Texture.The percentage of sand, silt, and clay in a soil determines its texture. Soil
texture influences how fast water can move through it. The more sand there is in the
soil, the easier it is for water and any contaminants (that is, pesticides) to move into
groundwater.
Acidity (pH).Soil acidity, or pH, affects the chemical properties of many pesti-
cides. As soil pH decreases, pesticides bind more to the clay in the soil and are fil-
tered out of the percolating water. Also, pesticides are usually less soluble in water at
lower pH values. Acidity is more important with some types of pesticides than others,
but is less important overall than organic matter and texture.
Other geologic and environmental factors also affect pesticide leaching to ground-
water. Depth from the soil surface to groundwater is very important. The closer the
water is to the surface, the less chance there is for a pesticide to be filtered and broken
down in the soil. Weather also plays an important role in many ways. Pesticides de-
grade faster in warm, moist soil than in cooler or drier soil. If heavy rainfall or irriga-
tion occurs soon after a pesticide application, the percolating water can carry the
pesticide deep into the soil, where it breaks down more slowly. Also, types of tillage
practices can affect soil temperature, moisture, and water infiltration, all of which
have an impact on pesticide degradation and leaching.^52
Adsorption.A soil-adsorbed pesticide is less likely to volatilize, leach, or degrade.
When pesticides are tightly bound to soil particles in highly adsorptive soil, they are
less available for absorption by plants and microorganisms. However, soil-adsorbed
pesticides can be lost by erosion. Understanding adsorption factors can reduce dam-
age to sensitive plants, leaching to groundwater, and the presence of illegal residues in
a food or in feed crops.
The behavior of pesticides under local environmental conditions is determined
with special reference to soil movement and persistence. Studies have shown that the
behavior of pesticides differs in various soils and under local conditions. It has been
shown, for example, that pesticides tend to move more readily in certain soils, and
extended half-lives of these compounds may be expected. Prolonged persistence of
soils combined with high soil mobility is indicative of greater pollution potential of a
pesticide.^53
Pesticides Disrupt Agriculture
If pesticides interrupt or destroy the microbiotic activity in the soil, it becomes
merely an anchor for plant material. In this ‘‘conventional’’ method of agriculture,
which has been in use for only the past seventy-five years out of 10,000 years of
recorded agriculture, plants can receive only air, water, and sunlight from their envi-
ronment; everything else must be distributed to the plant by the farmer, often from
inputs transported thousands of miles to reach the farm. Plants are commonly fed
only the most basic elements of plant life and so are dependent on the farmer to fight
all of nature’s challenges: pests, diseases, and drought.
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