Soil Management
Soil Health, Plant Health, and Pests
SOIL
PROPERTIES AND THEIR INTERRELATIONSHIPS
Healthy soils occur when they're 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 to 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 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 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 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 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). 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 induced resistance. However, there currently are no reliable and
cost-effective indicators to
determine
whether a soil amendment or soil is enhancing 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
protection 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
