Nutrient Cycles and Flows
Nutrient Cycles and Flows
Nutrient Cycles and Flows
We
used the term cycle earlier when
discussing the flow of nutrients from soil to plant to animal to soil, as well
as global carbon and nitrogen cycles (chapter 2).
Some farmers minimize their use of nutrient
supplements and try to rely more on natural soil nutrient cycles—as contrasted
with purchased commercial fertilizers—to provide fertility to plants. But is it
really possible to depend forever on the natural cycling of all the nutrients
to meet a crop’s needs? Let’s first consider what a nutrient cycle is and how
it differs from the other ways that nutrients move from one place to another.
When nutrients move from one
place to another, that is a flow.
There are many different types of nutrient flows that can occur. When you buy
fertilizers or animal feeds, nutrients are “flowing” onto the farm. When you
sell sweet corn, apples, alfalfa hay, meat, or milk, nutrients are “flowing”
off the farm. Flows that involve products entering or leaving the farm gate are
managed intentionally, whether or not you are thinking about
Nutrient Cycles and Flows
nutrients. Other flows are
unplanned—for example, when nitrate is lost from the soil by leaching to
ground-water or when runoff waters take nutrients along with eroded topsoil to
a nearby stream.
When crops are
harvested and brought to the barn to feed animals, that is a nutrient flow, as
is the return of animal manure to the land. Together these two flows are a true
cycle, because nutrients return to the fields from which they came. In forests
and natural grassland, the cycling of nutrients is very efficient. In the early
stages of agriculture, when almost all people lived near their fields, nutrient
cycling was also efficient (figure 7.1a). However, in many types of
agriculture, especially modern, “industrial-style” farming, there is little
real cycling of nutrients, because there is no easy way to return nutrients
shipped off the farm. In addition, nutrients in crop residues don’t cycle very
efficiently when the soil is without living plants for long periods, and
nutrient runoff and leaching losses are much larger than from natural systems
The first major
break in the cycling of nutrients occurred as cities developed and nutrients
began to routinely travel with farm products to feed the growing urban
populations. It is rare for nutrients to travel many miles away from cities and
return to the soils on which the crops and animals were originally raised
(figure
7.1b,c). Thus, nutrients have accumulated in
urban sewage and polluted waterways around the world. Even with the building
of many new sewage treatment plants in the 1970s and 1980s, effluent containing
nutrients still flow into waterways, and sewage sludges are not always handled
in an environmentally sound manner.
The trend toward
farm specialization, mostly driven by economic forces, has resulted in the
second break-in nutrient cycling by separating animals from the land that grows
their feed. With specialized animal facilities (figure 7.1c), nutrients
accumulate in manure while crop farmers purchase large quantities of fertilizers
to keep their fields from becoming nutrient deficient.
DIFFERING
FLOW PATTERNS
Different types of farms may have distinctly
different nutrient flow patterns. Farms that exclusively grow grain or
vegetables have a relatively high annual nutrient export (figure 7.2a).
Nutrients usually enter the farm as either commercial fertilizers or various
amendments and leave the farm as plant products. Some cycling of nutrients
occurs as crop residues are returned to the soil and decompose. A large nutrient
outflow is com-mon, however, because a large portion of the crop is usually
exported off the farm. For example, an acre of tomatoes or onions usually
contains over 100 pounds of nitrogen, 20 pounds of phosphorus, and 100 pounds
of potassium. For agronomic crops, the annual export of nutrients is about 100
pounds of nitrogen, 6 pounds of phosphorus, and 50 pounds of potassium per acre
for corn grain and about 150 pounds of nitrogen, 20 pounds of phosphorus, and
130 pounds of potassium per acre for grass hay.
It should be
fairly easy to balance inflows and out-flows on crop farms, at least
theoretically. In practice, under good management, nutrients are depleted a bit
by crop growth and removal until soil test levels fall too low, and then they’re
raised again with fertilizers or manures (see chapter 21).
A grass-fed beef operation
that uses little to no imported feed should also be able to easily balance
imports and exports because few nutrients leave the farm (as animals) and few
nutrients are brought on to the farm (figure 7.2b). Most of the nutrients on
this type of operation complete a true cycle on the farm—they are taken up from
the soil by plants, which are eaten by the animals, and most of the nutrients
are then returned to the soil as manure and urine. The same type of flows will
occur on all integrated crop and livestock farms that produce all of their own
feed.
A contrasting situation occurs
on dairy farms if all of the forage is produced on the farm but grains and
minerals are purchased (figure 7.2c). Many dairy farms in the northeast U.S.
do not have the land base to grow all the needed feed and tend to emphasize
growing forage crops. In this situation, there are more sources of nutrients
coming onto the farm—with concentrates (common mixtures containing corn grain
and soy) and minerals usually comprising a larger source of nutrient inputs
than fertilizers. In a study of forty-seven New York state dairy farms, an
average of 76% of N came onto the farms as feeds and 23% as fertilizers. The
percentages were pretty much the same for P (73% as feeds and 26% as fertilizers).
Most of the nutrients consumed by animals end up in the manure—from 60% to over
90% of the nitrogen, phosphorus, and potassium. Compared with crop farms, where
a high percentage of the crop grown is sold, fewer nutrients flow from dairy farms
per acre. Under this situation, nutrients will accumulate on the farm and may
eventually cause environmental harm from excess nitrogen or phosphorus. This
same problem exists for any ani-mal farm that imports a high percentage of its
feed. To
put it another way, these farms have an
inadequate land base to produce all their feed and therefore also have an
inadequate land base on which to apply their manure at environmentally safe
rates. Animal operations that import all feeds and have a limited land base to
use the manure have the greatest potential to accumulate high
amounts of nutrients.
Contract growers of chickens are an example of this situation.
Two different nutrient flows
occur when manure on livestock farms is applied to the fields used for growing
the feeds. The nutrients in manure that came from farm-grown feed sources are
completing a true cycle. The nutrients in manure that originally entered the
farm as purchased feeds and mineral supplements are not participating in a
true cycle. These nutrients are completing a flow that might have started in a
far-away farm, mine, or fertilizer factory and are now just being transported
from the barn to the field.
If there is
enough cropland to grow most of the grain and forage needs, low amounts of
imported nutrients and export per acre will result. Relatively low amounts of
nutrients exported per acre as animal products make it easier to rely on
nutrient cycling on a mixed livestock-crop farm that produces most of its feed
then on a farm growing only crops.
IMPLICATIONS
OF NUTRIENT FLOW PATTERNS
Long-distance transportation
of nutrients is central to the way the modern food system functions. On
average, the food we eat has traveled about 1,300 miles from field to processor
to distributor to consumer. Exporting wheat from the U.S. Pacific Northwest to
China involves an even longer distance, as does the import of apples from New
Zealand to New York. The nutrients in concentrated commercial fertilizers also
travel large distances from the mine or factory to distributors to the field.
The specialization of the corn and soybean farms of the Midwest and the hog and
chicken mega-farms centralized in a few regions, such as Arkansas, the East
Coast’s Delmarva Peninsula, and North Carolina has created a unique situation.
The long-distance flows of nutrients from crop farms to animal farms require
the purchase of fertilizers on the crop farms; meanwhile, the animal farms are
overloaded with nutrients.
Of course, the very purpose of agriculture in
the
Nutrient Cycles and Flows
modern world—the growing of
food and fiber and the use of the products by people living away from the farm—results
in a loss of nutrients from the soil, even under the best possible management.
In addition, leaching losses of nutrients, such as calcium, magnesium, and
potassium, are accelerated by natural acidification, as well as by
acidification caused by the use of fertilizers. Soil minerals—especially in the
“young” soils of glaciated regions and in arid regions not subject to much
leaching—may supply lots of phosphorus, potassium, calcium, and magnesium and
many other nutrients. A soil with plentiful active organic matter also may
supply nutrients for a long time. Eventually, however, nutrients will need to
be applied to a continually cropped soil. Nitrogen is the only nutrient you can
“produce” on the farm—legumes and their bacteria working together can remove
nitrogen from the atmosphere and change it into forms that plants can use.
However, sooner or later you will need to apply some phosphorus or potassium,
even to the richest soils. If the farm is in a mixed crop-livestock system that
exports only animal products, it may take a long time to deplete a rich soil because so few nutrients per acre are exported with those products. For crop
farms, especially in humid regions, the depletion occurs more rapidly, because
more nutrients are exported per acre each year.
The issue eventually becomes
not whether nutrients will be imported onto the farm, but rather, what source
of nutrients, you should use. Will the nutrients brought onto the farm be commercial
fertilizers; traditional amendments (limestone); biologically fixed nitrogen;
imported feeds or minerals for livestock; organic materials such as manures,
composts, and sludges; or some combination of sources?
Three
Different Flow Patterns
There are three main
nutrient flow patterns, each one with implications for the long-term
functioning of the farm and the environment: Imports of nutrients maybe
less than exports, imports
maybe greater than exports, or imports may equal exports.
Imports are less than exports. For farms “living
off capital” and drawing down the supplies of
nutrients from minerals and organic matter, nutrient concentrations
continually decline. This can continue for a while, just like a person can live
off savings in a bank account until the money runs out. At some point, the
availability of one or more nutrients becomes so low that crop yields
decrease. If this condition is not remedied, the farm becomes less and less
able to produce food, and its economic condition will decline. This is clearly
not a desirable situation for either the farm or the country. Unfortunately,
the low productivity of much of Africa’s agricultural lands is partially caused
by this type of nutrient flow pattern, as increasing population pressure
elevated land-use intensity, and fertilizer prices are too high for poor
farmers. In previous times under the system of shifting cultivation, agricultural
fields would have been allowed to return to forest for 20 or more years, during
which time there would have been a replenishment of nutrients in the topsoil.
One of the greatest challenges of our era is to increase the fertility of the
soils of Africa, both by using fertilizers and by building up healthier soils.
Imports are
larger than exports. Animal farms with inadequate land bases to produce all needed feed
pose a different type of problem (figure 7.2c). As animal numbers increase
relative to the available cropland and pasture, larger purchases of feeds
(containing nutrients) are necessary. As this occurs, there is less land
available—relative to the nutrient loads—to spread manure. Ultimately, the
operation exceeds the capacity of the land to assimilate all the nutrients, and
pollution of ground and surface waters occurs. For example, in a study of New
York dairy farms, as animal density increased from around 1/4 of an animal unit
(1 AU = one 1,000-pound animal or a number of animals that together weigh
1,000 pounds) per acre to over 1 AU
per acre, the amount of N and P remaining on
farms increased greatly. When there was 1/4 AU per acre, imports and exports
were pretty much in balance. But at 1 AU per acre, around 150 pounds of N and
20 pounds of P remains on the farm per acre each year. The nutrient flow pattern
on farms with high animal densities— with large imports, mainly as feeds,
greatly exceeding exports—is not environmentally acceptable, although under
current conditions it may be more economical-
cal than a more balanced pattern. In addition,
some farmers, mainly organic ones, try to build up their soil organic matter
and nitrogen supply by annual applications of manure or compost. This also
causes an unacceptable buildup of nutrients in soils. In a survey from 2002
through 2004 of thirty-four organic farms from seven states in the Northeast,
encompassing 203 fields, it was found that approximately a third of the soils
had below-optimal levels of nutrients. However, about half of the fields were
found to have excessive levels of P. Other ways need to be found to add organic
matter through on-farm practices such as intensive use of cover crops and
rotations with perennial forages.
Imports and exports are close to balanced.
From the environmental
perspective and for the sake of long-term soil health, fertility should be
raised to—and then maintained at—optimal levels. The best way to keep desirable
levels once they are reached is to roughly balance inflows and outflows. Soil
tests can be very helpful in fine-tuning a fertility program and making sure
that levels are not building up too high or being drawn down too low (see
chapter 21). This can be a challenge and may not be economically possible for
all farms. This is easier to do on a mixed crop-livestock farm than on either a
crop farm or a livestock farm that depends significantly on imported feeds. As
discussed above, because such a high percentage of the nutrients in feeds are
excreted, animal products end up exporting relatively low amounts of nutrients
off the farm. So if all the feeds are farm grown, adding an animal enterprise
Nutrient Cycles and Flows
to a crop farm
will tend to lower the nutrient exports. In order to help balance nutrient
imports and
exports, routine soil tests should become a
part of every farm’s practices, because they will indicate whether nutrients
are being depleted or accumulating to higher levels than needed.
tend to be more
readily lost to the environment. Even midwestern U.S. cash grain farms that
have balanced nutrient imports and exports lose nutrients. Nitrogen-leaching
losses from these farms are having negative environmental effects on the
Mississippi River and Gulf of Mexico ecosystems
