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 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 makes sand a somewhat expensive soil conditioner if a sizable area is to be treated.
OTHER MECHANICAL CONDITIONERS
One of my favorite low-cost soil amendments is steam cinders. These sharp black particles are the product of burning coal in steam
power plants. Because of high temperatures reached in the firebox, individual particles are partially vitrified. They are quite porous, which allows them to absorb a lot of moisture.
They can often be had for hauling. Municipal and private power plants accumulate them much more rapidly than they can be used for construction work, roads, and so forth. I pay about a dollar a yard, which makes steam cinders about the cheapest soil amendment I can buy of any kind.
They are particularly useful in improving lawn soils. They might not be so desirable in a vegetable garden where their harsh, gritty particles would be a nuisance to the gardener working around root crops. I have, however, used fine steam cinders in bulb and perennial beds.
The cinders have two drawbacks. One is that they must be purchased many months before they are needed. When fresh, they contain sulfur impurities which must be leached out before the cinders can be used. I buy them in late summer, pile them behind the garage over winter and use them the following spring. Weathering does a much better job of purifying them than can be done with a hose and water under pressure.
The other defect is that hard, rough clinkers of considerable size are often formed by some coals. This means that the pile must be screened to separate the unusable particles from the fine ones.