay, 0.5 inches per hour for a loam with good structure, and 15 inches per hour for a loamy sand.