Agriculture Reference
In-Depth Information
Delmarva Peninsula (Delaware, Maryland and
Virginia) between 1988 and 2001, and the
median concentration of nitrate in groundwater
in this region exceeded the federal standard.
Excess N or P in water causes algae popula-
tions to grow rapidly, or to 'bloom'. The subse-
quent decomposition of the algae consumes
dissolved oxygen in the water. Lack of dissolved
oxygen is a major factor affecting the growth
and reproduction of fish, clams, crabs, oysters
and other aquatic animal life. An algae bloom
and subsequent decrease in dissolved oxygen is
known as eutrophication , and may be caused by
runoff or leaching of P or N from land when
application is in excess of crop requirements.
Nitrogen is considered the first limiting nutrient
for algae growth in saltwater systems, and P the
first limiting nutrient in freshwater systems.
A classic series of whole-lake experiments
established that P was the first limiting nutrient
for eutrophication of freshwater (Schindler, 1977).
Before this, small-scale bottle experiments had
suggested that carbon may be the major limiting
nutrient for eutrophication in some lakes, and
the role of N in freshwater systems was unclear.
But Schindler (1977) observed that phytoplank-
ton growth was not compromised in lakes when
less carbon was supplied by fertilizers; invasion
of atmospheric CO 2 was sufficient to support nor-
mal growth of phytoplankton. Also, the growth
of phytoplankton was not reduced when N sup-
plementation was restricted; fixation of N by
blue-green algae compensated. Instead, phyto-
plankton growth was proportional to total P
concentration. The author concluded that a
complex series of biological, geological and physi-
cal processes were adequate to maintain the N
and C concentration in natural waters but that
neither external nor internal mechanisms were
evident to correct P concentration required for
phytoplankton growth. This series of studies
established firmly that nutrient management
plans to limit eutrophication in freshwater sys-
tems should be based primarily on P.
areas to areas with a high concentration of ani-
mal agriculture. Livestock utilize N and P ineffi-
ciently, excreting 60-80% of that consumed.
Therefore, the majority of nutrients brought on
to the farm in feed stay on the farm rather than
being exported in meat, eggs or milk. Animal
manure is typically land-applied to supply nutri-
ents for crop growth, but application in excess
of crop needs results in nutrient losses and
contamination of groundwater, surface water or
air. More detail on manure application and sus-
tainable use of manure in animal agriculture is
given in Chapter 7, this volume.
Concentrated animal agriculture has been
identified as a significant source of N and P con-
tamination of surface water (median contri-
bution 6.8-48% of P export, and 5.2-23% of
N export depending on watershed; Smith and
Alexander, 2000). The relative importance of
different nutrient sources varies greatly in dif-
ferent regions. Animal agriculture is a minor
source of nutrient pollution in the populous
Northeast and Great Lakes regions of the USA,
for instance. In contrast, the Shenandoah Valley
of Virginia is an example of an area of intensive
animal agriculture associated with increa-
sed contamination of surface water. The
Shenandoah Valley has the highest population
of both dairy cattle and poultry in the state, and
as many as 20% of the dairy farms also have at
least one poultry house. Estimated manure
nutrient production in the Shenandoah Valley
exceeds crop requirements on a yearly basis.
Manure phosphate per acre of cropland
increased by 90% between 1978 and 1992, and
an analysis of soil tests in 2007-2010 indicated
that nearly 90% of samples were ranked 'high'
or 'very high' in P (Pease et al ., 1998). Although
there is controversy as to the threshold level of
soil test P that leads to P runoff, these soils clearly
do not need additional P.
The link between animal numbers, manure
application to a limited land area and P contami-
nation of surface water was also demonstrated
in the Lake Okeechobee watershed in Florida.
From 1973 to 1988, P concentration in the
water of Lake Okeechobee in Florida increased
by 250% (Negahban et al ., 1993). During this
same period, dairy cow numbers in the three
counties surrounding the lake increased by
more than 900 cows per year (Boggess et al .,
1997), and dairies were identified as the source
Nutrient Imbalance with Intensive
Animal Agriculture
Increased specialization and concentration of
livestock and crop production has led to the net
export of nutrients from major crop-producing
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