Sand is sometimes suggested as an amendment material for clay soils. However, there is good reason to be cautious about using sand. In order for sand to be effective in breaking up a clay soil, sand grains must touch one another so there are pore spaces between grains that can hold air and/or water. If the grains do not touch, the clay fills in all the voids between the sand particles leaving no room for pores.
This is the same principle used to make concrete and the result is somewhat the same. You end up making a bad situation worse. So how much sand does it take for it to be effective? Normally, we consider about 80 percent sand to be sufficient. In most cases this makes the use of sand impractical. The addition of organic matter is a much better choice. (Ward Upham)
