Agroecosystem Diversity and Stability - Agroecology: The Ecology of Sustainable Food Systems - page 225

effect, where a crop grown after another does better
than when grown in continuous monoculture. By adding
residues of different species of plants to the soil, rotations
help maintain the biological diversity of soil microorgan-
isms. Each residue type varies chemically and biologically,
stimulating and/or inhibiting different soil organisms. In
some cases, the residue from one crop is able to promote
the activity of organisms that are antagonistic to pests or
diseases for a subsequent crop. Rotations also tend to
improve soil fertility and soil physical properties, reduce
soil erosion, and add more organic matter. The well-known
advantages of soybean/corn/legume-hay rotations in the
Midwestern U.S. are based in part on the way that greater
temporal diversity aids nutrient and disease management.
Research on the impact of rotations on the dimensions of
diversity can improve the effectiveness of this important
practice (Altieri, 1999).

Fallows
A variation of the rotation practice is to allow a period in
the cropping sequence where the land is simply left uncul-
tivated, or fallow. The introduction of a fallow period
allows the soil to “rest,” a process that involves secondary
succession and the recovery of diversity in many parts of
the system, especially the soil. Shifting cultivation, dis-
cussed in Chapter 10, is probably the most well-known
fallow system; the long rest period allows the reintroduc-
tion of native plant and animal diversity and the recovery
of soil fertility. In some systems, the fallow principle is
used to create a mosaic of plots in different stages of
succession, from farmed fields to second growth native
vegetation. In dry-farmed regions, fallow may occur
in alternate years to allow rainfall to recharge soil
moisture reserves, while at the same time promoting the
recovery of diversity in the soil ecosystem during the
uncultivated cycle. Another variation on the use of
the fallow is to make it productive in addition to being
protective; in swidden-fallow agroforestry, specific crop
plants are introduced just before the fallow begins, or
intentionally allowed to reestablish, so that harvestable
products can be obtained during the fallow period
(Denevan and Padoch, 1987). Wherever a fallow period
is incorporated into the cropping cycle, it is the lack of
human-induced disturbance, not just the absence of a crop,
which allows the diversity recovery process.

FIGURE 16.5 A multiple-use hedgerow around a home
garden in Tepeyanco, Tlaxcala, Mexico. Cactus form a barrier
to animals, and chayote squash and apricot trees provide food.

Cover Cropping
A cover crop is a noncrop species planted in a field to
provide soil cover, usually in-between cropping cycles.
Cover crops range from annuals to perennials, and include
many different taxonomic groups, although grasses and
legumes are used predominantly. Increasing the diversity
of a system by planting one or more cover crop species
has a variety of important benefits. Cover cropping
enhances soil organic matter, stimulates soil biological
activity and diversity of the soil biota, traps nutrients in
the soil left over from previous crops, reduces soil erosion,
contributes biologically fixed nitrogen (if one of the cover
crop species is a legume), and provides alternate hosts for
beneficial enemies of crop pests. In some systems, such
as orchards, cover crops may serve the additional purpose
of inhibiting weed development (Sullivan, 2003).

Reduced or Minimum Tillage
Since disturbance in an agroecosystem has a major role
in limiting successional development, diversity, and
stability, a practice that minimizes disturbance may help
enhance diversity. Reducing the intensity of soil cultiva-
tion and leaving residues on the surface of the soil is a
primary method of effecting reduced system disturbance.
The many advantages to be gained from reducing both
the frequency and intensity of tillage were discussed in

Rotations
Growing crops in rotation is an important method of
increasing the diversity of a system over the dimension
of time. Rotations usually involve planting different crops
in succession or in a recurring sequence. The greater the
differences between the rotated crops in their ecological
impact on the soil, the greater the benefits of the method.
Alternating crops can create what is known as a rotational