the pest to be controlled, and other soil management considerations. A pesticide can
be injected into the soil as a fumigant, or into irrigation water, or it can be sprayed
onto the soil surface. Crops can be sprayed, for example, with boom sprayers or tun-
nel sprayers or by aerial application, or they can be treated with specific pesticides.
Seeds are sometimes treated with pesticides prior to planting. Pesticides also can be
incorporated into other materials so that release of the active ingredient occurs over a
longer period of time.
Soil fumigants are a special category of pesticides that are highly mobile in the
soil-water-air environment. Because of environmental and health concerns, several
fumigants, notably methyl bromide, have been banned during the last decade.
Pesticides also have the potential to damage important organisms in the soil.
Research has shown that less than 1 percent of pesticides that are applied to crops
actually reach their targets. The remainder can often, therefore, end up in soil.^45
Although this problem has not been widely researched, it is known that pesticides
have the capacity to destroy earthworms, fungi, and bacteria. Soil organisms are vital
to the proper functioning of agricultural systems. Most importantly, earthworms and
microorganisms break down organic matter and make nitrogen and other nutrients
accessible to plants. Some earthworm species are particularly vulnerable to the toxic
effects of pesticides.^46
Earthworms Are Beneficial
Although one acre of soil may hold up to 8 million earthworms, most people pay
little attention to these productive and beneficial animals. They mostly go unnoticed
from day to day, unless a heavy rain forces them to the surface of the soil, an angler
needs some bait, or their casts (fecal matter) disrupt a game of golf.
Earthworms benefit the soil in many ways, primarily due to the physical and
chemical effects of their casts and burrows. Earthworm casts, consisting of waste
excreted after feeding, are composed mostly of soil mixed with digested plant resi-
dues. Casts modify soil structure by breaking larger structural units (plates and
blocks) into finer, spherical granules. As plant material and soil passes through an
earthworm’s digestive system, its gizzard breaks down the particles into smaller frag-
ments. These fragments, once excreted, are further decomposed by other worms and
microorganisms. Earthworm casts can contribute up to 50 percent of the soil compo-
sition in some soils.
Many species of earthworms deposit their casts beneath the soil surface within their
burrows, where casts contribute to the formation and development of the soil. Species
that excavate permanent, vertical burrows, however, deposit their casts on the soil sur-
face, where they play a greater role in soil development. In addition to benefiting soil
structure, casts also provide nitrogen in a usable form for other organisms that
decompose organic matter on the soil surface.^47
Earthworms are generally found in the top twelve to eighteen inches of soil because
this is where food is most abundant. Pesticides applied to control turf diseases or
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