Sand and Soil
Sand is one of the oldest materials used to increase porosity by mechanically opening soils to air and water. Mixing sand in garden soil is a favorite with British gardeners.
The addition of enough sand to a stiff clay soil should, in theory, separate the particles so that air and moisture can move in freely and thus "correct" the soil so it will crumble readily when squeezed into a ball. Sand should also provide pore spaces in which bacteria and fungi can thrive. This in turn would gradually improve the humus content so that a clay soil would turn into a clay loam.
Unfortunately, this end is not always reached when sand is added to clay. Large amounts are needed to bring about any worthwhile improvement. In the final mixture of the two there should be at least one third sand and not more than two thirds clay. If sand is used too sparingly it will, instead of separating the clay particles, merely act like the aggregate in a concrete mixture. The individual grains will be cemented together by the much finer clay particles to form an almost impervious solid. I once saw a stiff clay to which 20 per cent sand had been added; the mixture was so hard it resembled a cement sidewalk. But when another inch layer of sand was spread on top of the soil and worked in with a rotary tiller, the whole mass of sand and soil crumbled and fell apart as if by magic.
For this reason, if sand is to be used to modify clay, say to a depth of 6 inches, at least a 3-inch layer of sand should be spread over the entire area.
This means mixing sand in garden soil can be a somewhat expensive soil conditioner if a sizable area is to be treated.