The Need for Sustainable Food Production Systems - Agroecology: The Ecology of Sustainable Food Systems - page 9

per year in Africa, South America, and North America,
and almost 30 tons/ha annually in Asia. In comparison,
soil is created at the rate of about 1 ton/ha per year,
which means that in just a short period, humans have
wasted soil resources that took thousands of years to be
built up.
The cause-effect relationship between conventional
agriculture and soil erosion is direct and unambiguous.
Intensive tillage, combined with monoculture and short
rotations, leaves the soil exposed to the erosive effects of
wind and rain. The soil lost through this process is rich
in organic matter, the most valuable soil component. Simi-
larly, irrigation is a direct cause of much water erosion of
agricultural soil.
Combined, soil erosion and the other forms of
soil degradation render much of the agricultural soil

of the world increasingly less fertile. Some land —
severely eroded or too salty from evaporated irrigation
water — is lost from production altogether. The land
that can still produce is kept productive by the artificial
means of adding synthetic fertilizers. Although fertil-
izers can temporarily replace lost nutrients, they cannot
rebuild soil fertility and restore soil health; moreover,
their use has a number of negative consequences, as
discussed above.
Since the supply of agricultural soil is finite, and
because natural processes cannot come close to renewing
or restoring soil as fast as it is degraded, agriculture cannot
be sustainable until it can reverse the process of soil
degradation. Current agricultural practices must undergo
a vast change if the precious soil resources we have
remaining are to be conserved for the future.

FIGURE 1.5 Severe soil erosion on a sloping hillside following intense winter rains. In this strawberry growing region in the
Elkhorn Slough watershed of central California, soil losses exceed 150 tons/acre in some years.