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Physical Properties of Soils

Important physical properties of soils, in addition to texture and structure, are bulk density, infiltration rate, permeability, and available water capacity. Bulk density is defined as the mass or weight of the soil per unit mass, usually given as grams per cubic centimeter (g/cm3). Bulk density affects seedling emergence, root growth, infiltration, permeability, and water holding capacity. As bulk density increases, the total pore space of the soil decreases.

Infiltration rate is the maximum rate at which water enters the soil. It is affected most by the properties of the surface horizon. Clayey soils and soils with weak structure will tend to have lower infiltration rates than soils with coarser textures and moderate or strong structure. Total infiltration also is affected by soil slope. Water that does not infiltrate the soil will run off. High runoff may lead to soil erosion if the soil surface is not adequately covered.

Permeability is the movement of water through the soil profile. Soil properties affecting permeability include structure, texture, and the presence of restricting layers such as fragipan. A fragipan is a dense, brittle layer that restricts both water movement and root penetration and growth.

Available water capacity is the capacity of the soil to hold water that is usable by most plants. It is commonly defined as the difference between the water held at field capacity and the amount held at wilting point. Field capacity is that point at which water stops flowing freely from the soil after a saturation event. The large pores in the soil drain so rapidly that plants are unable to use the water. At field capacity, the medium-sized and small pores are holding water. Wilting point is that point at which so little water is left in the soil that plants wilt. Usually some water will be held in small pores, but it will be held so tightly that plants cannot use it.

Additional information about physical properties of soil may be found at:



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