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Competency Area 2: Soil hydrology AEM

PO 13. Understand permeability and infiltrability, and how they are affected by soil type, weather, and management practices.

Permeability is the ease with which gases, liquids, or plant roots can penetrate or pass through a layer of soil or porous media. Permeability is thus both a feature of the soil pore size and distribution for allowing something to pass through it; and a feature of the gas, liquid, or plant root in its ability to diffuse, flow, or penetrate through these soil pore openings.

One analogy of intrinsic permeability is screens with different size openings. The screen with the biggest opening is more intrinsically permeable than one with the smallest opening. The opening size may have little effect on a gas passing through, may effect on how water can move through, and a major effect on what size roots might grow through.

Since soil gases primarily move through the larger (non-water filled) pores, compaction which compresses the larger pores first can substantially diminish the soils ability for gaseous exchange. Saturation of the pores also severely restricts gas permeability and the exchange of soil air with the atmosphere.

Liquids differ greatly from air with regards to flow properties. Water is much denser than air, and water viscosity changes more dramatically over normal environmental temperatures. Because of changes in density and viscosity, water at 35 degrees Fahrenheit will not infiltrate soil quite as quickly as warmer water. Liquid manure is thought to move more quickly than water through soil via gravity because it has a higher density and the suspended organics alter its viscosity.

Management practices which reduce the soil's permeability at the surface (makes soil pores smaller or causes them to be blocked or less connected) will reduce infiltrability. Practices such has heavy traffic loads or compressive tillage (plow, disk) which reduce pore size or connectivity at deeper depths may also reduce the soils overall permeability at deeper depths in the soil.

Management practices that reduce the soil's permeability often have an effect on the ease with which plant roots can penetrate the soil and grow too. Plant roots may have difficulty penetrating and thriving in soils with smaller pores, not only because of changes in air-water relationships, but also because of alterations to the soils bulk density and soil strength. Plant roots have a more difficult time penetrating dry soils, compared to moist soils, because of differences in soil strength at different soil water contents.