microbes perform va rious oxidations and reductions by which mine rals s uch as iron,
manganese, and sulfur are transformed and made available to plants.
Also present in prodigious numbers are microscopic mites and primitive wingless insects called
springtails. Despite their small size they play an important part in breaking down the residues of
plants , aiding in the s low conve rs ion of the litter of the forest floor to soil. The specialization of
some of these minute creatures for their task is almost incredible. Several species of mites, for
example, can begin life only within the fallen needles of a s pruce tree. Sheltere d he re, they
diges t out the inner tissues of the needle. When the mites have completed their development
only the outer layer of cells remains. The truly s taggering tas k of dealing with the treme ndous
amount of plant material in the annual leaf fall belongs to s ome of the s mall ins ects of the s oil
and the f ores t floor. The y macerate a nd diges t the leaves , and aid in mixing the decompos ed
matter with the surface soil.
Besides all this horde of minute but ceaselessly toiling creatures there are of course many
larger forms, for soil life runs the gamut from bacteria to mammals. Some are pe rma nent
res idents of the dark s ubs urface layers ; s ome hibernate or s pend definite parts of their life
cycles in underground chambe rs ; s ome freely come and go between their burrows and the
uppe r worl d. In general the effect of all this habitation of the soil is to aerate it and improve
both its drainage and the penetration of wate r throughout the layers of plant growth.
Of all the larger inhabitants of the soil, probably none is more important than the earthw or m.
Over three qua rte rs of a century ago, Charles Darwin publis hed a book titled The Formation of
Vegetable Mould, through the Action of Worms, with Observations on Their Habits. In it he gave
the world its firs t unders tanding of the funda mental role of ea rthw or ms as geologic agents for
the trans port of s oil—a picture of s urface rocks being gradually covered by fine s oil brought up
from below by the worms , in annual amounts running to many tons to the acre in mos t
favorable areas. At the same time, quantities of organic matter contained in leaves and grass
(as much as 20 pounds to the s quare yard in s ix months ) are drawn down into the burrows and
incorporated in soil. Darwin’s calculations showed that the toil of earthworms might add a layer
of soil an inch to an inch and a half thick in a ten-year period. And this is by no means all they
do: their burrows aerate the soil, keep it well drained, and aid the penetration of plant roots.
The presence of earthworms increases the nitrifying powers of the s oil bacteria and decreas es
putrefaction of the soil. Organic matter is broken down as it passes through the diges tive tracts
of the worms and the s oil is enriched by their excretory products. This s oil community, then,
cons is ts of a web of interwove n lives , each in s ome way related to the othe rs—the living
creatures depending on the s oil, but the s oil in turn a vital element of the earth only s o long as
this community within it flouris hes.
The proble m that concerns us here is one that has received little cons ideration: What happens
to thes e incredibly nume rous and vitally necess ary inhabitants of the s oil whe n pois onous
chemicals are carried down into their world, either introduced directly as soil ‘sterilants’ or
borne on the rain that has picked up a lethal contamination as it filters through the leaf canopy
of fores t and orchard and cropland? Is it reas onable to s uppos e that we can apply a broad-
s pectru m insecticide to kill the burrowing larval stages of a crop-des troying ins ect, for example,
without also killing the ‘good’ ins ects whos e function may be the ess ential one of breaking
down organic matte r? Or can we us e a nons pecific fungicide without also killing the fungi that
backadmin
(backadmin)
#1