of nitrogen. Research in Iowa found that even using
240 pounds of N per acre when growing corn after
corn, yields were not as good as corn grown following
alfalfa with little or no N applied. In addition, following
a nonlegume crop with another nonlegume produces
higher yields than a monoculture using recommended
fertilizer rates. For example, when you grow corn follow-
ing grass hay, or cotton following corn, you get higher
yields than when corn or cotton is grown year after year.
a
b
This yield benefit from rotations is sometimes called a
rotation effect. Another important benefit of rotations
is that growing a variety of crops in a given year spreads
out labor needs and reduces risk caused by unexpected
climate or market conditions. Other benefits may occur
when perennial forages (hay-type crops) are included
in the rotation, including decreased soil erosion and
nutrient loss.
ROTATIONS AND SOIl ORgANIc MATTER lEVElS
You might think you’re doing pretty well if soil organic
Figure 11.1. Corn roots: (a) continuous corn with mineral fertilizer, (b) corn matter remains the same under a particular cropping
following alfalfa with dairy manure compost. Photos by Walter Goldstein
(Michael Fields Institute).
system. However, if you are working soils with depleted
organic matter, you need to build up levels to counter
sod-type forage grasses, legumes, and grass-legume
the effects of previous practices. Maintaining an inad-
mixes as part of the rotation also increases soil organic
equate level of organic matter won’t do.
matter. When you alternate two crops, such as corn and
The types of crops you grow, their yields, the amount
soybeans, you have a very simple rotation. More com-
of roots produced, the portion of the crop harvested,
plex rotations require three or more crops and a five- to
and how you manage crop residues will all affect soil
ten-year (or more) cycle to complete.
organic matter. Soil fertility itself influences the amount
Rotations are an important part of any sustainable
of organic residues returned, because more fertile soils
agricultural system. Yields of crops grown in rotations
grow higher-yielding crops, with more residues.
are typically 10% higher than those of crops grown in
The decrease in organic matter levels when row
monoculture in normal growing seasons, and as much
crops are planted on a virgin forest or prairie soil is very
as 25% higher in droughty growing seasons. When
rapid for the first five to ten years, but, eventually, a
you grow a grain or vegetable crop following a forage
plateau or equilibrium is reached. After that, soil organic
legume, the extra supply of nitrogen certainly helps.
matter levels remain stable, as long as production
However, yields of crops grown in rotation are often
practices aren’t changed. An example of what can occur
higher than those of crops grown in monoculture,
during twenty-five years of continuously grown corn is
even when both are supplied with plentiful amounts
given in figure 11.2. Soil organic matter levels increase
116
Building SoilS for Better CropS: SuStainaBle Soil ManageMent