Corn & The Environment

Corn & Soil Organic Matter

A portion of the organic matter produced by corn is removed as the harvested crop. With grain corn production, the harvested portion represents about 40% of all dry matter produced - with 60% being left on or in the soil after harvest. With forage corn, where the entire above- ground plant is harvested, the ratio is about 80% removed: 20% left.

About 80% of Canadian grown corn is used for animal feed. Where the harvested corn is used to feed livestock animals, about 1/5 to 1/3 of the harvested biomass is returned to the soil in the form of manure. With manure composting, the amount of organic matter returned to the soil is less. (About half of the organic matter in manure is converted back into carbon dioxide during composting.) Some of the byproducts of industrial corn processing, particularly those components high in protein such as "corn gluten feed" and "distillers grains," are also used as feed, and are returned, in part, to the soil as livestock manure.

Benefits of organic matter returned to soil

The organic matter returned to the soil, directly from crop residues or indirectly as manure, consists of many different organic compounds. Some of these are digested quickly by soil micro- organisms. The result of this is a rapid formation of microbial compounds and body structures - important in holding soil particles together to provide soil structure and limit soil erosion - and the release of carbon dioxide back to the atmosphere through microbial respiration.

Many of the organic compounds returned to the soil after corn harvest, are digested more slowly by soil organisms. Some of these compounds don't break down for many years. Longer- lasting compounds, such as lignin, also help preserve and improve soil structure by creating pores for air penetration, by holding soil water, and by helping soil resist the compaction caused by field equipment, tramping animals, and rainfall.

The rate of decomposition of dead corn plants is slower than for many other plants - this is one reason why corn stalks left on the surface provide good, long-term protection against wind and rainfall.

Soil tillage increases organic matter decomposition

Recent research has shown that soil tillage ("working" of the soil by farm and garden implements) has a major effect on the rate of soil organic matter decomposition. More tillage means faster rates of organic matter oxidation (breakdown). Conversely, the techniques of minimum tillage and "no-till" (also called zero tillage and no tillage), which are becoming popular on Canadian farms, mean slower rates of organic matter decay.

In a University of Guelph experiment comparing different tillage practices for corn, the topsoil (0-15 cm depth) of plots devoted to no-till corn production had an average of 19.8 tonnes/hectare more organic matter after 18 years of research than did plots where corn was grown using traditional tillage (fall moldboard plowing plus secondary tillage in springtime) methods (T.J. Vyn, Department of Crop Science, referenced in Ontario Corn Producer, February 1994). (See table below. There was no difference among tillage treatments in this experiment in soil organic matter levels at lower depths in the soil.)

A total of 9,730 Ontario farmers used conservation (minimum) tillage and 3,850 used no- till in 1991 according to the 1991 Census. The number of Ontario farmers using no-till has increased substantially since 1991.

Other research results have shown that soil organic matter levels can be expected to increase as corn yield levels increase (for example, Barber, 1979).

Traditional corn production practices, characterized by high amounts of soil tillage and relatively low crop yields, often meant decreases in soil organic matter percentages. The combination of newer, higher-yielding hybrids, and the use of minimum or no-till techniques, should mean the reverse.

The greatest reductions in soil organic matter levels occur when soil is tilled regularly during the summer with no crop being produced - the traditional summer fallowing practice (see, for example, Campbell and Zentner, 1993). Surprisingly, many people still believe, erroneously, that such a "resting process" is good for the soil, and, also, that cropping programs which produce high yields of organic matter are inherently "bad." Fortunately, summer fallowing is now used only rarely in Ontario.