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Soil Health, Plant Health, and Pests
[H1]SOIL PROPERTIES AND THEIR INTERRELATIONSHIPS
Healthy soils occur when their biological, chemical, and physical conditions are all
optimal (figure 8.1), enabling high yields of crops. When this occurs, roots are able to
proliferate easily, plentiful water enters and is stored in the soil, the plant has a sufficient
nutrient supply, there are no harmful chemicals in the soil, and beneficial organisms are
very active and able to keep potentially harmful ones in check as well as stimulate plant
growth.
[place fig. 8.1 about here]
A soil’s various properties are frequently related to one another, and the interrelationships
should be kept in mind. For example, when a soil is compacted, there is a loss of the large
pore spaces, making it difficult or impossible for some of the larger soil organisms to
move or even survive. In addition, compaction may make the soil waterlogged, causing
chemical changes such as when nitrate (NO3–) is denitrified and lost to the atmosphere as
nitrogen gas (N2). When soils contain a lot of sodium, common in arid and semiarid
climates, aggregates may break apart and cause the soils to have few pore spaces for air
exchange. Plants will grow poorly in a soil that has degraded tilth even if it contains an
optimum amount of nutrients. Therefore, to prevent problems and develop soil habitat
that is optimal for plants, we can’t just focus on one aspect of soil but must approach crop
and soil management from a holistic point of view.
[H1]PLANT DEFENSES, MANAGEMENT PRACTICES, AND PESTS
Before discussing the key ecological principles and approaches to soil management, let’s
first see how amazing plants really are. They use a variety of systems to defend
themselves from attack by insects and diseases. Sometimes they can just outgrow a small
pest problem by putting out new root or shoot growth. Many plants also produce
chemicals that slow down insect feeding. While not killing the insect, it at least limits the
damage. Beneficial organisms that attack and kill insect pests need a variety of sources of
nutrition, usually obtained from flowering plants in and around the field. However, when
fed upon—for example, by caterpillars—many plants produce a sticky sweet substance
from the wounds, called “extra-floral nectar,” which provides some attraction and food
for beneficial organisms. Plants under attack by insects also produce airborne (volatile)
chemicals that signal beneficial insects that the specific host it desires is on the plant. The
beneficial insect, frequently a small wasp, then hones in on the chemical signal, finds the
caterpillar, and lays its eggs inside it (figure 8.2). As the eggs develop, they kill the
caterpillar. As one indication of how sophisticated this system is, the wasp that lays its
eggs in the tomato hornworm caterpillar injects a virus along with the eggs that
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deactivates the caterpillar’s immune system. Without the virus, the eggs would not be
able to develop and the caterpillar would not die. There is also evidence that plants near
those with feeding damage sense the chemicals released by the wounded leaves and start
making chemicals to defend themselves even before they are attacked. [add some
qualification here, such as “sometimes” or “some”? NOT NEEDED]
[fig. 8.2 about here]
Leaves are not the only part of the plant that can send signals when under attack that
recruit beneficial organisms. When under attack by the western corn rootworm—a major
pest—the roots of some varieties of corn have been shown to release a chemical that
attracts a nematode that infects and kills rootworm larvae. During the process of breeding
corn in the U.S., this ability to signal the beneficial nematode has apparently been lost.
However, it is present in wild relatives and in European corn varieties and is, therefore,
available for reintroduction into U.S. corn varieties.
Plants also have defense systems to help protect them from a broad range of viral,
fungal, and bacterial attacks. Plants frequently contain substances that inhibit a disease
from occurring whether the plant is exposed to the disease organism or not. In addition,
antimicrobial substances are produced when genes within the plant are activated by
various compounds or organisms—or a pest—in the zone immediately around the root
(the rhizosphere) or by a signal from an infection site on a leaf. This phenomenon is
called “induced resistance.” This type of resistance causes the plant to form various
hormones and proteins that enhance the plant’s defense system. The resistance is called
systemic because the entire plant becomes resistant to a disease, even far away from the
site where the plant was stimulated.
There are two major types of induced resistance: systemic acquired resistance (SAR)
and induced systemic resistance (ISR) (figure 8.3). [both are “induced” even though
ISR means induced? THEY ARE BOTH INDUCED TO OCCUR] SAR is induced
when plants are exposed to a disease organism or even some organisms that do not
produce disease. Once the plant is exposed to the organism, it will produce the hormone
salicylic acid and defense proteins that protect the plant from a wide range of pests. ISR
is induced when plant roots are exposed to specific plant growth–promoting rhizobacteria
(PGPR) in the soil. Once the plants are exposed to these beneficial bacteria, hormones
(jasmonate and ethylene) are produced that protect the plants from various pests. Some
organic amendments have been shown to induce resistance in plants. Therefore, farmers
who have very biologically active soils high in organic matter may already be taking
advantage
of
Plant Defense Mechanisms
induced resistance.
Plants are not passive in the face of attack by insects, nematodes,
However,
there
or diseases caused by fungi and bacteria. Genes activated when
currently are no
plants are attacked or stimulated by organisms produce chemicals
reliable and costthat
effective indicators
• slow insect feeding
to
determine
• attract beneficial organisms
whether
a
soil
• produce structures that protect uninfected sites from nearby
amendment or soil is
pathogens
enhancing a plant’s
• produce chemicals that provide a degree of resistance to
defense
pathogenic bacteria, fungi, and viruses
mechanisms. More
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research needs to be conducted before induced resistance becomes a dependable form of
pest management on farms. Although the mechanism works very differently from the
way the human immune system works, the effects are similar—the system, once it’s
stimulated, offers protection from attack by a variety of pathogens and insects.
[fig. 8.3 about here]
Managing Soils and Crops to Minimize Pest Problems
It is well established—and known by most farmers—that crop rotation can decrease many
disease, insect, nematode, and weed pressures. A few other examples of management
practices that reduce [reduce?YES—CHANGED IT] pest pressure follow:
• Insect damage can be reduced by avoiding excess inorganic nitrogen levels in soils by
using better nitrogen management.
• Adequate nutrient levels reduce disease incidence. For example, calcium
applications have reduced diseases in crops such as wheat, peanuts, soybeans, and
peppers, while added potassium has reduced the incidence of fungal diseases in crops
such as cotton, tomatoes, and corn.
• Damage from insect and disease (such as fungal diseases of roots) [do you mean fungal
diseases of roots are the only disease you’re talking about here?NO—but probably
the most important] can be decreased by lessening soil compaction.
• Severity of root rots and leaf diseases can be reduced with composts that contain low
levels of available nitrogen but still have some active organic matter.
• Many pests are kept under control by having to compete for resources or by direct
antagonism from other insects (including the beneficials feeding on them). [edits ok? OK]
Good quantities of a variety of organic materials help maintain a diverse group of soil
organisms.
• Root surfaces are protected from fungal and nematode attack by high rates of beneficial
mycorrhizal fungi. Most cover crops help keep mycorrhizal fungi spore counts high and
promote higher rates of infection by the beneficial fungi.
• Parasitic nematodes can be suppressed by selected cover crops.
• Weed seed numbers are reduced in soils that have a lot of biological activity, with both
microorganisms and insects helping the process.
• Weed seed predation by ground beetles is encouraged by reduced tillage and
maintenance of surface residues. Reduced tillage also keeps the weed seeds at the surface,
where they are accessible to predation by other organisms, such as rodents, ants, and
crickets.
• Residues of some cover crops, such as winter rye, produce chemicals that reduce weed
seed germination.
When plants are healthy and thriving, they are better able to defend themselves from
attack and may also be less attractive to pests. When under one or more stresses, such as
drought, nutrient limitations, or soil compaction, plants may “unwittingly” send out
signals to pests saying, in effect, “Come get me, I’m weak.” Vigorous plants are also
better competitors with weeds, shading them out or just competing well for water and
nutrients.
Many soil management practices discussed in this chapter and the other chapters in
part three help to reduce the severity of crop pests. Healthy plants growing on soils with
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good biological diversity can mount a strong defense against many pests. For examples of
the effects of soil management on plant pests, see the box on the next page. [Is this -Strong Ecosystem Characteristics -- the title of the box that’s meant? NO—it’s the one above
Managing Soils and Crops to Minimize Pest Problems ] The issue of plant health is so
critical to ecological soil and plant management because it also influences, as we have
just seen, the ability of plants to resist pests. Developing optimal soil health is, therefore,
the basis for management of crop pests on farms—it should be a central goal that
underpins crop integrated pest management (IPM) programs.
[H1]ECOLOGICAL PRINCIPLES FOR AGRICULTURE
Approaching agriculture and soil management from an ecological point of view means
first
Strong Ecosystem Characteristics
understanding
Efficiency. Efficient energy flows are characteristic of natural systems. The
the
sun’s energy captured by green plants is used by many organisms, as fungi
characteristics
and bacteria decompose organic residues and are then fed upon by other
organisms, which are themselves fed upon by others higher up the food web.
that
comprise
Natural ecosystems also tend to be efficient in capturing and using rainfall
strong
natural
and in mobilizing and cycling nutrients. This helps to keep the ecosystem
systems.
Let’s
from “running down” because of excessive loss of nutrients and at the same
time helps maintain the quality of the groundwater and surface waters.
take a look at
Rainfall tends to enter the porous soil, rather than run off, providing water to
overall strategies
plants as well as recharge to groundwater, slowly releasing water to streams
that
can
and rivers.
Diversity. High biological diversity, both above ground and in the soil,
contribute
to
characterizes many natural ecosystems in temperate and tropical regions.
similar strength
This [“this” refers to high biological diversity? YES] provides nutrients to
of
crops,
plants, checks on disease outbreaks, etc. For example, competition for
resources and specific antagonisms (such as antibiotic production) [this is an
animals,
and
example of “antagonisms”? YES] from the multitude of soil organisms
farms.
Then
usually keep soilborne plant diseases from severely damaging a natural
we’ll
briefly
grassland or forest.
Self-sufficiency. A consequence of efficiency and diversity in natural
discuss practices
terrestrial ecosystems is that they become self-sufficient—requiring only
that contribute to
inputs of sunlight and rainfall.
creating vital and
Self-regulation. Because of the great diversity of organisms, outbreaks (or
huge population increases) of diseases or insects that severely damage plants
strong
or animals are uncommon. In addition, plants have a number of defense
agricultural
mechanisms that help protect them from attack.
systems
Resiliency. Disturbances, such as climate extremes, occur in all
ecosystems—natural or not. The stronger ones are more resistant to
(discussed
in
disturbances and are able to bounce back more quickly.
more
detail
in
—Modified from Magdoff (2007).
later chapters).
Ecological crop
and
soil
management practices can be grouped under one or more of three overall strategies:
• grow healthy plants with strong defense capabilities
• stress pests
• enhance beneficial organisms
These overall strategies are accomplished by practices that maintain and enhance the
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habitat both above ground and in the soil. Ecological approaches call for designing the
field and farm to take advantage of the inherent strengths of natural systems. Most of this
is done prior to, and during, planting a crop and has the goal of preventing problems from
developing by contributing to one or more of the three overall strategies. However, there
are also routine management practices that occur during the season even if you have done
a lot of preventive management. For example, irrigation is frequently needed for highvalue crops such as fresh market vegetables—even in humid regions. Also, scouting for
pest problems and beneficials [phrasing ok? YES] should be part of routine management
during the season. If an unanticipated problem, such as an insect outbreak, arises,
remedial action, such as applying the most ecologically sound pesticide or releasing
purchased beneficials into the field, may be required to save the crop.
Ecological principles provide a good framework for sustainable management, but we
must also recognize that crop production is inherently an “unnatural” process because we
favor one organism (the crop plant) over the competing interests of others. With currently
available pesticides, the temptation exists to simply wipe out competitors—for example
through soil fumigation—but this creates dependency on purchased materials from off
the farm and weakens the overall resiliency of the soil and cropping system. The goal of
ecological crop and soil management is to minimize the extent of reactive management
(which reacts to unanticipated occurrences) by creating conditions that help grow healthy
plants, promote beneficials, and stress pests. The discussion below and in the rest of this
book focuses on ways to maintain and enhance habitat in order to promote one or more of
the three strategies listed above.
[H1]ECOLOGICAL CROP AND SOIL MANAGEMENT
We’ll discuss ecological crop and soil management practices as part of a general
framework for approaching ecological crop management (figure 8.4). The heart of the
matter is that the strength of the system is improved by creating improved habitat both
above ground and in the soil. Although it is somewhat artificial to talk separately about
aboveground and soil habitat—many practices help both at the same time—it should
make many issues clearer. Not all of the above-ground discussion refers directly to
management of soil, but most does. In addition, the practices we’ll discuss [what
practices? modified] contribute to one or more of the overall strategies: (a) growing
healthy plants with strong defense capabilities, (b) stressing pests, and (c) enhancing
beneficial organisms.
[fig. 8.4 about here]
[H2]Above-Ground Habitat Management
There are numerous ways that the above-ground habitat can be improved to help grow
healthy plants, stress pests, and enhance beneficial organisms:
• Select crops and varieties that are resistant to local pests (in addition to other qualities
such as yield, taste, etc.).
• Use appropriate planting densities (and companion crops) to help crops grow
vigorously, smother weeds, and (with companion crops) provide some protection against
pests. In some cases, blends of two or more varieties of the same crop (one susceptible to
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a pest but with a higher yield potential, and one that’s resistant) have shown potential for
increasing total yields for wheat and rice. Even though the farmer is growing the same
crop, increased genetic diversity due to using different varieties (cultivars) seems to
provide some protection. Perhaps there are possibilities for growing mixes of other crops
as well.
• Plant perimeter (trap) crops that are more attractive to a particular pest than the
economic crop(s) growing in the middle of the field and so can intercept incoming
insects. (This has been successfully practiced by planting blue Hubbard squash on the
perimeter of summer squash fields to intercept the striped cucumber beetle.)
• Create field boundaries and zones within fields that are attractive to beneficial insects.
This usually involves planting a mix of flowering plants around or as strips inside fields
to provide shelter and food for beneficials.
• Use cover crops routinely for multiple benefits, such as providing habitat for beneficial
insects, adding N and organic matter to the soil, reducing erosion and enhancing water
infiltration into the soil, retaining nutrients in the soil, and much more. It is possible to
supply all of the nitrogen to succeeding crops by growing a vigorous winter legume cover
crop, such as crimson clover in the South and hairy vetch in the North.
• Use rotations that are complex, involve plants of different families, and, if at all
possible, include sod crops such as grass/clover hay that remain without soil disturbance
for a number of years.
• Reduce tillage. This is an important part of an ecological approach to agriculture.
Tillage buries residues, leaving the soil bare and more susceptible to the erosive effects of
rainfall, and at the same time breaks up natural soil aggregates that help infiltration,
storage, and drainage of precipitation. (The use of practices that reduce erosion is critical
to sustaining soil productivity.)
Some of these practices—use of cover crops and more complex rotations and reducing
tillage—will also be mentioned below under “Enhancing Soil Habitat” and discussed in
detail in later chapters.
[H2]Enhancing Soil Habitat
The general practices for improving the soil as a place for crop roots and beneficial
organisms to thrive are the same for all fields and farms and are the focus of our
discussions in the next chapters. However, the real questions are which ones are best
implemented, and how are they implemented on a specific farm? [where are these
questions answered?] There are many practices outlined below that may make the soil a
better environment for growing healthy plants, stressing pests, and enhancing beneficial
organisms: [reordering of sentences has been done to provide an intro sentence for
the following list; smooth out the paragraph as needed OK!]
• Add organic materials—animal manures, composts, tree leaves, cover crops, rotation
crops that leave large amounts of residue, etc.—on a regular basis (see chapters 10
through 13). [ch. 14 ok here? changed]
• Use different types of organic materials because they have different positive effects on
soil biological, chemical, and physical properties (chapter 9). [chapter ok?YES]
• Keep soil covered with living vegetation and/or crop residues by using cover crops, sod
crops in rotation, and/or reduced tillage practices (chapters 10, 11 and 16). [add ch. 10?
YES] This encourages water infiltration, reduces erosion, promotes organisms that feed
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on weed seeds, and increases mycorrhizal numbers on the roots of the following crops.
• Reduce soil compaction to a minimum by keeping off fields when they are too wet,
redistributing loads, using traffic lanes, etc. (chapter 15).
• Use practices to supply supplemental fertility sources, when needed, that better match
nutrient availability to crop uptake needs (chapters 18 through 21). This helps to reduce
both weed and insect damage as well as pollution of surface and ground waters.
• For soils in arid and semiarid climates, reduce salt and sodium contents if they are high
enough to interfere with plant growth (chapter 20).
• Evaluate soil health status (chapter 22) so that you can see improvement and know what
other soil-improving practices might be appropriate.
• Use multiple practices that improve the soil habitat (chapter 23). Each one may have a
positive effect, but there are synergies that come into play when a number of practices—
such as reduced tillage and cover crops—are combined.
Conflicting Disease Management Advice?
In this book we promote reduced tillage and [reduced?NO—OK AS IS]
retention of crop residues at the soil surface. But farmers are often
encouraged to incorporate crop residues because they can harbor disease
organisms. [sentence ok?changed, ok now] Why the conflicting advice? The
major difference is in the overall approach to soil and crop management. In a
system that involves good rotations, conservation tillage, cover crops, other
organic matter additions, etc., the disease pressure is reduced as soil
biological diversity is increased, beneficial organisms are encouraged, and
crop stresses are reduced. In a more traditional system, the susceptibility
dynamics are different, and a disease organism is more likely to become a
dominant concern, necessitating a reactive approach. A long-term strategy of
building soil and plant health reduces the need to use short-term cures.
[H1]SUMMARY
The overall strategies of ecologically sound crop and soil management focus on
prevention of factors that might limit plant growth. These three strategies are to grow
healthy plants with enhanced defense capabilities, stress pests, and enhance beneficial
organisms. There are a variety of practices that contribute to these overall goals and have
been discussed in this chapter as enhancing both above-ground habitat and soil habitat.
There is some overlap, because cover crops, crop rotations, and tillage have effects both
above and below ground. The various practices that improve and maintain soil habitats
are discussed in detail in the following chapters of part three.
As indicated in figure 8.4 [figure number correct?NO-HAS BEEN CHANGED], in
addition to the work of prevention (mainly accomplished before and during planting),
there are routine management practices that are carried out during the season, and
remedial or reactive approaches may need to be used if prevention practices are not
enough to take care of some potential threat to the crop. However, just as with human and
animal health, prevention is preferred to curing a problem after it develops. For this
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reason, the orientation of the remaining sections of the book are on practices that help
prevent problems from developing that might limit the growth or quality of plants.
SOURCES
Borrero, C., J. Ordovs, M.I. Trillas, and M. Aviles. 2006. Tomato Fusarium wilt
suppressiveness. The relationship between the organic plant growth media and their
microbial communities as characterised by Biolog. [last word ok?] Soil Biology &
Biochemistry 38: 1631–1637.
Dixon, R. 2001. Natural products and plant disease resistance. Nature. 411: 843–847.
Gurr, G.M., S.D. Wratten, and M.A. Altieri, eds. 2004. Ecological Engineering for Pest
Management: Advances in Habitat Management for Arthropods. Ithaca, NY:
Comstock Publishing Association, Cornell University Press.
Magdoff, F. 2007. Ecological agriculture: Principles, practices, and constraints.
Renewable Agriculture and Food Systems 22(2): 109–117.
Magdoff, F., and R. Weil. 2004. Soil organic matter management strategies. In Soil
Organic Matter in Sustainable Agriculture, ed. F. Magdoff and R.R. Weil, pp. 45–65.
Boca Raton, FL: CRC Press.
Park, S-W., E. Kaimoyo, D. Kumar, S. Mosher, and D.F. Klessig. 2007. Methyl salicylate
is a critical mobile signal for plant systemic acquired resistance. Science 318: 313–
318.
Rasmann, S., T.G. Kollner, J. Degenhardt, I. Hiltpold, S. Toepfer, U. Kuhlmann, J.
Gershenzon, and T.C.J. Turlings. 2005. Recruitment of entomopathic nematodes by
insect damaged maize roots. Nature 434: 732–737.
Sullivan, P. 2004. Sustainable management of soil-borne plant diseases. ATTRA,
http://www.attra.org/attra-pub/PDF/soilborne.pdf.
Vallad, G.E., and R.M. Goodman. 2004. Systemic acquired resistance and induced
systemic resistance in conventional agriculture. Crop Science 44: 1920–1934. [correct
page numbers--HAVE CORRECTED]
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