Physical Properties of Soil
Soil Porosity
Soil porosity, pore space, or void space, in a soil consists of that portion of the soil volume not occupied by solid particles, either mineral or organic. Poor spaces are the portion of the soil occupied by air and water. The number and size of pore spaces are determined by the size of the soil particles (soil texture) and the arrangement of the soil particles into aggregates (soil structure). The solid portion is made up of sand, silt, and clay, and a small fraction of organic matter. Under field conditions, air and water occupy the pore space between the mineral particles at all times.
Pore Size
Next to soil texture, pore size is probably one of the most important physical features of a soil. It controls water and air movement and storage. The number and size of pore spaces are determined by the size of the soil particles (soil texture) and the arrangement of the soil particles into aggregates (soil structure). Most soils are a mixture of sand, silt, and clay particles, so there is a mixture of different sized soil pores. Pore size are either referred to as macropores (larger than about 0.075 mm) and micropores (smaller than about 0.075 mm).
Soil Conditions Affecting Soil Porosity
Total porosity varies widely among soils for the same reasons that bulk density varies. In an ideal soil, air and water fill the pore space and compose about 50 percent of the volume; organic matter accounts for about 1 to 5 percent of the soil volume; and mineral matter accounts for the remaining 45 to 49 percent. The partitioning of these four components varies considerably. As is the case for bulk density, there are a number of factors that can exert a decided influence on the pore space of soils. These include soil texture, soil structure, soil horizons, soil compaction, soil organic matter, and soil organisms.
Factors Affecting Soil Porosity
Management practices such as tillage, rain, and irrigation alter soil physical properties, such as porosity and pore size distribution, which play an important role in infiltration.
Irrigation
Sprinkler irrigation just like rain can beat apart aggregates exposed at the soil surface. In some soils the dilution of salts by this water stimulates the dispersion of clays. Once the aggregates become dispersed, small particles and dispersed clay tend to wash into and clog the soil pores. The remaining coarse particles at the soil surface become densely packed with little pore space.
Tillage
The pore size distribution which determines the water transmission properties in a soil, and therefore water infiltration, is influenced by soil tillage. Use of heavy machineries for tillage may compact the soil, particularly below the plow layer. Compaction results in a decrease in the total pore space and an increase in the bulk density.
Rain
The surface soil is often exposed to rain and sun for varying periods of time during crop production. At this time, raindrop impacts break soil aggregates at the soil surface, and small soil particles are released. Rainfall may disrupt soil aggregates by two processes: slaking and raindrop impact. Slaking is the breakdown of soil aggregates into smaller sized micro-aggregates when immersed in water. Dispersed soil particles clog the pores. The net result is a reduction in porosity.
Determining Soil Porosity
Porosity can also be calculated from bulk density (BD) and particle density (PD). If there were no pore space, then bulk density would be the same as particle density. The ratio BD/PD would be equal to 1. The more the pore space, the smaller the BD and smaller the ratio BD/PD. In fact, the ratio BD/PD is simply the percentage of the soil that is solid matter. If one subtracts that percentage from 100 percent, the difference is the percentage of pore space.
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