Soil Health, Plant Health, and Pests
SOIL PROPERTIES AND THEIR INTERRELATIONSHIPS
Healthy soils occur when their biological, chemical, and physical conditions are all optimal , 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.
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.
PLANT DEFENSES, MANAGEMENT PRACTICES, AND PESTS
Before discussing the key ecological principles and approaches to soil management, let’s first see how amaz-ing 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 feed-ing. 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 benefi-cial 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 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.
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 root-worm—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 reintroduc-tion 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) . 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 tak-ing advantage of induced resistance. However, there currently are no reliable and cost-effective indicators to
determine whether a soil amendment or soil is enhanc-ing a plant’s defense mechanisms. More 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 protec-tion from attack by a variety of pathogens and insects.
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 3 help to reduce the severity of crop pests. Healthy plants growing
