Soil Pollution

Soil pollution is the build-up in soil of substances -- chemical, biological, or radioactive --

at concentrations high enough to harm plant growth, soil organisms, or the humans and animals that

ultimately depend on that soil, either directly or through the food chain. Two sources dominate.

Pesticides. Agricultural pesticides, especially older chlorinated organic compounds such as DDT

(dichlorodiphenyltrichloroethane), are applied deliberately to kill insect pests, but a large

fraction of what is sprayed ends up in the soil rather than on the target pest. Chlorinated

pesticides of this kind are described as persistent pollutants because their carbon-chlorine

bonds resist the normal microbial and chemical breakdown pathways that degrade most organic matter

in soil -- so instead of disappearing over weeks or months, they can remain chemically intact in

soil for years or even decades.

This persistence is what drives bioaccumulation: because the pesticide is fat-soluble and is

not readily excreted, an organism exposed to it (for example, an earthworm or insect absorbing it

from soil) accumulates it in its tissues faster than its body can eliminate it, so the concentration

inside the organism steadily rises above the (low) concentration in the surrounding soil. When a

predator eats many such contaminated organisms, it accumulates their combined pesticide burden, and

this effect compounds at each successive step up a food chain -- a process called

biomagnification. As a result, a top predator (a fish-eating bird, for instance) can end up with

pesticide concentrations in its tissues many times higher than the concentration ever present in

the original soil or water, even though it was never directly exposed to a high dose.

Industrial waste and heavy metals. Industrial and mining activity contaminates soil in two main

ways: solid waste and slag dumped or landfilled directly onto land, and airborne particulate

fallout or liquid effluent that settles into or is absorbed by soil over time. A particularly

persistent form of this contamination is heavy-metal pollution -- soil contaminated with metals

such as lead, cadmium, mercury or arsenic from smelting, battery manufacture, mining tailings or …