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SOIL HEALTH AND SUSTMNABILITY
21
inefficiencies. Soils which are incapable of storing nutrients require excessive or
continuous addition of soluble nutrients for crop growth, to compensate for
losses and inefficiencies. Soluble nutrients in excess of plant and microbial needs
will pass beyond the reach of plant roots, with potential consequences to the
groundwater already discussed, compounded by the loss of valuable and possibly
dwindling nutrient resources.
Soil organisms must be acknowledged as key architects in nutrient turnover,
organic matter transformation, and physical engineering of soil structure (see
Fig. 1). The microbial populations of the soil alone encompass an enormous
diversity of bacteria, algae, fungi, protozoa, viruses, and actinomycetes. As
many as 10,000 different species may be found in a single gram of soil (Torsvik
et al., 1990), just a small sample of the nearly two million species of microorganisms thought to exist worldwide, with a range of form and function beyond
current capacity for comprehensive study. While the specific functions and interactions of the majority of these organisms are as yet poorly elucidated, their role
as functional groups in soil health regeneration and maintenance is becoming
increasingly clear (Kennedy and Papendick, 1995).
The microbial biomass is largely responsible for mineralization and turnover
of organic substrates (Killham, 1994). It includes both primary and secondary
decomposers, aerobic, anaerobic, and switch-hitting digestors, highly specialized consumers of gourmet delicacies and feeding trough generalists, finicky
occupants of outlandish environmental niches and highly adaptable opportunists,
hard-driven frenzied achievers and slow-metabolizing plodders, diners of rich,
fatty substrates and those eking out an existence gnawing on tough lignaceous
scrap. As a group, the community of microbial populations acts without regard
for the future, but instead responds quickly to favorable conditions, reproducing
and consuming with wild abandon until substrate limitations cause population
declines, victims of their collective gluttony. They are in turn cannibalized by
their surviving compatriots. The result is a continuous cyclic ballet of nutrient
uptake and release that enables less ephemeral life forms in the soil to be supplied
with their nutritional needs in a somewhat regulated way.
The role of larger soil organisms in maintenance of soil quality and health has
finally begun to receive much deserved attention in soil science circles with the
publication of several excellent review articles in recent years (Berry, 1994;
Linden et a l . , 1994; Stork and Eggleton, 1992). Soil fauna cover a range of soil
functions beyond that of the soil microbial community. Anderson ( 1988) classifies soil invertebrates into three categories, based primarily on size. Microfauna
are those less than 100 p m in diameter and include protozoa, nematodes, and
rotifers. They are the aquanauts of the soil, existing in water films around soil
particles and free water in soil pores. They function as secondary consumers,
feeding largely on bacteria and fungi, thereby speeding the turnover of microbial
biomass and their associated nutrients. The diversity in nematode function is
22
J. W. DORAN ET AL.
vast, spanning many different trophic levels, and nematode identification has
been suggested as an indicator of soil organism diversity and soil quality (Bohlen
and Edwards, 1994; Bongers, 1990; Neher et al., 1995; Parmelee and Alston,
1986).
The mesofauna, according to Anderson’s classification, are those invertebrates
100-200 p m in diameter and include mites, Collembola (springtails), and the
Enchytraeidae, or pot worms, as well as thousands of species of insects and
spiders. They tend to be omnivores, dining on microflora and fauna, as well as
other mesofauna and decomposing plant residues. In this way they speed organic
matter turnover directly, as well as indirectly, by fragmenting residues, thereby
increasing the surface area available for colonization by smaller organisms.
Enchytraeidae affect soil structure through creation of aggregates resulting from
fecal pellets and through burrowing activities.
Macrofauna are greater than 200 pm and include ants and termites as well as
the box-office stars of the underworld, the earthworms. Ants and termites can
have localized profound effects on soil structure, but earthworms are more ubiquitous and have become unwitting symbols of a healthy, living soil. They can
contribute in several ways to soil health. Most notably, earthworm burrows can
occupy as much as 1% of the soil volume (Kretzchmar, 1982), aiding in infiltration and flowthrough of water (Lee, 1985), as well as providing pathways for
root exploration and faunal habitat. Their feeding habits can help homogenize the
topsoil and, in the case of surface feeders, incorporate large amounts of surface
litter into deeper soil levels. Their digestive process releases nutrients and fragments of plant residues, leaving behind fertile casts and mucus burrow linings
(Berry, 1994).
The conditions favoring high earthworm populations overlap to a great degree
with those considered indicative of a healthy soil-good soil structure, adequate
moisture, sources of fresh organic material, and absence of certain pesticides.
There are several studies which in fact show them to be in considerably greater
abundance in natural ecosystems than in cultivated land (Barnes and Ellis, 1979;
Mackay and Kladivko, 1985) and higher under “sustainable” than conventional
management. Unfortunately, their use as an indicator of soil health is complicated by the fact that the conditions which cause them to be absent, or in low
numbers, do not correlate entirely with other indicators of soil health. For example, many will burrow into deep soil layers during cold or extended dry periods.
Although earthworms are usually present in highly productive soils, some highquality soils may be devoid of earthworm activity due to such factors as tillage or
environmental restrictions (Linden et al., 1994).
It is becoming increasingly obvious what the consequences of soil organic
matter loss are, that soil organisms are both the preservers and the destroyers of
soil organic matter, and that human intervention has a profound effect in orchestrating their activities. Clearly, a new vision of the fragile soil resource is needed.
SOIL HEALTH AND SUSTNNABILITY
23
The concept of the soil as a living organism, as discussed earlier, is not new
(Balfour, 1948). It is complementary to the Gaia Hypothesis articulated by James
Lovelock and Lynn Margulis in the 1960s (Lovelock, 1991) which envisions the
whole planet as a living creature, continually manipulating and adjusting existing
conditions to favor its own survival. The soil-as-organism model is useful for
conceptualizing the various functioning systems in the soil as analogous to animal respiratory, digestive, and circulatory systems. Perhaps a slightly more
appropriate paradigm is of the soil as a community. The difference in the community model is that it is largely self-contained-the outputs and waste products of
one group become the inputs and energy sources of another. There can then be
more complementarity of function than can occur at the single organism level.
Within stable communities, there is little loss of nutrients from the system and
outflows of water and energy are balanced by inflows, mostly from rainfall and
solar radiation. In this context the need for complex, diverse and overlapping
functional groups in the soil becomes apparent. There is a need for both generalists, which perform the bulk of everyday chores, and the specialists in the
community which fill specialized niches. In such a model, diversity itself may
serve as an indicator for soil health.
A follow-through of the soil-as-community paradigm is the idea of plant
nutrition being more efficient if cycled through a complex web of organisms and
their natural environment which is governed by rules that ensure the survival of
the whole community. This has been called the “feed the soil, not the crop” tenet
of sustainable or regenerative agriculture. Such plant nutrition seeks to mimic
natural ecosystems and relies on the yearly mineralization of organic materials by
soil microorganisms in response to fluctuating food sources, moisture, aeration,
and temperature. To function properly, it requires a continuing commitment to
adding sufficient and diverse organic residues, and to maintaining crop rotations
that maximize the presence of living roots throughout the year and synchronize
nutrient availability closely with crop needs. It demands a more complex management than current conventional agriculture, and necessitates a higher degree
of planning, but theoretically will lead to more efficient and environmentally
benign nutrient use.
A perhaps more profound outcome of a soil that functions as a living community would be the degree of resilience and stability that develops over time. The
dynamic combination of diverse-function populations, sufficient energy supplies,
and tight nutrient cycling would be expected to provide the basis in agricultural
soils for the kind of buffering capacity against environmental stresses seen in
equilibrium level natural ecosystems (Hillel, 1991; Soule and Piper, 1992). In a
system following this model, shortfalls in yearly nutrient inputs could be supplemented by stored nutrients in the organic matter or microbial populations. The
effect of disease and insect invasions would be minimized by a diverse group of
antagonistic organisms, and by the presence of a limited proportion of suscepti-
24
J. W. DORAN ET AL.
ble plants present at any given time. At a physical level, years of drought stress
would be ameliorated by higher water-holding capacity and more favorable conditions for root growth due to high organic matter, just as the impact of floods
would be lessened by good infiltration and drainage.
Swift (1994) proposed that assessments of production sustainability should be
based on two components-nondeclining crop yield trends, and stability of yield
from crop cycle to crop cycle. There is evidence that this sort of stability can in
fact be achieved in agricultural systems. After a 5-year transition period, a
comparison of conventionally grown crops and organically grown crops showed
that all systems had equivalent yields averaged over 9 years, but the organic
systems had less year-to-year variation (Hanson et a l . , 1990; Peters, 1994).
Dormaar et al. (1988) reported improved tolerance to drought stress in degraded
soils receiving animal manures relative to soils receiving only commercial fertilizer. In the Western Corn Belt of the U.S.A., Sahs and Lesoing (1985) found that
yields of rainfed corn (Zea mays L.) for organic management systems using
animal manures and/or crop rotations were higher than those for conventional
monoculture management with fertilizers and pesticides, especially during years
of high temperatures and water stress.
B. REGENERATIVE
AGRICULTURE
While terms for an agriculture that seeks to mimic natural ecosystems are
abundant, the term regenerative agriculture, coined by Robert Rodale, is perhaps
the most descriptive. Regenerative agricultural theory assumes that food production systems have caused some degradation of the natural resource base, and
seeks ways to restore or regenerate them toward their original state through
making maximum use of the internal resources available on farms (Rodale,
1984, 1995). The tenets of regenerative agriculture have never been explicitly
laid out. However, laying aside the social and economic aspects, in terms of
production systems alone, they are essentially the same as those associated with
sustainable or “biological” farming, namely:
Soil organic matter replenishment is the cornerstone to regenerating soil
health. Plant residues are left in the field or returned as compost as much as
possible. Animal production systems are designed to return manures to the
soil, either directly by pasturing, or by more efficient manure handling and
spreading systems. The necessary removal of organic material in the form of
harvested crops is compensated for by growing green manure crops or by
amending with compost, which may actually be composed of community food
waste, thus tightening the nutrient loop.
Living cover should be maintained throughout all or most of the year. This
SOIL HEALTH AND SUSTAINABILITY
25
provides plant roots which can take up soluble nutrients throughout the year,
further tightening nutrient cycles and decreasing loss. Living cover also protects
against erosion, provides habitat and substrate for soil organisms, and increases soil organic residue inputs. Although the feasibility of cover crops may
be limited in drier climates by the potential for competition for available water
with a grain crop, perennial soil cover is still an ideal to use as a guideline.
Diversity is critical at every level. Crops may be grown in polycultures, or
in alternating strips, or diversity may be achieved at the whole farm scale, with
complex rotations occumng in numerous small fields. Rotations are based on
progressions of plants with complementary water and nutrient needs, pest
susceptibilities, and root system types. This above-ground diversity may be
expected to harbor below-ground diversity in the soil microbial, and faunal
communities as there is a greater variety in food and nutrient sources available. Farm animals may also contribute to the diversity, fulfilling various
niches in nutrient cycling and waste disposal.
Inorganic fertilizers and pesticides should be reduced or eliminated. While
inorganic fertilizers may provide nutrients in similar or identical forms as
mineralized organic sources, they are discouraged because they have no direct
long-term soil enhancing properties. Certain plant nutrients need to be provided in inorganic form to restore losses from crop removal; in such cases
naturally occurring minerals are preferred because they can be applied in less
concentrated slow-release forms and commonly require less nonrenewable
energy for production and distribution. Pesticide reduction has a twofold
purpose-to protect farm employees from exposure to harmful substances,
and to avoid creating imbalances in communities of soil organisms.
Tillage should be minimized. Excessive tillage leads to increases in organic
matter decomposition due to physical disruption of aggregates, increased aeration and warming. While some form of soil disturbance may be required to
control weeds, less disruptive cultivation implements are favored and multiple
strategies for dealing with weed pressure are employed.
The theory and the practice of regenerative agriculture are rarely, if ever,
entirely meshed, but there are some signs that movement toward these ideals
does in fact lead to improvements in soil health. A comparison of organically and
conventionally managed tomato agroecosystems in California (Drinkwater et a l . ,
1995) showed that soils managed organically for at least 4 years had slightly
greater percentages of soil organic matter, lower soluble N concentrations, and
higher levels of microbial activity and potentially mineralizable N. A similar
study in New Zealand on paired biodynamically managed and conventional
farms found higher levels of microbial activity, soil organic matter contents, and
soil nutrient supplying capabilities on the biodynamic farms (Reganold et al.,
1993). When compared to a continuous grain system, an 8-year agroecological
26
J. W. DORAN ET AL.
rotation in Alberta, Canada, showed evidence of increases in total C, N , and P,
available N , P and K, CEC, microbial biomass, and microbial respiration (Wani
et al., 1994). The legume-based cropping system in the Rodale farming system
trial now exhibits higher organic matter content and microbial biomass (Wander
et al., 1994), greater water stable aggregates (Friedman, 1993), and reduced
nitrate leaching (Harris et al., 1994) as compared to the conventional system,
while maintaining equivalent yields.
Other authors have reported improvements in soil characteristics following
transition to more complex rotations including legumes (Angers and Mehuys,
1988; Doran and Smith, 1991; Doran and Werner, 1990; Kay et al., 1988), from
reducing tillage (Doran and Linn, 1994; Karlen et al., 1989; Angers et al.,
1992), or adding organic soil amendments (Dormaar et al., 1988).
C. NATURAL
RESOURCEACCOUNTING
Current agricultural practices provide an abundant and generally safe supply of
food and fiber at an inexpensive cost to the consumer. The cost of agricultural
products at the market, however, does not reflect the full cost of the agricultural
system. Environmental costs relating to deleterious consequences of contemporary agriculture, such as soil erosion, polluted water supplies, and poisoned
wildlife, are currently ignored under conventional agricultural accounting methods. Though these negative consequences may be more an attribute of a larger
economic model affecting agriculture than of agriculture itself, the environmental costs are nevertheless transferred from farmers to people in other places or
future time periods (Domanico et al., 1986).
Estimated environmental costs of agricultural production are significant. Annual off-site damage from soil erosion by water in the United States has been
estimated at over $7 billion (Pimentel et al., 1995; Ribaudo, 1989). Damage
includes costs associated with the loss of water’s value for recreation, decreased
water storage capacity, flooding, dredging ports and navigable rivers, and treating water for industrial and household use. Of the total soil erosion caused by
water in the United States, as much as 75% has been attributed to agricultural
sources (Pimentel et al., 1976). Wind erosion damage is generally considered to
be less severe than that by water, but may be substantial in arid regions. Damage
by wind erosion to households and businesses in New Mexico, where two-thirds
of the land is used for agriculture, has been estimated to range from $260 to $466
million annually (Piper and Huszar, 1989). Contamination of water by agrichemicals may be the most costly environmental consequence of agricultural
production. Annual damage by pesticides and fertilizers to water quality is suspected to range in the billions of dollars (Duda, 1985; Nielsen and Lee, 1987).
Costs associated with surface and ground water contamination from agrichemi-
SOIL HEALTH AND SUSTAINABILITY
27
cals include remediation and replacement of contaminated water, impairment of
human and animal health, and loss of water fauna and flora (CAST, 1992b;
National Research Council, 1993b).
Compared to off-site environmental damage, the value of changes in soil
health is much more difficult to quantify. This is primarily due to advances in
agricultural technology that have masked much of the yield-reducing impact of
soil degradation (Crosson, 1982, p. 184). Calculation of nutrient replacement
costs from erosion, however, shed some light on the economic magnitude of
agriculture’s impact on soil health. Using the approach of Willis and Evans
(1977), estimated loss of nitrogen, phosphorus, and potassium in soil eroded by
water would amount to over $6 billion annually in the United States. [This
calculation assumes an average soil nitrogen, phosphorus, and potassium content
of 0.15, 0.12, and 2.2921, respectively. Current fertilizer prices were used for
NH,NO,, P20,, and K 2 0 . Soil erosion by water estimated to be 6.9 metric tons
per hectare per year on 155 million hectares of cropland (Kellogg et af., 1994).]
These costs reflect just a portion of the economic burden that must be incurred by
farmers and consumers alike. The costs of soil organic matter loss and soil tilth
deterioration are also likely significant, but remain undefined (Bauer and Black,
1994).
Given contemporary agriculture’s estimated cost to the environment and soil
health, economic consideration of natural resources is clearly necessary to
achieve agricultural sustainability. This has motivated scientists to call for the
application of natural resource accounting methods to agricultural production
(Domanico et a / . , 1986; Tangley, 1986). This call has been addressed through
efforts by the World Resources Institute (Faeth rt ul., 1991), who have employed
natural resource accounting to incorporate factors of soil health, regional environmental impacts, farm profitability, and governmental policy to evaluate agricultural sustainability.
The method used by Faeth et ul. ( I99 I ) to quantify changes in soil health relies
upon interconnected ideas of sustainability, business income, and natural resource depreciation. Sustainability implies that economic activity should meet
current needs without foreclosing future options (WCED, 1987). Business income encompasses this notion of sustainability when defined as “the maximum
consumption in a certain period that does not reduce potential consumption in
future periods” (Edwards and Bell, 1961, after Faeth, 1993). By this standard,
then, agricultural accounting methods can only be accurate if depreciation in
natural resource assets (i.e., soil) is subtracted from net revenues along with the
more common forms of farming assets, like machinery and buildings. Faeth et
a / . (1991) followed this standard by calculating a soil depreciation allowance in
evaluating the economic performance of agricultural production systems. By
incorporating output from the Erosion-Productivity Impact Calculator (EPIC)
model, the allowance estimated future income losses over a 30-year period from
28
J. W. DORAN E T AL.
the impact of production on the soil resource as declines in crop yield (Williams
et al., 1989). Inclusion of the soil depreciation allowance in their evaluation of
economic performance resulted in a reduced net farm income of $62 per hectare
per year for Pennsylvania’s best conventional corn-soybean management. This
cost represents a significant loss of wealth in the natural resource base: a loss,
represented by degraded soil health, that is currently ignored by conventional
agricultural accounting methods.
VI. ASSESSMENT OF SOIL QUALITY AND HEALTH
Establishing an ongoing assessment of the condition and health of our soil
resources is vital to maintaining the sustainability of agriculture and civilization.
As discussed earlier, the failure of several earlier civilizations was sealed by their
disregard for the health of finite soil resources. In today’s energy- and
technology-intensive world, the need for maintaining the health of our soil resources is imperative to sustaining productivity for increasing populations and in
maintaining global function and balance. Assessment of soil quality and health is
invaluable in determining the sustainability of land management systems. A
framework for evaluation or an index of soil quality and health is needed to
identify problem production areas, to make realistic estimates of food production, to monitor changes in sustainability and environmental quality as related to
agricultural management, and to assist government agencies in formulating and
evaluating sustainable agricultural and other land-use policies (Acton, 1993;
Granatstein and Bezdicek, 1992). Effective identification of appropriate indicators for soil health assessment depends on the ability of any approach to consider
the multiple components of soil function, in particular, productivity and environmental well-being. Identification of indicators and assessment approaches is
further complicated by the multiplicity of physical, chemical, and biological
factors which control biogeochemical processes and their variation in intensity
over time and space (Larson and Pierce, 1991). Realistic assessment of soil
quality and health, however, requires consideration of the multiple functions of
soil and their relative importance as dictated by societal and ecological needs.
There is a great need both to determine the status of and to enhance our soil
resources. Assessment and monitoring of the quality and health of soils must also
provide opportunity to evaluate and redesign soil and land management systems
for sustainability. Standards of soil quality and health are needed to determine
what is sustainable and what is not, and to determine if soil management systems
are functioning at acceptable levels of performance. Recently, Doran and Parkin
(1994) identified nine research needs critical to assessment and enhancement of
soil quality. The two highest priority needs were: (i) Establishment of reference
SOIL HEALTH AND SUSTAINABILITY
29
guidelines and thresholds for indicators of soil quality that enable identification
of relationships between measured soil attributes and soil function which permit
valid comparisons across variations in climate, soils, landuse, and management
systems; and (ii) development of a practical index for on-site assessment of soil
quality and health for use by farmers, researchers, extension, and environmental
monitors that can also be used by resource managers and policy makers to
determine the sustainability of land management practices.
A. USEOF INDICATORS
Assessing the health or quality of soil can be likened to a medical examination
for humans where certain measurements are taken as basic indicators of system
function (Larson and Pierce, 1991). In a medical exam, the physician takes
certain key measurements of body system function such as temperature, blood
pressure, pulse rate, and perhaps certain blood or urine chemistries. If these basic
health indicators are outside the commonly accepted ranges, more specific tests
can be conducted to help identify the cause of the problem and find a solution.
For example, excessively high blood pressure may indicate a potential for system
failure (death) through stroke or cardiac arrest. The problem of high blood
pressure may result from the lifestyle of the individual due to improper diet, lack
of exercise, or high stress level. To assess a dietary cause for high blood pressure, the physician may request a secondary blood chemistry test for cholesterol,
electrolytes, etc. Assessment of stress level as a causative factor for high blood
pressure is less straightforward and generally involves implementing some
change in lifestyle followed by periodic monitoring of blood pressure to assess
the effectiveness of the change. This is a good example of using a basic indicator
both to identify a problem and to monitor the effects of management on the health
of a system.
Applying this human health analogy to soil health is fairly straightforward.
Larson and Pierce (1991) proposed that a minimum data set (MDS) of soil
parameters be adopted for assessing the health of world soils, and that standardized methodologies and procedures be established to assess changes in the
quality of those factors. A set of basic indicators of soil quality and health has not
previously been defined, largely due to difficulty in defining soil quality and
health, the wide range over which soil indicators vary in magnitude and importance, and disagreement among scientists and soil and land managers over which
basic indicators should be measured. Acton and Padbury (1993) defined soil
quality attributes as measurable soil properties that influence the capacity of soil
to perform crop production or environmental functions. Soil attributes are useful
in defining soil quality criteria and serve as indicators of change in quality.
Attributes that are most sensitive to management are most desirable as indicators