Soil Density & Porosity

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Soil Density & Porosity
Sandy soil has a higher density and lower porosity than clayey soil. (Photos.com/Photos.com/Getty Images)

Density and porosity are two physical properties of a soil. Soil scientists discuss density as both bulk density and particle density. The measurement of bulk density of a soil sample has an indirect relationship to the porosity. Density is a weight per volume of soil while porosity is a percentage of the total volume.

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Soil Bulk Density

Bulk density in a soil is a measurement of the total volume of both the particles and pore space within a sample. Sands have higher bulk densities than clay soils because the clays have more pore space and are lighter by volume. Loamy soils, a mixture of sand, silt and clay have moderate bulk densities between that of sands and clays. The addition of organic matter to a soil is typically a low percentage that does not significantly influence the measurement of bulk density. Soil scientists use one of several methods to determine the bulk density of a sample including the clod method, core method, excavation method or the radiation method, which all involve determining the mass and volume of the sample.

Soil Particle Density

Soil particle density is the mass of the solid particles in a given sample. Particle density does not include the volume of the pore space. Unlike bulk density, the organic content of a soil may greatly affect its particle density because it is usually lighter than rock and mineral particles. Therefore, soils like loam with higher organic content have a lower particle density than other soils. Similarly, surface soils are generally lighter than deeper soils due to the reduction of organic matter with depth below the land surface. Another factor that affects soil particle density is the type of mineral present in the soil since some minerals, such as quartz and feldspars, have a lower mass than dense minerals like magnetite and hornblende. Soil scientists use Archimedes' Principle to calculate particle density.

Soil Porosity

Porosity is the volume of void, or open space, in a soil sample. The open pore space in a soil contains water or air. The open, available pores control how effectively the soil moves water and air through the layers. Two types of porosity include total porosity and effective porosity. Total porosity is a measure of all pore spaces while effective porosity is a measure of the connected pore spaces that allow air and water to flow through the soil system. Sands have larger pore spaces than clays, while loams have a mixture of pore sizes. Soil scientists calculate the porosity as a percentage of the sample. Clays have the highest porosity values, with silts, sands and gravels having increasingly lower percentages.

Relationship Between Density and Porosity

Soil porosity is inversely proportional to the measurement of bulk density, which means that porosity decreases as bulk density increases. You can calculate porosity if you know the bulk density and particle density of a soil sample. Since porosity is expressed as a percentage, you must divide the bulk density by the particle density then multiply that result to determine the percentage of solid space in the sample. Since per cent of solid space and per cent porosity equal the total volume, you can subtract the per cent solid space from 100 per cent to determine the per cent porosity in the soil.

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