Topsoil vs. subsoil

Dig down about 50 cm in most soils and you will have exposed 2 distinct layers: the topsoil and part of the subsoil. The topsoil is the uppermost layer and has these features:

• It's usually darker in color than the subsoil since it contains more organic matter from decaying plants and their roots.

• It's more fertile than subsoil, due to having more organic matter and because fertilizers are usually added to the topsoil only.

• It's usually looser and less compacted than the subsoil, mainly due to its higher organic matter content and to plowing (or hoeing).

• The topsoil is usually about 15-25 cm thick. On cultivated soils, topsoil depth is about equal to tillage depth since this determines how deep organic matter and fertilizers are worked into the soil.

• About 60-80% of the roots of most crops are found in the topsoil since it's a better environment for root growth than the subsoil (i.e. more fertile, less compact).

The subsoil is located between the topsoil and the parent rock (or material) below. Aside from being lighter in color, less fertile, and more compact, it's usually more clayey; that's because downward water movement has transported some of the tiny clay particles from the topsoil into the subsoil.

The role of subsoil: It would seem that we could dismiss subsoil as not having much influence on crop growth. However, this isn't so for 2 good reasons:

• Subsoil is an important storehouse of moisture, especially since it's usually much thicker than the topsoil, and the moisture isn't lost as easily by evaporation. The higher clay content of subsoils makes for higher water holding capacity, too. This moisture reserve is very useful during dry spells, even though there are fewer roots in the subsoil. For example, it's estimated that half the moisture needed to grow a maize crop in the U.S. Corn Belt is already stored in the subsoil at planting time; rainfall during the crop's growth provides the rest but would fall far short by itself to produce good yields.

• Subsoil characteristics like clay content and compaction have a big influence on drainage (the ability to get rid of excess water).

Making Topsoil out of Subsoil: If little topsoil remains due to erosion, you can convert subsoil into productive topsoil. All it takes is hefty additions of organic matter like compost, manure,or green manure (see Chapter 8 on organic fertilizers) for a few years, but this isn't often feasible on large plots.