Teacher background information
Soil type affects how well plants grow, and how effectively we can water them. Soil is a mixture of mineral particles, water, air, living organisms and decomposed organic matter.
The size of the mineral particles determines the spaces available for the other elements and, hence, the fertility, water-holding capacity and drainage capability of the soil.
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.
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.