The Subsoil

The soil immediately beneath the richest part of the surface soil is called the subsoil. It may be of any depth, and extends to the underlying rock. The distinction between the soil and the subsoil, as the two names are commonly used, lies almost entirely in the colour and texture, due to the greater amount of humus near the surface. In cultivated land there is usually a more or less distinct line between the rich, black surface soil and the poorer and lighter-coloured subsoil. In most soils, especially in the East, this line marks the depth of plowing. The depth at which the vegetation that gives the surface soil its black colour and looser texture has been buried is about nine inches. Many soils, especially those made by wind or built by water, and peat and muck soils, show very little if any difference in colour or texture between the first nine inches of soil and that below.

In nearly all cases the subsoil contains less available plant food than the soil above because it is not affected as much by weathering, being protected, and because it is less affected by acids resulting from the decay of vegetation, since it contains less humus. We might call the subsoil rotting rock, and the soil rotted subsoil. This is a providential arrangement. If the plant food in all the soil, down to bed-rock, were as easy to lose as that in the first nine inches of soil our fields would become unproductive much sooner than they do now. The subsoil is a store of plant [food that is held in reserve. We should look upon the rocks, stones, pebbles and subsoil of our fields as so much potential plant food. It is being doled out to us from year to year as fast as it can be used to advantage. As the surface soil slowly wears away and is carried off in crops, the subsoil gradually becomes surface soil. The roots of deep-feeding plants, as clover and alfalfa, bring up plant food that they secure below the roots of ordinary crops. When these crops are cut, and the stubble and roots plowed under, a part of the plant food that the subsoil has contributed to their growth is returned to the surface soil, enriching it.

Earthworms bring to the surface subsoil that has never seen the light of day and this adds richness. A plowing somewhat deeper than usual may mix an inch or more of light subsoil with the surface soil. This may reduce the crop for a year or two, or until the raw plant food in the subsoil has been acted upon by air, water, and soil acids, but eventually the surface soil is enriched by the fresh material. It is advantageous for a sandy soil to rest upon an impervious clay subsoil, and for a clay soil to be underlaid with a sand or gravel subsoil; both subsoils help to correct the defects of the soil above them. A deep gravel or sandy subsoil, however, is usually a disadvantage, as it allows plant food to leach down beyond the roots of plants.