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Chapter 1. Nutrient Cycling, Transformations, and Flows: Implications for a More Sustainable Agriculture

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)

VIII. Conclusions



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.



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-



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-



tries in temperate regions as well as developing nations in both the temperate and

tropical regions.





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



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.

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Chapter 1. Nutrient Cycling, Transformations, and Flows: Implications for a More Sustainable Agriculture

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