porosity,weight,soils
Porosity means total space, that is, the soil volume not occupied with solid materials, varying from 35% to 50% in dry soils. As texture becomes finer, the size of individual pore spaces decreases, but total pore space increases. Any change in structure or organic matter, also influences the size of pores and porosity. The sizes of pores and porosity are reduced by compaction, often necessary in sandy soils. Porosity is increased in fine-textured soils by tillage and organic matter, because they influence a granular structure in silty and clay soils. Dry mineral soils weigh more than water, organic soils weigh less. The numerical ratio between the weights of equal volumes of dry soil and water represents the volume weight, or bulk density of the soil. Specific gravity is also a comparison of the weights of equal volumes of dry soil and water, but differs from volume weight, as the equal water volume is determined by displacement and represents soil volume without pore space. Specific gravity is practically constant for most mineral soils with values on 2.65 to 2.70. Dark-coloured soils are often lighter because of more organic matter; organic soils have specific gravity values from 1.5 to 2.0, which vary with their mineral content. In comparison with volume weight, specific gravity of soil is less important. Volume weight usually ranges from 1.0 to 1.5 for fine-textured oils and from 1.2 to 1.7 for coarse-textured soils.
