Agriculture Reference
In-Depth Information
example, some AM fungi produce more of the soil-aggregating compound glomalin than
others (Wright and Upadhyaya, 1996) or differ in the distance from the root in which they
explore for P (Smith et al., 2000).
7.2.2.1
Agronomic background
7.2.2.1.1 Brief history and definition.
Crop rotation is the planned succession of dif-
ferent crops planted in the same field. Farmers have been using crop rotations since Roman
times, when a 3-year sequence of “food-feed-fallow” was common. Land was used to grow a
food crop, planted for livestock feed, or left fallow to replenish its nutrients. This simple early
rotation was improved over centuries by eliminating the fallow and adding legumes and
animal manures into the cycle, which in turn protected the soil from erosion and increased
yields (“Crop Rotation History,” 2011). However, the advent of chemical fertilizers and pes-
ticides after World War II caused cropping systems in industrialized countries to become
increasingly simplified, with large tracts of land being taken up by monocultures. In more
recent years, farmers have been reverting from monocultures to systems that include several
different crops. Reasons to do so might originate from an economic need to diversify in order
not to depend on one commodity alone or could be driven by efforts to combat problems that
are often attributed to monocultures, such as declining yields and increased pest pressures.
Rising interest in sustainable agricultural practices and organic systems is probably also
a factor. The U.S. Department of Agriculture National Organic Program (NOP) Standards
require, for example, that “the producer must implement a crop rotation, including but not
limited to sod, cover crops, green manure crops and catch crops.” The NOP further instructs
that the rotation must provide ecosystem functions, including maintaining or improving
soil organic matter content, providing for pest management in annual and perennial crops,
managing deficient or excess plant nutrients, and providing erosion control (
http://www.
ams.usda.gov/nop/
).
Rotations can be very simple, alternating between two different crops, for example,
or last more than 5 or 6 years. In general, farmers will alternate between heavy feeders
and light feeders (nonlegumes and legumes), cool and warm season crops, annuals and
perennials, or row crops, drilled crops, and sod-forming crops. In vegetable production
systems, growers often rotate according to plant families or root crops with nonroot crops.
The exact sequence and type of crops will depend on an area's climate, culture, or market
trends but will also be influenced by the grower's preference or need for certain crops.
7.2.2.1.2 Effects of rotations.
Numerous studies have shown that longer rotations can
have many benefits. Smith et al. (2008) conducted a field experiment in Michigan compar-
ing continuous monocultures of three row crops (maize, soybean, and winter wheat) and
2- and 3-year rotations with and without cover crops (no cover crop, one or two legume/
small-grain species), encompassing a range of crop diversity from one to six species. All
rotations were grown without chemical inputs. Maize grain yields increased linearly with
the number of crops in the rotation and were highest in the rotation with six crop species
(more than twice as high as in monocultured maize). Soybean and wheat yields were also
significantly different between treatments but did not follow a linear trend like maize.
A long-term crop rotation experiment in Hungary tested the effect of seven crop
sequences and five fertilization treatments on maize and wheat yields. Rotations included
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