amounts of nutrients. Contract growers of chickens are
modern world—the growing of food and fiber and the
an example of this situation.
use of the products by people living away from the
Two different nutrient flows occur when manure on
farm—results in a loss of nutrients from the soil, even
livestock farms is applied to the fields used for growing
under the best possible management. In addition, leach-
the feeds. The nutrients in manure that came from farm-
ing losses of nutrients, such as calcium, magnesium, and
grown feed sources are completing a true cycle. The
potassium, are accelerated by natural acidification, as
nutrients in manure that originally entered the farm as
well as by acidification caused by the use of fertilizers.
purchased feeds and mineral supplements are not par-
Soil minerals—especially in the “young” soils of glaci-
ticipating in a true cycle. These nutrients are completing
ated regions and in arid regions not subject to much
a flow that might have started in a far-away farm, mine,
leaching—may supply lots of phosphorus, potassium,
or fertilizer factory and are now just being transported
calcium, and magnesium and many other nutrients. A
from the barn to the field.
soil with plentiful active organic matter also may supply
If there is enough cropland to grow most of the grain
nutrients for a long time. Eventually, however, nutrients
and forage needs, low amounts of imported nutrients
will need to be applied to a continually cropped soil.
and export per acre will result. Relatively low amounts
Nitrogen is the only nutrient you can “produce” on the
of nutrients exported per acre as animal products make
farm—legumes and their bacteria working together can
it easier to rely on nutrient cycling on a mixed livestock-
remove nitrogen from the atmosphere and change it
crop farm that produces most of its feed than on a farm
into forms that plants can use. However, sooner or later
growing only crops.
you will need to apply some phosphorus or potassium,
even to the richest soils. If the farm is in a mixed crop-
IMPlIcATIONS OF NUTRIENT FlOW PATTERNS
livestock system that exports only animal products, it
Long-distance transportation of nutrients is central to
may take a long time to deplete a rich soil, because so
the way the modern food system functions. On average,
few nutrients per acre are exported with those products.
the food we eat has traveled about 1,300 miles from
For crop farms, especially in humid regions, the deple-
field to processor to distributor to consumer. Exporting
tion occurs more rapidly, because more nutrients are
wheat from the U.S. Pacific Northwest to China involves
exported per acre each year.
an even longer distance, as does the import of apples
The issue eventually becomes not whether nutrients
from New Zealand to New York. The nutrients in
will be imported onto the farm, but rather, what source
concentrated commercial fertilizers also travel large
of nutrients you should use. Will the nutrients brought
distances from the mine or factory to distributors to the
onto the farm be commercial fertilizers; traditional
field. The specialization of the corn and soybean farms
amendments (limestone); biologically fixed nitrogen;
of the Midwest and the hog and chicken mega farms
imported feeds or minerals for livestock; organic materi-
centralized in a few regions, such as Arkansas, the East
als such as manures, composts, and sludges; or some
Coast’s Delmarva Peninsula, and North Carolina, has
combination of sources?
created a unique situation. The long-distance flows of
nutrients from crop farms to animal farms require the
Three Different Flow Patterns
purchase of fertilizers on the crop farms; meanwhile, the
There are three main nutrient flow patterns, each one
animal farms are overloaded with nutrients.
with implications for the long-term functioning of the
Of course, the very purpose of agriculture in the
farm and the environment: Imports of nutrients may be
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Building SoilS for Better CropS: SuStainaBle Soil ManageMent