Soil porosity is a measure of how much space in the soil is not occupied by soil particles or organic matter. The spaces in between soil particles are filled with air and water. The density of a soil, or porosity, determines how much water it will absorb and how long it will hold it.
Soil texture determines the porosity of a soil. Finely textured soil, such as sandy soil, has more pore space than rough-textured soils, according to the University of Wisconsin. Pores in a soil are created when contacts are made between particles in the soil. The more soil particles there are in a soil, the greater the amount of contacts available.
Permeability is how connected the soil is between pores. Soil that is highly permeable has loose connections and allows lots of water and air through the soil. Coarse-textured soils have large particles that connect well and allow a high level of permeability. Clay soils that have tightly packed particles have pores that let through little water.
Soil comes in four basic structures: granular, platy, blocky and prismatic. Granular soil tends to be crumbly and have a broken appearance. Granular allows a great amount of porosity. Platy structures look like daggers, or plates stacked on top of each other. They reduce the downward movement of water and reduce porosity. Blocky structures, says NASA, are irregular blocks that are between 1.5 and 5 cm in diameter. Prismatic or columnar structures are vertical columns of soil. Columnar soil has a salt cap at the top of the vertical structure. A fifth structure is massive soil structure, but this is a single block of rock that has zero porosity.
Improving Soil Porosity
Adding organic matter to a soil improves the soil structure. Several years of adding soil amendments may be necessary before soil structure improves itself. Organic matter is available in compost form. Debris from the garden, such as dead material from plants and trees, added to the soil will improve soil porosity over time.
Organic matter creates microorganisms that break down soil into fine granules, as well as adding nutrients to the soil. As the organic material breaks down, it becomes granular and more porous. Soil tests are available from university extension services that tell you how to improve both your soil's nutrient content as well as the soil structure.