Permeability
Permeability refers to the movement of air and water through the soil. Permeability is affected by many soil characteristics. It is extremely important because it affects the supply of air, moisture, and soil nutrients in the root zone available to the plant. A soil's permeability is determined by permanent characteristics such as texture, structure, and consistence. It may be increased or decreased and still remain within the range of each permeability rating.

Each soil layer has a permeability rating, but the soil's permeability is determined by the relative rate moisture and air move through the most restricting layer within the upper 40 inches of the effective root zone. For contests, the subsoil texture sample in the box will be used to determine the permeability. Subsoil texture and structure are of primary importance in determining permeability. Four levels of permeability are recognized in Oklahoma land judging.

Rapidly permeable --- Soils with coarse-textured subsoils that are granular or single-grained are rapidly permeable. Subsoils tend to be very friable or loose when moist, and exhibit little restriction of water or air.

Moderately permeable --- Moderately coarse- and medium-textured subsoils are moderately permeable regardless of structure. They are friable to very friable with large pores. Roots are abundant. A few soils with moderately fine texture and granular subsoils, are also moderately permeable.

For contest, the subsoil texture sample in the box will be used to determine the permeability.

Slowly permeable --- Soils that have moderately fine-textured subsoils with angular and subangular blocky structure are slowly permeable. The subsoils are firm when moist and hard when dry. Roots are common. Length differences of vertical and horizontal cracks are negligible. Soil peds often have thin, discontinuous, clay films on surfaces and tend to break more easily along the vertical axis than in the very slowly permeable soils. Soils frequently have thick surface and transitional horizons from the surface to the most clayey horizon in the profile.

Very slowly permeable --- Soils that have dense, fine-textured subsoils and claypan soils are very slowly permeable. Their structure is coarse, angular blocky or massive with very few visible pores. Roots are few and generally follow ped faces and cracks. These subsoils are very firm when moist and very hard when dry. Soil peds have thick, continuous, clay films on surfaces. Horizontal cracks are longer than vertical ones. (Moddling and grayish subsoil colors are not required for a soil to have a very slow permeability in Oklahoma.)