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VII. Promoting a More Sustainable Agriculture through Changes Influencing Nutrient Cycles and Flows

VII. Promoting a More Sustainable Agriculture through Changes Influencing Nutrient Cycles and Flows

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NUTRIENT CYCLING, TRANSFORMATIONS, AND FLOWS



57



fuels to manufacture fertilizers or other amendments and to transport fertilizers

and feeds over long distances. With the fertilizer industry (especially N) so intimately tied to the price of energy, a sharp increase in the cost of fuel will make

agriculture especially vulnerable. Ways also need to be found to reduce nutrient

pollution of surface and groundwater. Enhanced efficiency of nutrient cycling and

uptake by plants can increase individual cash crop farm profitability by decreasing the amount of purchased inputs.

Finally, although there are large reserves of K- and P-bearing mineral deposits,

the pace of mining these nonrenewable deposits may increase, thus decreasing the

time needed to exhaust the deposits. This indicates that the nutrients should be used

carefully and efficiently.

Changes in agriculture and society that are related to nutrient flows and that

might promote a more sustainable system include those that can happen quickly

as well as those that will take longer to accomplish (Fig. 9). Some of these suggestions appear to hold more promise than others. In general, short-term changes

help to “tighten up” the efficiency of the use of nutrient sources and the cycling of

nutrients in the field setting. Those changes that might occur at the farm level will

take longer to put into effect but will have a more long-lasting impact. However,

farm-based changes may not turn out to be appropriate if the scale of the problem,

such as the transfer of nutrients between specialized farms, is not addressed. Fi-



centuries

8d



7

decades



8a 8b

8c



8



b



4c 6 6



years



4a



2



current

field season



Ib

la



3



4b



field



farm



1c



I



plantkoil



regionhation



earth



Spatial Scale

Figure 9 Relationship between spatial scale of possible changes affecting nutrient flows and estimated time needed to complete changes (see text for explanation of numbers).



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FRED MAGDOFF ETAL.



nally, long-term changes at the societal level are those that will eventually have

the greatest potential impact on the development of sustainable nutrient flows.

These changes are also the most difficult to ultimately carry out. A variety of possible changes that might impact nutrient flows are discussed in the following sections.



A. FIELD-LEVEL

CHANGES

(SHORTTERM)

1. Enhance the Uptake Efficiency of Already Available Nutrients in

the Soil

This promotes cycling by reducing nutrient loss and the flow of added nutrients

from off the farm, might reduce costs, and may also maintain environmental quality.

a. Increase the Degree of Synchronization between Nutrient Availability and

Uptake Needs of Plants

The idea behind this approach is to decrease the amount of time when there are

large quantities of available nutrients present (mainly N) and thereby decrease the

potential loss of nutrients by leaching or runoff or through gaseous loss. Poor synchronization resulting from the application of fertilizers applied long in advance

of crop needs may lead to higher amounts of fertilizer use. Also, when well-drained

soils rich in organic matter are intensively worked, high amounts of nitrate are produced (Magdoff, 1991a,b). If the crop being grown does not have a high N need,

much of the NO;-N that accumulates cannot be used and is subject to leaching

and denitrification losses.

Greater synchronization between supply and demand of nutrients decreases the

opportunity for loss to the environment and allows nutrients to be taken up by

plants more efficiently. Nutrients have greater potential for participating in a true

soil+plant+soil, soil+plant+human+soil, or soil+plant+animal-+human+

soil cycle when taken up by plants than when lost from the soil as runoff or leaching water or as gaseous forms. Along these lines, there is interest in planting of

spring wheat instead of winter wheat on northern plains to take advantage of better synchronization between the supply and demand for available nutrients.

Reducing the intensity of tillage operations will also reduce the burst of mineralization that follows. In addition, keeping the soil more evenly moist during the

growing season may reduce minibursts of mineralization that occur after rewetting a dry soil. Mulches (organic residues or plastic) are commonly used in vegetable crop and small fruit production and the surface of no-till soils usually will

have plentiful residues, thus helping reduce evaporation from the soil surface. Although higher moisture soils may have less SOM mineralization than soils that dry



NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS



59



down and then rewet, NOT-N may be more prone to denitrification in the moist

soil. However, more available C for microbes to use following rewetting also allows for a burst of denitrification in alternately dry and wet soils.

b. Optimize the Root Environment

By optimizing conditions such as soil structure or pH, enhanced root health and

exploration may help plants take up nutrients more efficiently. Soil biological activities produce a multitude of active plant hormones (Frankenberger and Arshad,

1995). Humic materials have been shown to promote root growth and branching

(Lee and Bartlett, 1976). Also, practices that enhance mycorrhizal associations

such as using cover crops will increase the ability of plants to extract available nutrients from the soil. Using diverse residue sources to maintain high levels of SOM

may reduce soil pathogens and other pests.

However, if root systems are healthier and explore a greater portion of the soil,

native stocks will be drawn down quicker than would have occurred otherwise.

Thus, although this may help from an environmental point of view and reduce the

need for imported nutrients from off the farm and enhance profitability in the short

run, it only delays the need for nutrient inputs.

c. Use Cover Crops

A vigorous grass cover crop such as winter rye when the soil would otherwise

be bare (fall, winter, and early spring for temperate region summer annuals) will

minimize leaching losses of N (Meisinger et al., 1991) and runoff losses of both

available and unavailable N and P.



2. Increase the Availability of Soil Nutrients (from SOM, Newly

Added Residues, or Soil Minerals)

By more reliance on the soil to supply nutrients, there will be less need to use

commercial fertilizers or other nutrients from off the farm.

Emphasizing practices that result in building up and maintaining high levels of

SOM will promote the more efficient use of nutrients. Use of cover crops, reduced

tillage, and rotations that include sod-type crops are practices that enhance soil

structure and thereby reduce loss of nutrients by both runoff and leaching. The use

of cover crops also helps to reduce loss of inorganic N and can enhance myccorhizal development in the following crop, thus helping the following crop obtain needed nutrients. In addition, active microbial populations help make P and

micronutrients more available to plants.

It must be kept in mind that if large quantities of nutrients are removed from

soils in harvested crops and not returned (as manure or other residues), this strategy plus the ones discussed under Section VII,A, 1 must cause a decrease in total

stocks of the available nutrients. Thus, although for a period of time (which might



60



FRED MAGDOFF ET AL.



be quite long or short) it will be possible to replace fertilizer nutrients with those

derived from soil, this strategy of more efficient exploitation of soil nutrients cannot go on forever.



3. Utilize Fertilizers and Amendments More Efficiently

If fertilizers and amendments are used more efficiently, application rates can be

decreased.

a. Use Manures More Efficiently

Available on-farm sources of nutrients such as manures should be distributed

according to soil/crop needs. This best management practice calls for reliance on

regular soil testing and on making sure that manures are applied where they are

needed and not just where it might be most convenient to put them. This will help

build up nutrients in depleted fields and not oversupply nutrients on fields with

large amounts of available nutrients.

Other best management practices include better timing of applications, more

rapid incorporation, and recommendation systems that take better account of

(credit) nutrients in manures.

b. Better Use of Synthetic Fertilizers

Better soil and tissue tests and/or recommendations based on tests are needed to

ensure that only the amounts of fertilizers needed for crop production are actually applied (Beringer, 1985). Most routinely used soil tests do not adequately account for contributions from organic matter mineralization (Beringer, 1985).

As discussed previously, better timing and methods of fertilizer applications

may enhance the percentage of fertilizer nutrients that are taken up by crops.



B. FARM-LEVEL

CHANGES

(MJZDIUM

TERM)

1. Use Nutrient Sources That Are More Efficiently Taken Up and/or

Require Lower Energy Inputs Instead of Synthetic Fertilizers

a. Rely Primarily on N, Fixation by Symbiotic and Nonsymbiotic Organisms

This will allow the substitution of biologically fixed N for the energy-intensive

chemical fixation process, as well as decreasing transportation and application energy and machinery requirements. This is also the only example of a soil biological process that results in increasing the total stock of a nutrient. However, it is the

forage legumes that are the best in terms of fixing N, and contributing available N

to following crops. Perennials such as alfalfa can be grown as an economic crop

in a rotation, whereas annuals such as hairy vetch can be used as a cover crop.



NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS



61



However, if there are insufficient animals on the farm or nearby farms, the use of

perennial legumes may not be profitable.

b. Use Only Sparingly Soluble Nutrient Sources

One of the suggestions for enhancing nutrient cycling efficiency is to rely exclusively on sources of applied nutrients that are slowly available, such as composts and partially processed rock minerals. This will keep losses from the available stock lower than when large amounts of available nutrients are used and could

potentially allow nutrients to be used more efficiently.

During the period of high plant demand mineralization and/or dissolution of minerals must proceed rapidly enough to keep up with plant needs. Thus, to make sure

that crop production does not suffer when first using sparingly soluble sources, it

will be necessary to build up total stocks to higher levels than would be needed with

more soluble sources. It must also be kept in mind that many of these low-solubility materials, such as rock phosphate and greensand, travel significant distances and

require energy for application. Thus, although processing rock phosphate requires

only 9 GJ tonp1 P compared to the 22 GJ ton-' P for producing concentrated super phosphate, they both consume an additional 22 GJ ton- I P for packaging, transportation, and application (Boswell et al., 1985). Using such partially processed

mineral sources may decrease field losses and may require less energy for production but may not be contributing to cycling as a closed-loop pattern of flow in any

meaningful sense, except that losses may be decreased somewhat.

c. Utilizing Nearby Nutrient Flows for Agriculture

The agriculture of a number of societies has taken advantage of nearby flow nutrients. The Aztecs, as well as other indigenous peoples of Central and South America, and people in parts of China built mounds for cropping in naturally swampy

regions or in shallow lakes. The fertility for these raised bed chinampas came primarily from the sediments used for building and maintaining the mounds, from

weeds harvested from the water, and from the water itself as it flowed from upstream into and through the system. Fish were commonly produced in the water

and a mixture of crops was grown on the beds. Animals raised on crop residues

also provided some nutrients to cycle back onto the beds. Other important early

agricultural systems, such as those along the Nile and the Euphrates, relied on another type of nutrient flow-sediments from upriver. The rich alluvial soils were

replenished by the annual floodwaters. From the point of view of nutrients, although these systems relied on upriver soil erosion, they may have been able to

function for a very long time. However, salinization resulting from insufficient internal soil drainage is believed to have had a serious adverse influence on productivity. In modern times, the building of dams to reduce flooding and to gain better

control over water distribution has greatly decreased the amounts of sediments

available for adding nutrients to many alluvial soils around the world.



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FRED MAGDOFF ETAL.



2. Change Miu of Crops or the Animal-Crop Mix on a Particular Farm

so That Fewer Nutrients Are Exported

Because the export (sale) of some crops removes a particularly large amount of

nutrients, it is possible to change the mix of crops grown on a farm and place more

emphasis on those that result in smaller quantities of nutrient removal. In addition,

increasing the importance of animal enterprises on the farm relative to crop sales

will result in less nutrient loss in agricultural products. However, economic barriers and personal preferences of farmers make it difficult for selection of the farm

enterprise mix to be made only on the basis of nutrient management issues.



3. Change Animal Feeds to Promote More Efficient Use of Nutrients

If ways can be found to make the nutrients in the feeds more available to the animals, less N and P will be required in the diet, and thus less N and P will occur in

the manure. The use of an enzyme that increases P availability and balancing the

correct proportions of essential amino acids in feeds can reduce the total amount

of P and N in feeds (Cromwell et al., 1993; Chase, 1994). Because these approaches only deal with a small part of the problem, they are likely to provide only

incremental improvements.



C. SOCIETAL-LEVEL

CHANGES(LONGTERM)

1. Stop Using Practices or Growing Crops That Promote Large

Nutrient Leaks and Wasteful Use of Soil Resources

a. Promote the Philosophy of Eating “Lower” on the Food Chain

This is a euphemism for eliminating or greatly reducing consumption of animal

products. The interest in this approach is because of (i) the belief that animals are

treated inhumanely in conventional agriculture, (ii) the belief that consumption of

large quantities of animal products are potentially harmful, and (iii) the concern

for using resources to more efficiently supply food to hungry people.

If a significant portion of the U.S. population started eating fewer animal products, there should be a decreased demand for feed grains. Thus, less land would be

planted for crops such as corn and soybeans. The way these crops are currently

grown tends to promote especially large leaks due to inefficiencies in nutrient

flows and to breakdown of nutrient cycles. Because less hayland would be needed, the total area in crop production would be decreased even more. There would

also be fewer large animal production facilities, which can cause considerable environmental damage resulting from the concentration of animal wastes. The mas-



NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS



63



sive interregional transfer of nutrients for the production and utilization of grains

for animals would be dramatically decreased.

However, there are a number of potential negative side effects to the reduction

of animal products in the diet as well as countertrends that may offset potential

benefits. Most important, the market for feed grains is now global and trends in

many developing countries such as China are toward more animal products in the

diet. Thus, decreased internal use of feed grains may be countered by increased

exports abroad. The remaining livestock enterprises in the United States may be

of even larger size than the enormous units that currently exist. The reduction of

hayland that would occur with less animal production also removes an important

practice for build up of soil organic matter and N as well as improvement of soil

structure. There would also need to be at least some increase in the area devoted

to production of grains, vegetables, and fruits for humans that substitute plant for

animal products (and, thus, not as large an area removed from production as would

be estimated from the reduced use of animal products). Exporting crops from individual farms (for direct human consumption in this scenario) also entails a much

larger loss of nutrients than export of animal products from cropanimal farms. In

addition, as opportunities shrink in animal production and less farmland is used

for animal feeds, the numbers of farmers would probably decrease.

The net effect of decreasing the extent of animal agriculture may be to decrease

overall nutrient flows (assuming increased feed grain exports are less than the decreased use in the United States), although nutrient loss from an individual crop

farm may be larger than would have occurred on an integrated croplivestock

farm. Thus, even if there is a widespread movement toward eating lower on the

food chain, it may be important to enhance the possibility of nutrient cycling by

promoting ways to lessen distances between production and consumption of human food crops.

Another similar idea is to promote production systems in which ruminants eat

lower on the food chain with a decreased amount of grain and an increased amount

of perennial forage crops in their diets. This would diminish the flow of nutrients

for production and utilization of feed grains, promote the utilization of perennial

forages, and may also lead to decreased numbers of very large production facilities. This change could occur whether or not large numbers of people choose to

follow new diets. However, it may take longer and be more costly to bring animal

products to market with reduced feed grain inputs.



2. Overcoming the Physical Separation of the Plant-Animal-Human

Trophic Pyramid Segments

When consumption of food by farm animals or humans is in relatively close

proximity to where the crops are grown there is a great potential for true cycling

of nutrients.



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FRED MAGDOFF ETAL.



a. Integrate (or Reintegrate) Animals into Many Cropping Systems

When crops leave a farm, normally a relatively high percentage of the nutrients

in the aboveground biomass are exported. The extreme situation occurs with crops

such as hay and corn silage, but for grain crops such as corn and soybeans a high

percentage of the N and P,although less than half of the K, in the aboveground

biomass is still exported off the farm. By raising animals on farms that produce

the bulk of their feeds, true cycling of nutrients from animal agriculture becomes

more feasible. In an integrated livestock and crop farm, fewer nutrients are exported than on strictly crop farms. Because a high percentage of the nutrients eaten by animals are excreted in urine and feces (Azevedo and Stout, 1974), only approximately 1 ~ 3 0 %

of nutrients in crops harvested on integrated animal-crop

farms end up in exported animal products, with the remainder available for cycling

back onto cropland. Thus, there is less depletion of nutrient stocks in animal-based

systems than in cash grain or cash forage systems.

Another reason for the proposal to reintegrate animal and crop production is to

give more rotation options to crop farmers. Without animals present on the farm

or on nearby farms, there is little incentive to put land into legume or grass-legume

hay. Growing sod crops help make nutrient cycles operate more efficiently because

they help to maintain or increase organic matter (and hence CEC) and promote better soil structure and reduce loss of nutrient stocks in leaching waters or as eroded sediments. Using a forage legume as part of the rotation also helps to build

stocks of soil N. Use of manures also helps to build up and maintain soil structure

and CEC.

Although there will always be niche local markets for animals produced on

small- to medium-sized farms, reversing the national trend to concentrated animal

production units and reintegrating animal and crop farms over large areas will require major national political and economic changes.

b. Consume Mainly Locally Produced Food

Increased reliance on locally produced food has been proposed as a means to

better support regional farmers and, it is hoped, create a more viable agriculture that

relies less on long-distance shipping to markets. In addition, it will be easier to

“close the loop” by returning nutrients to the farms if more products are sold locally

and in return local organic waste materials are applied to farmland. Regardless of

the extent that locally grown food becomes a greater part of people’s diets, there

will always be the need to import certain foods produced only in other regions.

c. Transport Nutrients from Urban Areas (Contained in such Materials as

Leaves and Sewage Sludge) and from Livestock Production Facilities “Back” onto

Farmland

This is suggested because it will help build up andor maintain SOM and at the

same time help municipalities deal with waste materials. It is also a way of di-



NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS



65



verting nutrients from the solid or liquid waste stream and promoting their flow to

cycle back to agricultural land. In most cases this would entail long-distance transport of wastes. However, because of the significant costs (and energy use) associated with this practice as well as the concern for toxic material accumulation when

sludges are used, long-distance waste transport is probably not a sustainable solution to these problems.

d. Reduce the Number of People Living in Cities and Suburbs and Encourage

Living in Smaller Communities Close to Farmland

As mentioned previously, the concentration of large numbers of people into urban areas creates an overabundance of nutrients in these areas while nutrient-exporting crop farmers must rely heavily on commercial fertilizers to maintain soil

fertility. However, the problems associated with nutrient cycling are but one aspect of the issues arising from concentrating large numbers of people in cities and

their associated sprawling suburban developments. These other problems include

excessive reliance on personal automobiles for transportation (wasting energy and

construction materials and labor for highways, gas stations, auto construction,

etc.), concentrating social problems in urban areas, etc. By promoting the revitalization of small towns and encouraging a reverse migration back to rural communities, nutrient cycling problems resulting from the separation of people from the

land can be more easily resolved.



WII. CONCLUSIONS

The purpose of agriculture is to provide food and fiber to people, and when people live removed from the land a flow of nutrients off the farm in the form of harvested products is unavoidable. Cycling from plant residue or animal or human

wastes back to soils can never be 100%efficient, even under the most favorable

conditions. Thus, external sources of nutrients will, over the long run, continue to

be an essential part of agriculture.

The efficiency of nutrient use can be controlled through on-farm decisions and

changes in soil and crop management will certainly help tighten up the nutrient cycle and build stocks of nutrients to optimal levels. This can foster a decreased reliance on imported nutrients onto farms and reduce environmental damage and resource depletion caused by excess nutrients as well as reduce the use of fossil fuel.

However, the control of nutrient flow patterns lies beyond the farm boundaries.

Thus, promoting long-term sustainable nutrient management will ultimately require radical changes in the way agriculture and society are organized. These include reintegration of livestock and crop farms and the encouragement of closer

physical association of people with farmland. Establishing and implementing per-



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FRED MAGDOFF ETAL.



formance expectations for contemporary patterns of nutrient flow is especially difficult because the scale of transactions is broad and control of most aspects is far

removed from the farm level. As long as the nutrient cycle is perceived to be only

within the boundaries of a farm and the responsibility for its management exclusively within the purview of the farmer, there will be little progress in substantive

modification and movement toward a sustainable nutrient flow pattern.



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