As discussed in point 13 under “General Principles,” it may be best for
water-limiting regions such as the Great Plains. In dryland agriculture low
many farmers to adapt more “dynamic” crop sequences rather than
water availability is usual y the greatest limitation to crop growth. In such
strictly adhere to a particular sequence. Many things change from year
regions, where much of the water needed for a crop is stored in the soil
to year, including prices paid for crops, pest pressures, and climate. And
at planting time, growing of two heavy water users in a row may work out
many farmers do deviate from plans and change what they plant in a
well if rainfall was plentiful the first year. However, if rainfall has been low, particular field—for example, in a wetter than normal field a dry spring
following a heavy-water-using crop (such as sunflowers or corn) with one
opens the opportunity for a vegetable farmer to plant an early-season
that needs less water (such as dry pea or lentil) means that water stored
crop, thus potential y enhancing the diversity of crops grown in that
in the soil may be enough, along with rainfall during the growing season,
field. However, this issue is especial y important for dryland farmers in
to result in a reasonable yield.
Table 11.2
Comparison of Monoculture, Fixed-Sequence Rotations, and Dynamic Cropping Systems Monoculture
Fixed-Sequence Rotations
Dynamic Cropping Systems
Multiple crops; number depends on
Multiple crops; number depends on
Numbers and types of crops
Single crop
regional y adapted species, economics,
regional y adapted species, economics,
farmer knowledge, infrastructure.
farmer knowledge, and infrastructure.
Diversity high due to annual
Crop diversity
N/A
Diversity depends on length
variation in growing conditions
of fixed sequence.
and marketing opportunities, as well
as changes in producer goals.
None, although fixed-sequence
Crop-sequencing flexibility
N/A
cropping systems that incorporate
High. All crops, in essence,
opportunity crops increase flexibility.
are opportunity crops.
Extended knowledge of
Biological and ecological knowledge
Basic knowledge
Some knowledge of crop
of agronomy
interactions is necessary.
complex, multiyear crop and
crop-environment interactions.
General y low, though
Complexity variable depending on
Complexity inherently high due to
Management complexity
variable depending
length of fixed sequence and
annual variation in growing conditions,
on crop type
diversity of crops grown.
markets, and producer goals.
Source: Modified from Hanson et al. (2007).
cause it to decrease too much. On prairie soils, with their
unintentionally encouraged farmers to narrow produc-
very high original contents of organic matter, levels still
tion to just two feed grains. The two-year corn-soybean
probably decreased with this rotation.
rotation is better than monoculture, but it has a number
In the corn belt region of the Midwest, a change in
of problems, including erosion, groundwater pollution
rotations occurred as pesticides and fertilizers became
with nitrates and herbicides, depletion of soil organic
readily available and animals were fed in large feedlots
matter, and in some situations increased insect prob-
instead of on crop-producing farms. Once the mixed
lems. Research indicates that with high yields of corn
livestock farms became grain-crop farms or crop-hog
grain there may be sufficient residues to maintain
farms, there was little reason to grow sod crops. In addi-
organic matter. With soybeans, residues are minimal.
tion, government commodity price support programs
The Thompson mixed crop-livestock (hogs and beef)
121
Building SoilS for Better CropS: SuStainaBle Soil ManageMent