How deep should land be tilled?

Most seedbed preparation methods use a plow or digging hoe to loosen the soil to a depth of 15-20 cm (i.e. the topsoil). There are 2 situations where deeper tillage may be costeffective, though not always, by any means:

• Encouraging roots to enter the subsoil by breaking up a pan (compacted layer) may enable them to tap into a valuable moisture reserve; this can make a crucial difference in a drought, especially for deep-rooted crops (e.g. maize, sorghum) grown under rainfed conditions. Loosening a pan may also improve soil drainage.

• In very hot conditions (e.g. the Sahel during the period from March through June) deep tillage may allow roots to grow deeper into cooler, more hospitable soil.

The value of deep tillage is commonly overrated for a number of reasons:

• Power and labor requirements increase greatly, especially since the subsoil tends to be more compact. With hand tools, deep tillage is seldom practical on anything but small plots.

• About 60-80% of most crops' roots are found in the topsoil, even in high-yielding fields.

• Subsoils that are poorly drained or too acid won't allow much root growth, no matter how well loosened or enriched. Also, it's not unusual for the soil below a hard pan or clay pan to be compacted and poorly drained.

• Loosening a compacted subsoil is likely to be only temporarily effective unless a soil conditioner (sand, rice hulls, etc.) is added, as well as fertilizer. This may be feasible using hand tools but is very laborious.

• On large fields, tractor-drawn subsoilers (long, narrow shanks that penetrate up to 60 cm deep) can be used to break up compacted layers (pans). Results vary from poor to good, depending on the type of pan, its wetness, and the characteristics of the soil below it. Hardpans (natural layers that remain cemented whether wet or dry) and traffic Pans (compacted layers right below normal tillage depth that are caused by machinery traffic) can usually be successfully fractured while dry. However, subsoiling claypans (natural, dense, clayey layers) often gives disappointing results for 2 reasons. First, they're often continually wet (unless the dry season is long) and aren't subject to fracture in this condition. Secondly, soil below such claypans is often compacted and poorly drained, too.