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Nutrient Cycles and Flows
Increasingly ... emphasis is being laid on the
direction of natural forces,
on the conservation of inherent richness, on the acquirement of
plant
food supplies from the air and subsoil.
J.L. Hills, C.H. Jones, and C. Cutler, 1908
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
4). Some farmers depend more on natural soil nutrient cycles
as contrasted with purchased commercial fertilizers to provide fertility
to plants. Is it really possible to depend forever on the natural
cycling of all the nutrients the crop needs? Let's first consider
what a cycle really is and how it differs from the other ways that
nutrients move from one location 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, 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 nutrients. Other flows are unplanned:
when nitrate is lost from the soil by leaching to groundwater 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, where 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.
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| Figure 7.1 The patterns of nutrient flows change
over time. From Magdoff et al., 1997. |
The first major break in
the cycling of nutrients occurred as cities developed and nutrients
began to routinely travel with the farm products to feed the growing
urban populations. Few nutrients now return to the soils that grew
them many miles away (figure 7.1b, 7.1c). The accumulated nutrients
in urban sewage have polluted waterways around the world. Even with
the building of many new sewage treatment plants in the 1970s and
1980s, effluent containing nutrients still flows into waterways, and
sewage sludges are not always handled in an environmentally sound
manner.
The trend
to farm specialization 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 at the same time that 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 are exclusively growing 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 common, however, because a large portion of the crop is
usually exported off the farm. For example, an acre of a good crop
of tomatoes or onions usually contains over 100 lbs. of nitrogen,
20 lbs. of phosphorus, and 100 lbs. of potassium. For agronomic crops,
the annual exports of nutrients is about 100 lbs. of nitrogen, 6 lbs.
of phosphorus, and 50 lbs. of potassium per acre for corn grain and
about 150 lbs. of nitrogen, 20 lbs. of phosphorus and 130 lbs. of
potassium per acre for grass hay.
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| Figure 7.2 Nutrient flows and cycles on a) crop
and b) dairy farms. |
It should be fairly easy to balance inflows and outflows 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 19).
A contrasting
situation occurs on dairy farms, if all of the forage is produced
on the farm, but grains and minerals are purchased (figure 7.2b).
In this situation, there are more sources for the nutrients coming
onto the farm with feeds and minerals for animal consumption usually
a larger source than fertilizers. Most of the nutrients consumed by
animals end up in the manure 60 to over 90 percent of the nitrogen,
phosphorus, and potassium. Compared with crop farms, more nutrients
flow onto many dairy farms and fewer flow off per acre. Under this
situation, nutrients will accumulate on the farm and may eventually
cause environmental harm from excess nitrogen or phosphorus.
Two different
nutrient flows occur when manure on livestock farms is applied to
the fields used for growing the feeds. The nutrients in the manure
that came from farm-grown feed sources are completing a true cycle.
The nutrients in the manure that 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 or mine and are now just being transported from the barn to the
field.
Animal operations
that import all feeds and that 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 practice.
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. The relatively
low amounts of nutrients exported per acre from animal products makes
it easier to rely on nutrient cycling on a mixed livestock-crop farm
that produces most of its feed, than on a farm growing only crops.
IMPLICATIONS
OF NUTRIENT FLOW PATTERNS
Long distance
transportation of nutrients is central to the way in which 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 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 Delmarva Peninsula and North Carolina, has created a unique situation.
The long distance flows of nutrients from crop farms to animal farms
requires 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 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 very 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, with
each one having implications for the long-term functioning of the
farm. Imports of nutrients may be less than exports, imports may
be greater than exports, and 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 awhile, just like a person can continue
to 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.
Imports are much larger than exports. Animal
farms with inadequate land bases pose a different type of problem.
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. This pattern of nutrient flow is not
environmentally acceptable. However, under current conditions, it
may be more economical than a more balanced pattern.
Imports and exports are close to balance. 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 to fine-tune a fertility program and make sure that levels
are not building up too high or being drawn down too low (see chapter
19). 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.
Sources
Magdoff, F., L. Lanyon, and W. Liebhardt. 1997. Nutrient cycling,
transformations, and flows: Implications for a more sustainable
agriculture. Advances in Agronomy 60: 173.
Magdoff, F., L. Lanyon, and W. Liebhardt. 1998. Sustainable
Nutrient Management: A Role for Everyone. Northeast Region Sustainable
Agriculture Research and Education Program. Burlington, VT.
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