Porosity is the open space between soil grains. Soil scientists use porosity measurements to determine how effectively air and water move through the subsurface. Large pore spaces within a soil column help promote infiltration allowing the water to uptake into the root system of plants without mounding and becoming boggy.
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Soil porosity is the open space, or voids, within the soil profile that is free for air and water. These opens spaces occur between grains of sand, silt, clay and organic matter in the soil. The size and types of the sediments in the soil will determine the amount of porosity. Homogeneous samples usually have greater porosity values than mixtures.
Soil Mixtures and Porosity
The type of sediments present in the soil largely determines the porosity. Soil with homogeneous grains have the largest porosity values because there is maximum open space between grains. When large grains, such as sand, are mixed with smaller grains of silt or clay the porosity is lower. The lower porosity is due to the small grains filling the voids between the larger grains.
Total and Effective Porosity
Scientists often measure two types of porosity in soil. The total porosity is the ratio of all of the volume of the voids to the volume of the soil sample. The effective porosity is the amount of connected void space available to move air and water through the soil. The effective porosity is always equal to or less than the total porosity. Isolated void spaces that do not connect to other open areas are not part of the effective porosity calculation.
Porosity of Common Soil Sediments
Soil scientists discuss porosity as a fraction or percentage. Porosity does not have dimensions. Generic porosity values have a range of values although geotechnical engineers test a soil sample to obtain a more precise value, if necessary. Gravel porosities range from 0.25 to 0.40, while sand values range from 0.25 to 0.50. The porosity of silt ranges from 0.35 to 0.50 while clay has the highest porosity range of 0.40 to 0.70. A sand and gravel mixture has a porosity range of 0.20 to 0.35 while glacial till has the lowest porosity range of 0.10 to 0.20 because of the mixture of large to small grains.