Advances in agronomy volume 60

V O L U M6 0E

8.
.

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Advisory Board
Martin Alexander

Eugene J. Kamprath

Cornell University

North Carolina State University

Kenneth J. Frey

Larry P.Wilding

Iowa State University

Texas A&M University

Prepared in cooperation with the

American Society of Agronomy Monographs Committee
William T. Frankenberger, Jr., Chaimnan
P. S. Baenziger

David H. Kral
Dennis E. Rolston
Jon Bartels
Sarah E. Lingle
Diane E. Storr
Jerry M. Bigham
Kenneth J. Moore
Joseph W. Stucki
M. B. Kirkham
Gary A. Peterson


DVANCES IN

Edited by

Donald L. Sparks
Department of Plant and Soil Sciences
University of Delaware
Newark, Delaware

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Contents
CONTRIBUTORS
...........................................
PREFACE
.................................................

vii
u

NUTRIENT
C Y C L ~ TRANSFORMATIONS.
G.
AND FLOWS:
IMPLICATIONS
FOR A MORESUSTATNABLE
AGRICULTURE
I.

I1.
111.

Fred Magdoff. Les Lanyon. and Bill Liebhardt
Introduction ..............................................
Framework for Evaluating Nutrient Dynamics . . . . . . . . . . . . . . . . . .
Soil-Plant System .........................................
Cycling and Flows a t the Field Level ..........................
Farm-Scale Cycling and Flows ...............................
Watershed, Regional, and Global Issues ........................

Iv.
v;
VI.
VII. Promoting a More Sustainable Agriculture through Changes

Influencing Nutrient Cycles and Flows ........................
VIII . Conclusions ..............................................
References ...............................................

2

5
13
23
38
47

56
65

66

ADAPTATIONOF PLANTSTO SALINITY
Michael C. Shannon

I . Invoduction ..............................................
I1. Rationale for Breeding for Salt Tolerance ......................

III. Selection for Salt Tolerance ..................................
IV Salt Tolerance Mechanisms ..................................

v. Genetic Variability .........................................
VI . Breeding Methods .........................................
VII . Novel Concepts. ..........................................
VIII . Summary and Conclusions ..................................
References ...............................................

V

76
77

78
84
88
101
105
107
108

vi

CONTENTS

INFLUENCEOF NO-TILL
CROPPINGSYSTEMS
ON MICROBIAL
RELATIONSHIPS

L . F. Elliot and D .E. Stott
I . Introduction ..............................................

I1. Decomposition of Surf-ace-Managed Crop Residues . . . . . . . . . . . . . .
I11. Modeling Crop Residue Decomposition .......................
n? Root-Microbial Relationships................................
v. Deleterious Rhizobacteria for Weed Control ....................
VI . Low-Input. On-Farm Composting ............................
References ...............................................

121
122
125
129
137
141
144

PRACTICAL
ETHICSIN AGRONOMICRESEARCH

Don Holt
I . Introduction ..............................................
I1. Basic Concepts ............................................
I11. Ethics of Choosing Research Subject Matter ....................
rv. Difficulties with the Utilitarian Approach ......................
v. Agricultural Ethics and the World Food Situation . . . . . . . . . . . . . . . .
VI. Ethics in the Conduct of Research ............................
VII . Ethics in Research Administration ............................
References ...............................................

150
151
154
158
162
165
184
190

AREAGROECOSYSTEMS
SUSTAINABLE
IN SEMIARID
REGIONS?

B . A. Stewart and C. A. Robinson
I . Introduction ..............................................
I1. Agroecosystems ...........................................
ILL Semiarid Regions ..........................................

n?

v.

VI.

VII .

The Issue of Sustainability ..................................
Technologies for Increasing Plant-Available Water . . . . . . . . . . . . . . .
Soil Organic Matter Maintenance .............................
Summary ................................................
References ...............................................

INDEX...................................................

191
193
194
198
205
223
224
225

229


Contributors
Numbers in parentheses indicate the pages on which the authors’ conlribulions begin

L. F. ELLIOT (12 l), National Forage Seed Production Research Center, Corvallis, Oregon 97331

DON HOLT (149), College of Agricultural, Consumer, and Environmental Sciences, Illinois Agricultural Experiment Station, University of Illinois, Urbana,
Illinois 61801
LES LANYON (l), Department of Plant and Soil Science, University of Vermont,
Burlington, Vermont 05405-0082
BILL LIEBHARDT (l), Department of Plant and Soil Science, Universityof Vermont, Burlington, Vermont OS405-0082
FRED MAGDOFF (l), Department of Plant and Soil Science, University of Vermont, Burlington, Vermont 05405-0082
C. A. ROBINSON (191), Dryland Agriculture Institute, West Texas A&M University, Canyon, Texas 79016
MICHAEL C. SHANNON ( 7 9 , United States Department of Agriculture,
Agriculture Research Service, U S . Salinity Laboratory, Riverside, California
92507
B. A. STEWART (191), Dryland Agriculture Institute, West Texas A&M University, Canyon, Texas 79016
D. E. STOTT (12 l), National Soil Erosion Research Laboratory, Purdue University, West Lajayette, Indiana 47906

vii


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Preface
Volume 60 contains five outstanding chapters that address cutting-edge research
and timely issues in the plant and soil sciences. Chapter 1 discusses nutrient cycling transformations and flows and the implications for a sustainable agriculture.
Topics that are included are the soil-plant system; cycling and flows at the field
level; farm scale cycling and flows; watershed; state, regional, and global issues;
and promoting a more sustainable agriculture. Chapter 2 is a state-of-the-art review on adapting plants to salinity. The most contemporary research on selection
for salt tolerance, salt tolerance mechanisms, genetic variability, breeding methods, and novel biotechnological tools for improving plant adaptation to salinity,
including tissue culture and molecular biology, is included. Chapter 3 discusses
the effects of no-tillage cropping systems on soil microbiological relationships, including decomposition of surface-managed crop residues, modeling crop residue
decomposition, root-microbial relationships, deleterious rhizobacteria for weed
control, and low-input, on-farm composting. Chapter 4 discusses the very timely

topic of ethics in agronomic research. This treatise should be of great interest to
students in the plant and soil sciences and to practicing professionals. The author
defines personal ethics and scientific conduct and then discusses the ethics of
choosing research subject matter, agricultural ethics and the world food situation,
and ethics in research and administration. Chapter 5 discusses the question of the
sustainability of agroecosystems in semiarid regions. Semiarid regions, the issue
of sustainability,and technologies for increasing plant available water are covered.
The editor expresses sincere gratitude to the authors for their fine contributions.

ix


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NUTRIENT
CYCLING,
TRANSFORZMATIONS,
AND FLOWS:
IMPLICATIONS
FOR A MORE
S U S T ~ A B LAGRICULTURE
E
Fred Magdoff, Les Lanyon, and Bill Liebhardt
Department of Plant and Soil Science
University of Vermont
Burlington, Vermont 05405-0082

1. Introduction
11. Framework for Evaluating Nutrient Dynamics

A. Historical Overview
B. Definitions
C. Implications of Spatial Scale and Ecosystem Relations, Seasonal Patterns,
and Landscape Position
111. Soil-Plant System
A. Plant Nutrition and Soil Nutrient Stocks
B. Ecology of Nutrient Flows, Transformations, and Cycles
C . Soil Chemical Properties
D. Soil Physical Properties
E. Biological, Chemical, and Physical Interactions
W. Cycling and Flows at the Field Level
A. Nutrient Losses
B. Nutrient Additions
C. Management Practices and Nutrient Flows
D. Changes in Field Nutrient Flows
E. Changing to Biologically Based Nutrient Sources
V. Farm-Scale Cycling and Flows
A. Within-Farm Nument Flows
B. Nutrient Flows to and from Farms
C. Nutrient Flows between Farms
D. Patterns of Farm Nutrient Flows
VI. Watershed, Regional, and Global Issues
A. Watersheds
B. Other Spatial Scales
C . Energy Use and Nutrient Flows
D. Possible Changes in Large-Scale Flows
E. Influences on Nutrient Flow Patterns

1
Adwmcrr in A p n a r y , Voliimr 60

Copyright 0 1997 by Academic Press. All rights of rcproducoon in any fnnn reserved

0065.2 1 11/97 $25.00


2

FRED MAGDOFF ETAL.
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
References

I. INTRODUCTION
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

3

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
runoff.
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-


4

FRED MAGDOFF ETAL.

puts such as synthetic fertilizers, pesticides, irrigation, fuel, equipment, feeds, and
labor.
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

5

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

II. FRAMEWORK FOR EVALUATING

NUTRIENT DYNAMICS
A. HISTORICAL
OVERVIEW
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

6

FRED MAGDOFF ETAL.

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.


NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS

7

b

a

nutrients

II

t

Acnnsumers

I

primary producers
(plants)

Figure 1 Changes in the spatial relationships of the trophic pyramid relating plants and animals
to humans. (a) Early agriculture (eighteenth to mid-nineteenth century); (b) urbanizing agriculture
(mid-nineteenth to mid-twentieth century); (c) industrial agriculture (mid- to late-twentieth century).

lc). The availability of low-cost N fertilizers after WW I1 rendered forage legumes
superfluous on farms producing grain crops. There was no longer the need to raise
animals to utilize the forages. In the United States, the conversion to enormous
production units is essentially complete for poultry, far advanced for beef cattle,
and well under way for hogs. This phenomenon has further exacerbated environmental problems associated with agriculture. The heart of the issues resulting from
the geographic separation of crops and animals can be summarized as two sides of

the same coin: (i) the decline of SOM and nutrients on crop farms (requiring the
application of large quantities of synthetic fertilizers as well as other inputs to compensate for organic matter depletion, and (ii) the simultaneous overabundance of
nutrients and organic matter at animal production facilities (with the resulting pollution of surface and groundwaters).

B. DEFINITIONS
Clarification of the definitions of some of the key terms that we will use will be
helpful for the discussion of issues and problems of crop nutrient management.
Stocks-Stocks refer to the quantity of nutrients within a defined part of a system. The total stock of nutrients may be of interest for many assessments. However, from the point of view of plant nutrition the maintenance of a sufficient stock
(pool) of nutrients that are either available or easily transformed into an available
state is essential for crop productivity. At the same time, available nutrient substocks must be low enough to moderate potential environmental effects of agriculture. Flows will both contribute to and be subject to the magnitude of the various stocks.


8

FRED MAGDOFF ETAL.

There are numerous biological and chemical reactions that change the state of
nutrients to more or less available forms. These transformations convert nutrients
from one stock of the element to another but do not change the quantity of the total stock of a nutrient. Although the total stock of a particular nutrient may be important for long-term sustainability, it will not usually be of interest for the shortrun concerns of soil fertility unless the net rate of transformation to an available
form is also known.
The size of a stock may exert an influence on susceptibility for nutrient flow.
For example, large stock of inorganic Nor of soluble P will permit significant flows
of these nutrients with leaching or runoff waters.
Flows-The flow of nutrients in an ecosystem is the most basic concept of nutrient movement. Nutrient flows represent linkages among various pools (or
stocks). Measurements of various types of nutrient flows can suggest control
mechanisms and indicators of system performance.
Some nutrient flows are managed pathways, where the purpose of the operation
entails the intentional addition or removal of nutrients. Managed flows occur when
fertilizer is applied to meet an estimated crop need, when manure is applied to certain fields, when a crop is harvested and sold, when animals graze on pastures, etc.
Although other flows, such as leaching of nitrate or nutrient losses in runoff waters, are not purposely managed, their magnitude is strongly influenced by management practices such as tillage systems, rotations, fertilizer application rates,

manure application rates and application methods, and animal stocking density.
Cycles-A nutrient cycle is an example of a closed loop pattern of flow in which
a particular atom ends up back in the same location from where it started. Where
a boundary is drawn surrounding the extent of the system has a significant impact
on deciding whether a true cycle or rather another pattern of flow is occurring.
Transformations-There are numerous processes that determine the “state” or
form in which nutrients occur in soils. These include mineralization from organic
matter, immobilization of inorganic ions by microbial uptake, precipitation of lowsolubility compounds, various oxidation reactions such as nitrification, various reduction reactions such as denitrification, dissolution from solid forms, etc.
The particular form that a nutrient is in influences its availability for plant uptake as well as susceptibility to leaching or gaseous losses. When a nutrient undergoes a transformation to another form, it is not usually considered a flow because the transformation normally occurs in place. However, one transformation,
biological N, fixation, is also a flow. Because soil N, is in equilibrium with the atmosphere, N, moves into the soil as N, fixation occurs, and the stock of total soil
N is increased. For purposes of discussion in this chapter we will refer to N, fixation as a flow of N rather than a transformation.
Boundaries-When discussing nutrient flows and cycles it is essential to define a boundary around the system of interest. The boundary becomes a reference
point for evaluating relative movement of nutrients. Different objectives may


NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS

9

cause one to define a boundary to be around a certain portion of the soil or a field,
farm, state, region, watershed, or country. If global-scale cycles are of interest, the
boundary then includes the entire earth.

C. IMPLICATIONS
OF SPATIAL
SCALEAND ECOSYSTEM
RELATIONS,
SEASONAL
PATTERNS,AND LANDSCAPE
POSITION

When discussing nutrient transformations, flows, and cycles it is important to
take into account implications of spatial scale, ecosystem relations, seasonal patterns, and landscape position. These various considerations can either influence
the nutrient flows and transformations themselves or our perception of them.

1. Spatial Scale and Ecosystem Relations
The extent of the system under consideration has a huge impact on how we view
and understand flows and cycles. The emphasis in the literature on nutrients has
been placed on the field scale because most tactical and operational management
decisions are field based. When viewing processes and flows at this scale, the issue of applying fertilizers or manures is relatively simple. When a specific nutrient application is believed necessary some is applied and this is a flow into the field
from somewhere outside. Likewise, when the crop is harvested, it seems to be a
simple flow of nutrients out of the field. However, the crop may be consumed on
the farm or leave the farm. Also, the nutrients in manure may come from inside
the farm (if animals are fed farm-grown feedstuffs without imported fertility
sources) or from off the farm (if animals are fed only imported feeds) or some mix
of the two (if farm-produced feeds are grown with imported fertilizers).
A greatly simplified diagram of a natural soil-plant-animal ecosystem (Fig. 2)
can aid the discussion of scale of consideration and nutrient cycling and flows in
agriculture.In this figure, the only input flows into the soil come from atmospheric
deposition while the only output flows result from erosion, leaching, and gaseous
losses. There are three stocks of nutrients (boxed in Fig. 2): in the soil (including
all living organisms), in living plants above ground; and in aboveground animals.
Nutrients are taken up from the soil by the plant as it grows and plant residues are
returned to the soil to complete a soil+plant+soil or a soil+plant+animal+
soil cycle.
In general,cycling of nutrients is very efficient under natural ecosystems (Crossley et af., 1984). In most undisturbed natural systems such as forests and grasslands, there is a high degree of synchronization of the supply of available nutrients with the uptake needs of plants. This results in a low level of nutrients in the
soil solution at any one time, promoting an efficient soil+plant+soil cycling of
nutrients. Continuous soil cover with little disturbance helps promote water infil-


10

FRED MAGDOFF ETAL.

residue

8

~

atmospKem
deposmn

erosto; kaching.
g~~~
LOSS

I

Figure 2 Simplified natural system nutrient cycle and flows in the soil-plant system.

tration and maintain low rates of soil erosion. There may be some spatial discontinuity between where nutrients are taken up by plants and where they are deposited in residues, such as when leaves fall on the forest soil surface while roots
may take up nutrients at 10 or 20 cm or greater depth. However, soil organisms,
such as earthworms, beetles, and termites, and leaching help to reintroduce the nutrients into the root zone.
Plants in natural systems sometimes appear to use different nutrient cycling
“strategies” to their own advantage. It is hypothesized that through an evolutionary selection process some species of plants developed characteristics that enhance
the fitness of their environment for themselves at the expense of other plant species
(van Breeman, 1993, 1995). For example, fast-growing species tend to have
residues that decompose and turnover nutrients rapidly. On the other hand, slowgrowing species often have residues that are high in lignin and secondary metabolites that slow microbial decomposition and, thus, reduce competition from fastgrowing species that require high levels of available nutrients.
Compared to a natural ecosystem, a managed agricultural ecosystem has greater
amounts of nutrients flowing in and out, less capacity for nutrient storage, and less

nutrient cycling (Hendrix et al., 1992). There are now inputs of nutrients from a
variety of animal feeds, synthetic fertilizers, inorganic amendments, manures, and
composts (Fig. 3). In this example, the boundary has been drawn around a plant
and the soil below to the bottom of the root system. A major nutrient output from
the field is harvested plant material, which is fed to an animal or used in another


NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS
4

1I

crop removed

PLANT

* erosion,I leaching,

'

fertlltzers.
manures, lime,
organlc residues,
ahospherlc

deposition

+

gaseous loss

Figure 3 Simplified managed system nutrient cycle and flows in the soil-plant system.

manner. In general, nutrient losses by runoff, erosion, volatilization, and leaching
are far greater in an agroecosystems than in a natural system. Compared to natural systems, there is normally a greater quantity of soluble nutrients present in
agroecosystems and more soil disturbance and longer times during the year when
the soil is not covered with living vegetation. These agroecosystem characteristics
stimulate SOM breakdown and lead to more compact soils with less porous infiltrative surfaces and more runoff and erosion than in natural ecosystems.
When looking at the soil-plant system level, it is difficult to tell whether or not
an input is completing a true cycle where the nutrients removed from that particular area of soil are being returned to the same location. For example, is the origin
of the nutrients in manure the location under consideration or is it another field or
farm? Thus, it is necessary to look at both field- and farm-level flows and cycles
to determine whether or not true cycles are occumng.
When looked at regionally (or globally), the location where the nutrients are
produced or mined and refined or incorporated into plants or animals and where
the agricultural products are shipped to, processed, and consumed all become important considerations in understanding intraregional and interregional flow patterns. These may be as important to a sustainable agriculture as field- and farmlevel flows. Nutrients commonly travel significant distances, as when fertilizer is
shipped from the manufacturer to the farm or when feed grains are transported
from the Midwest to the dairy farms in the Northeast, vegetables are shipped from
California to New York, or wheat is transported from the Northern Plains and the


12

FRED MAGDOFF ETAL.

Northwest of the United States to China. In these situations the flow is all one way
and there is no realistic means for the nutrients to cycle back to the farms and fields
from where they came.

2. Seasonal Patterns

Nutrient transformations and flows do not happen at a uniform rate during the
year. Mineralization of nutrients from organic matter is usually very slow during
the winter and at a standstill when soil is frozen. Peak rates of mineralization in
temperate region soils coincide with the warming in the spring and are probably
significantly enhanced by freezing and thawing over the winter (Magdoff, 1991a;
DeLuca et al., 1992). When soils dry down during the field season and are then
rewetted, there is also a burst of mineralization caused by the conversion of a certain portion of SOM to forms that are more susceptible to microbial attack.
Significant leaching and runoff losses of nutrients in most temperate annual
cropping systems are confined to the late fall, winter, and early spring when precipitation exceeds evapotranspiration and recharge requirements (Fig. 4).During
the summer season evapotranspiration is usually greater than precipitation and
leaching and runoff are usually minimal because of the drier soil conditions.
Managed flows also occur during distinct times of the year (Fig. 4).Large quantities of lime, fertilizer, and manure are normally applied when the crop is not in
the field-in the spring before the crop is planted or in the fall after the last crop
is harvested (some application during crop growth as side-dress and top-dress is
also common). The flow of nutrients leaving the field with the harvested crop usually occurs at a distinct time of the year-determined by climate, species and cultivar, and other management practices.
Thus, nutrients may be applied in the fall, taken up by plants during the following growing season, and removed from the field as the crop is harvested 10 or
11 months after application. Also, some portion of the applied nutrients may be
held by the soil so that they are taken up by plants and removed from the field only
years after application.
There are also changes in nutrient stocks that operate over decades and even
longer. Soil stocks of N in many midwestern soils were drawn down over decades
as organic matter was depleted (Hass et al., 1957). Also, the buildup of nutrient
levels by a few decades of heavy fertilizer and/or manure application by many
farmers has made it difficult to even find low P and low K soils in certain areas
(Engelstad and Parks, 1976; Sims, 1993).

3. Landscape Position
By increasing the scale of attention from the soil-plant system to the field and
then to the farm and watershed or subregion, issues relating to position in the land-

NUTRIENT CYCLING, TRANSFORIMATIONS, AND FLOWS

13

managed
flows
fertilizer

fertilizm

harvest

J

F

M I A
M
denitrification
leaching

J

J

A

S

0

N I D
denitrification
leaching

rUnOff

emion

I

unmanaged
flows

runoff

erosion

1

Figure 4 Seasonal aspects of nutrient flows into and out of fields for a northern hemisphere temperate region annual crop.

scape become apparent. For example, soil eroded from the slope of a field may or
may not leave the field or farm. The sediments may be deposited in a low-lying
depression in the field or in an adjacent field. Sediments might also flow from a
field to a stream and from there into a lake. In the first situation, there is only a redistribution within a field or a flow from one field to another. It is not the same net
loss to the field or farm that usually occurs after sediments enter a stream.

111. SOILPLANT SYSTEM

A. PLANTNUTRITION
AND Son, NUTRIENT
STOCKS
Within the soil, for each plant nutrient of interest there are three main types of
stocks that can potentially supply nutrients in forms that are available to plants: (i)
nutrients in the soil solution in forms that can be taken up by plants, usually as simple ions; (ii) nutrients associated with organic matter by being adsorbed on negative exchange sites or present as part of organic molecules; and (iii) nutrients as-


14

FRED MAGDOFF ETAL.

sociated with soil minerals, either adsorbed on exchange sites or as part of the
structure of the inorganic mineral.
Mineralization of organic compounds as well as cation exchange, solubilization,
desorption, and dissolution of minerals convert the soil nutrient stocks listed in (ii)
and (iii) into forms that can be immediately used by plants. Nutrients are also
added to the soil in a number of forms, such as fertilizers, manures, and crop
residues from other fields, in precipitation and dry deposition, and in the special
case of N by biological N, fixation.

1. Satisfying Short-Term Fertility Needs of Crops at
the Soil-Plant Level
To satisfy short-term needs of crops during the growing season the amount of
available nutrients must be greater than or equal to the uptake needs of the crop
(see soil-plant flow labeled 7 in Fig. 5). Using the numbering system in Fig. 5,
Solution stock + (1-2)

+ ( 3 4 ) + (5-6) + (10-13) 2 7,

(1)

where solution stock is the quantity of nutrient in soil solution at start, 1-2 are the
net mineralization, 3 4 are the net desorption from SOM, 5-6 are the net desorp-

[+output (flow)V-v I

fertilizers,

atmospheric
deposltlon

Figure 5 Simplified nutrient cycle, flows, and transformations in the soil-plant system with inputs and outputs indicated.