Soil is a unique three-phase system composed of solids (soil particles), liquid (water), and gases (the soil atmosphere). The characteristics and interactions among these three phases determine the soil's physical properties. Bulk density, particle density, and porosity are three soil physical properties exemplifying the relationships among the soil phases. The volume of voids in soil is related to the percentages of sand, silt and clay (soil texture) and to the arrangement of these soil particles soil structure.
The texture and structure of soil determine the size and distribution of the pores and the total porosity of the soil. This empty space in soil enables root growth, water retention, atmospheric gas exchange, and water drainage. A soil ideal for plant growth will have approximately 50% of its total volume as pore space. Sands contain less pore space than any of the other textures and clay usually has the most. The porosity of a soil can be reduced by compaction or be increased by the addition of organic matter to improve a soil's aggregation (structure).
Two factors must be known about a soil before its porosity can be calculated: bulk density and particle density (Figure 1).