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Transcript
8 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 1 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 2 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 3 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 4 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 5 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 6 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 7 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] 8