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Analytical Methods

TECH GUIDE

Physical Properties: Porosity

Soil porosity refers to that part of a soil volume that is not occupied by soil particles or organic matter. Pore spaces are filled with either air, other gases, or water. Large pores (macropores) allow the ready movement of air and the drainage of water. They are also large enough to accommodate plant roots and the wide range of tiny animals that inhabit the soil. Large pore spaces permit fast infiltration and percolation of water through a soil or soil horizon. Small pores (micropores) exhibit attractive forces strong enough to hold water in the pore. They are the water retention system of the soil which provides water storage for plant roots. During precipitation, macropores conduct water into the soil where it fills the micropores. At field capacity all pores small enough to retain water against the pull of gravity are filled (Brady and Weil, 1999; Munshower, 1994).

Clay soils have numerous micropores and hold large quantities of water, but since they have few macropores they produce very slow infiltration rates. The pores in the clays may be so small and hold water so tenaciously that the water is not available to plants. Sandy soils with numerous macropores but few micropores have higher infiltration and percolation rates but a lower water-holding capacity than other soil textures. A lower water-holding capacity can mean less available water for plant roots. For revegetation purposes, plants perform best in intermediate soil textures (loams) where soils contain mixtures of micro- and macropores (Munshower, 1994).

 

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