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Chapter 1. Nutrient Cycling, Transformations, and Flows: Implications for a More Sustainable Agriculture
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VII. Promoting a More Sustainable Agriculture through Changes Influencing
Nutrient Cycles and Flows
A. Field-Level Changes (Short Term)
B. Farm-Level Changes (Medium Term)
C. Societal-Level Changes (Long Term)
The many economic, environmental, and social problems associated with conventional agriculture have elicited calls for new approaches to agricultural science
as well as practices at the farm level. It is suggested that by relying on ecologically sound principles it will be possible to develop practices that enhance the economic viability of agriculture while at the same time helping to improve environmental quality (MacRae et al., 1990).
Among the environmental problems associated with conventional agricultural
practices are a number related to nutrient management. The most pressing of these
include pollution of groundwater with nitrates and surface water with both nitrates
and phosphates. Nutrients from agricultural activities have decreased drinking water quality as well as the usefulness of fresh water and estuaries for recreation and
commercial fisheries. This decline of water quality is caused by leakages from
farms that, although not desired, appear to be an integral part of conventional agricultural practices.
Part of the explanation for the large quantity of nutrients lost to leaching and
runoff waters is the use of more fertilizers and manures than are actually needed
by crops. For example, it has been estimated that farmers in the Midwest have used
about one-third more N fertilizer than actually needed (Swoboda, 1990). One of
the reasons for the overuse of nutrients may be insufficiently precise soil test and
fertilizerhanure recommendation systems. Other explanations for nutrient
overuse include insufficient available cropland area to properly utilize nutrients
from animal production facilities and the use of “rule of thumb” guidelines by
many farmers instead of regularly testing soils or plant tissue to determine nutrient needs. In addition, the heavy reliance on the readily available (soluble) nutrients in commercial fertilizers as well as in many manures may enhance nutrient
loss from soils by leaching and runoff compared to amounts lost from less soluble
sources. Finally, the decreased soil tilth associated with various crop and soil management practices can result in loss of large amounts of runoff, carrying with it
dissolved nutrients and eroded sediments.
The loss of nutrients from soils can also have significant economic consequence.
NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS
Any use of fertilizers above the economic optimum, where the value of increased
yields just balances the extra cost of applying an increment of fertilizer, is a direct
economic loss to farmers while at the same time it greatly increases the risk of pollution. This is especially important for low-value per hectare agronomic crops,
where the cost of fertilizer is a significant portion of input expenditures and the
margin between costs of production and crop value is very narrow. For example,
for high yielding corn and wheat the estimated expenditures of fertilizers and lime
in Michigan are approximately 18% of the crops’ value (including deficiency payments) and 33 and 44% of the costs of growing the crops, respectively (excluding
depreciation, insurance, rent, taxes, interest, and family labor) (Nott et al., 1995).
In contrast, similar data for bearing semi-dwarf apples for fertilizer and lime are
approximately 1 % of the crop’s value and 2% of the costs. Therefore, although a
little extra fertilizer above the economic optimum applied to an apple orchard will
have minimal effects on economic returns, the situation is very different for agronomic crops. For low-value per hectare crops, it is especially critical to ensure that
as little fertilizer as possible is used over that needed for maximum economic return.
There are also other nutrient management issues that potentially influence the
long-term sustainability of agriculture. Reliance on large amounts of energy to produce fertilizers, especially N, and to transport them significant distances to farms
as well as crops to animals and food to people depends on ready availability and
relatively low-cost fossil fuels. Also, runoff from agricultural land tends to carry
surface sediments that are enriched in organic matter in addition to readily available nutrients. This loss of organic matter, which may contribute to pollution of
surface waters, also decreases soil quality and long-term productivity. Erosion of
organic matter-enriched surface soil decreases the tilth as well as the fertility of
soil, decreasing water infiltration and storage for plant use and leading to more
The development of the synthetic fertilizer industry, which began in the 19th
century and vastly expanded during the post-WW I1 era, allowed agriculture to
avoid many of the obvious consequences of depleting the natural fertility of soils.
The introduction of low-cost N fertilizers also permitted the elimination of forage
legumes from rotations on many farms and lead to increased farm specialization
such as continual cultivation to grain crops. However, as soil organic matter
(SOM) was depleted, other problems developed such as decreased soil tilth, increased soil erosion, lower soil water holding capacity, decreased buffering with
respect to pH and nutrient availability, increasing plant pest problems, etc. (Magdoff, 1993). In response to these many problems as well as other powerful forces
and trends, practices and grower outlook developed during the last half of the 20th
century so that agriculture is now treated in a manner that mimics industry. Plant
and animal outputs of agriculture are thought of in almost the same way as nonbiological industrial products that require “assembling” by using various external in-
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puts such as synthetic fertilizers, pesticides, irrigation, fuel, equipment, feeds, and
As cities have grown more numerous and larger and an agriculture has developed that relies on specialized production of crops and animals and high application rates of readily available nutrients from synthetic fertilizers as well as manures, there has been a dramatic increase in the magnitude of problems resulting
from flows of nutrients that end up in surface and subsurface waters and in the air.
It is now clear that the economic and environmental impact of these nutrient
management issues is so large that a reevaluation of nutrient flows and cycles is
critical to the successful development of sustainable agricultural systems. Agriculture is practiced along a broad continuum of possibilities with farmers following many different practices and philosophies. Sustainability refers to agriculture
that is viable for a long period. It implies economic, environmental, and social
components that interact to a high degree and are not mutually exclusive. Because
humans have such a large impact on the globe, the social or human component of
agriculture is very important to the subject of nutrient cycling. Some current agricultural practices and ways in which agriculture and the rest of society interact appear to be sustainable; others do not. “Sustainability” is not a formula or a recipe;
rather, it may be more of a direction toward a “moving target” because society and
the earth are constantly changing. What may be considered sustainable at one time
may or may not be considered sustainable at another as new information is evaluated. Conventional agriculture is dependent on large quantities of synthetic chemical, capital, energy, and machinery inputs. It largely follows the theme of manipulation of nature-changing nature to suit humankind. Sustainable agriculture
practitioners attempt to work with natural systems as much as possible. They endeavor to develop economically and environmentally sound practices and reduce
depletion of nonrenewable resources. At the same time they strive to enhance their
quality of life, as well as that for rural communities and society as a whole.
This review will discuss characteristics of current nutrient flows, some of the
concerns about the condition of nutrient cycles in contemporary agriculture, and
opportunities for nutrient cycling in sustainable agriculture. We will view these issues at different geographic scales, including the soil-plant, field, farm, watershed,
regional, and global levels. We will also discuss features of nutrient cycles that influence the relationships of agriculture and society. As the character of nutrient
flows is evaluated and modified in the future, changes are likely to have implications for the nonfarm segment of society as well as on-farm practices. Thus, it is
important for nonfarm citizens to become familiar with features of nutrient cycles
that influence the relationship of agriculture to society. It may well be possible to
significantly “tighten-up’’ nutrient cycles and make them function more efficiently in individual soils or on the farm as a whole. This is a challenge for agriculture
and society. Although we will focus most of our attention on the conditions in the
United States, much of the discussion will be relevant to other developed coun-
NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS
tries in temperate regions as well as developing nations in both the temperate and
II. FRAMEWORK FOR EVALUATING
The flow of energy in an ecosystem can be represented by a pyramid with those
species higher on the pyramid consuming organisms or residues below. A simple
trophic pyramid involving plants at the base, providing all the primary products,
and humans at the top can be used to demonstrate connections within a system of
food production and consumption. The energy of sunlight captured and the nutrients taken up by plants flow upward in the pyramid as the products of plants are
consumed and utilized. Trophic pyramid diagrams can be used to highlight differences over time in the spatial connections between plants, animals, and humans
and indicate the potential for nutrient cycling and maintenance of soil nutrient levels or stocks. What follows are generalized abstractions of complex processes and
relationships that do not apply equally to all current or historical situations but help
to highlight major trends over time.
It is thought that for most of human history people lived in small bands that wandered over extensive territories as they spread out and eventually populated much
of the earth’s land area. As populations increased and became more sedentary,
preagricultural hunters and gatherers brought plants and animals back to villages
and dwellings and there was some spatial separation between humans and their
food sources. There was little possibility for return of nutrients to soils from where
they came except that animals would cycle nutrients in urine and manure as they
fed themselves prior to capture. However, because there were small numbers of
people relative to the territories being exploited for food and they constantly
changed the areas being used, effects on nutrient flows were probably small.
During the early stages of agriculture when crops were produced near dwellings
and animals were raised by seminomadic herding there was more potential for nutrient cycling. Animal manures were deposited as the animals grazed as before, but
crop and animal remains were now in or near fields. It was during this stage of development when a wave of episodes of erosion occurred, such as the one in Greece
and the Middle East, as a result of hillside deforestation and subsequent grazing
and cropping (Runnels, 1995; Hillel, 1991).This resulted in a massive transfer of
nutrients and soil from hills and mountains to valley floors.
It has been argued that the agricultural changes that occurred in medieval Europe were an essential precursor to the industrial revolution. The diversification of
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crops through the raising of forages, especially N,-fixing clovers, allowed continuous cropping to take the place of the alternate year or every third year fallow systems (Bairoch, 1973). It also permitted the integration of livestock into cropping
systems and ended nomadic husbandry. The enhanced productivity of the land allowed a significant increase in the annual agricultural production over the needs
for farm family consumption (Bairoch, 1973). Although the industrial revolution
began in England during the last half of the 18th century, it reached other countries in Europe and the United States only during the 19th century. Through much
of the 19th century, and well into the 20th century in pockets, most agricultural
products were consumed on the farm where produced. This was a common feature
of temperate region agriculture in what eventually became the advanced economically developed countries. In the less developed temperate and tropical regions,
with the important exception of plantation crops such as sugar and bananas, subsistence farming has been common through much of the 20th century, with only
small amounts of products exported off the farm.
In the diversified subsistence farming systems that developed in Europe and the
United States before the industrial revolution, most of the plant products were either consumed directly by people on the land or were consumed by animals that
were then consumed by humans (Fig. la). In this example the three parts of the
pyramid are physically connected and residues and waste products can easily return to the land.
The development of large cities and transportation systems to move food long
distances in the United States and the industrializing countries of northern Europe
created the first modern widespread physical break in the production-consumption chain. Crops and animal products were sent from the countryside to urban areas and even to other countries, decreasing the potential for on-farm nutrient cycling (Fig. 1b). In the last half of the 20th century, rapid urbanization has also been
occurring in most developing countries (usually without commensurate economic development), and this, together with the development of an “advanced” commercial agricultural sector oriented toward exports, has also had a significant negative impact on nutrient flows in those countries. Concern about the consequences
of interrupting the cycling of nutrients was expressed in the last century:
Capitalist production, by collecting the population in great centers, and causing
an ever increasing preponderance of town population . . . disturbs the circulation of matter between man and the soil, i.e., prevents the return to the soil of
its elements consumed by man in the form of food and clothing; it therefore violates the conditions necessary to lasting fertility of the soil.’’ (Marx, 1887; originally published in German in 1867)
Another physical break in the trophic pyramid resulted from the transformation
of animal agriculture based on small diversified farms to large specialized production units separated by long distances from the farms that produce feeds (Fig.