less than exports, imports may be greater than exports,
per acre, the amount of N and P remaining on farms
or imports may equal exports.
increased greatly. When there was 1/4 AU per acre,
Imports are less than exports. For farms “living
imports and exports were pretty much in balance. But at
off capital” and drawing down the supplies of nutrients
1 AU per acre, around 150 pounds of N and 20 pounds
from minerals and organic matter, nutrient concentra-
of P remain on the farm per acre each year. The nutrient
tions continually decline. This can continue for a while,
flow pattern on farms with high animal densities—
just like a person can live off savings in a bank account
with large imports, mainly as feeds, greatly exceeding
until the money runs out. At some point, the availabil-
exports—is not environmentally acceptable, although
ity of one or more nutrients becomes so low that crop
under current conditions it may be more economi-
yields decrease. If this condition is not remedied, the
cal than a more balanced pattern. In addition, some
farm becomes less and less able to produce food, and
farmers, mainly organic ones, try to build up their soil
its economic condition will decline. This is clearly not
organic matter and nitrogen supply by annual applica-
a desirable situation for either the farm or the country.
tions of manure or compost. This also causes an unac-
Unfortunately, the low productivity of much of Africa’s
ceptable buildup of nutrients in soils. In a survey from
agricultural lands is partially caused by this type of
2002 through 2004 of thirty-four organic farms from
nutrient flow pattern, as increasing population pressure
seven states in the Northeast, encompassing 203 fields,
elevated land-use intensity, and fertilizer prices are too
it was found that approximately a third of the soils
high for poor farmers. In previous times under the sys-
had below-optimal levels of nutrients. However, about
tem of shifting cultivation, agricultural fields would have
half of the fields were found to have excessive levels of
been allowed to return to forest for 20 or more years,
P. Other ways need to be found to add organic matter
during which time there would have been a replenish-
through on-farm practices such as intensive use of cover
ment of nutrients in the topsoil. One of the greatest
crops and rotations with perennial forages.
challenges of our era is to increase the fertility of the
Imports and exports are close to balanced.
soils of Africa, both by using fertilizers and by building
From the environmental perspective and for the sake
up healthier soils.
of long-term soil health, fertility should be raised
Imports are larger than exports. Animal
to—and then maintained at—optimal levels. The best
farms with inadequate land bases to produce all needed
way to keep desirable levels once they are reached is
feed pose a different type of problem (figure 7.2c). As
to roughly balance inflows and outflows. Soil tests can
animal numbers increase relative to the available crop-
be very helpful in fine-tuning a fertility program and
land and pasture, larger purchases of feeds (containing
making sure that levels are not building up too high or
nutrients) are necessary. As this occurs, there is less
being drawn down too low (see chapter 21). This can be
land available—relative to the nutrient loads—to spread
a challenge and may not be economically possible for
manure. Ultimately, the operation exceeds the capacity
all farms. This is easier to do on a mixed crop-livestock
of the land to assimilate all the nutrients, and pollu-
farm than on either a crop farm or a livestock farm that
tion of ground and surface waters occurs. For example,
depends significantly on imported feeds. As discussed
in a study of New York dairy farms, as animal density
above, because such a high percentage of the nutrients
increased from around 1/4 of an animal unit (1 AU =
in feeds are excreted, animal products end up exporting
one 1,000-pound animal, or a number of animals that
relatively low amounts of nutrients off the farm. So if all
together weigh 1,000 pounds) per acre to over 1 AU
the feeds are farm grown, adding an animal enterprise
73
Building SoilS for Better CropS: SuStainaBle Soil ManageMent