Teacher background information
What is porosity?
Porosity or pore space is the amount of air space or void space between soil particles. Porosity depends on both soil texture and structure. For example, a fine soil has smaller but more numerous pores than a coarse soil. A coarse soil has bigger particles than a fine soil, but it has less porosity or overall pore space.
Water can be held tighter in small pores than in large ones, so fine soils like clay can hold more water than coarse soils, such as sand. The shape, arrangement and packing of soil particles also help determine porosity. Particles exist in many shapes and these shapes pack in a variety of ways that may increase or decrease porosity. Generally, a mixture of grain sizes and shapes, results in lower porosity.
What is permeability?
Permeability refers to the movement of air and water through the soil, which is important because it affects the supply of root-zone air, moisture, and nutrients available for plant uptake.
Soil is a valuable resource that supports plant life, and water is an essential component of this system. As the foundation for any garden, soil provides nutrients and water for plants and anchors them to the ground. Soil is made up of a variable mix of minerals, organic matter, water, air and a whole range of living organisms.
The size of pore space and interconnectivity of the spaces help determine permeability, so shape and arrangement of grains play a role. Water can permeate between granular void or pore spaces, and fractures between rocks. The larger the pore space, the more permeable the material. However, the more poorly sorted a sample (mixed grain sizes), the lower the permeability because the smaller grains fill the openings created by the larger grains.
The most rapid water and air movement is in sands and strongly aggregated soils, whose aggregates act like sand grains and pack to form many large pores. Some of these particles fit closely together and some do not and this creates spaces of many different sizes in the soil. Such soils are more stable and less susceptible to erosion.
Clay has low permeability due to small grain sizes with large surface areas which results in increased friction and these pore spaces are not well connected. Clay often creates confining layers in the subsurface.