Soil - Agroecology: The Ecology of Sustainable Food Systems - page 107

and humic substances (humus). The nonliving component
is by far the greater proportion.
Interaction between the living and nonliving organic
matter is constant. The complex carbon compounds in
fresh plant litter are rapidly metabolized or decomposed,
undergoing a process known as humification that eventu-
ally imparts a darker color to the soil as it produces humic
residues, or humus. Humic residues consist of condensed
aromatic polymers that are normally relatively resistant to
further breakdown, and normally are capable of becoming
stabilized in soil. The organic matter fraction that becomes
stabilized, though, eventually undergoes mineralization,
releasing mineral nutrients that can be taken up by plant
roots. An equilibrium is reached between humification and
mineralization, but this equilibrium is subject to shifts
depending on farming practices.
During its life in the soil, organic matter plays many
very important roles, all of which are of importance to
sustainable agriculture (Magdoff and Weil, 2004). Apart
from providing the more obvious source of nutrients for
plant growth, organic matter builds, promotes, protects,
and maintains the soil ecosystem. As we have already
discussed, soil organic matter is a key component of good
soil structure, increases water and nutrient retention, is the
food source for soil microorganisms, and provides impor-
tant mechanical protection of the soil surface. Depending
on the cropping practices used, however, these traits can
be rapidly altered — for the better as well as for the worse.
Of all of the characteristics of soil, the factor that we can
manipulate the most is soil organic matter.
Once a soil is put under cultivation, the original
organic matter levels begin to decline unless specific steps
are taken to maintain them. After an initial rapid decline,
the drop slows. Several kinds of changes occur in the soil
as a consequence of the loss of organic matter. Crumb
structure is lost, bulk density begins to rise, soil porosity
suffers, and biological activity declines. Soil compaction
and the development of a hardened soil layer at the average
depth of cultivation, called a plow pan, can become prob-
lems as well.
The extent to which organic content declines in soil
under cultivation is dependent on the crop and cropping
practices. Some examples follow.
In one study, the organic matter content of the upper
25 cm of soil in two agroecosystems used for intensive
vegetable production in coastal central California were
compared with each other and to an unfarmed grassland
control. One system had been farmed for 25 years using
organic farming practices; the other for 40 years under
conventional practices. The study showed that the organic
matter content had been reduced from 9.869 to 8.705
kg/m 3 in the organic system and to 9.088 kg/m 3 in the
conventional system (Waldon, 1994). Even with the higher
inputs of organic matter in the form of composts and
winter cover crops in the organic system, cultivation and

cropping significantly reduced soil organic matter even
more than in the conventional system.
In another case, after 15 years of continual production
of grains such as corn and rice, organic matter in the
initial 15 cm of a heavy alluvial clay in the humid low-
lands of tropical Tabasco, Mexico, had lowered to less
than 2%, as compared to an organic matter content of
more than 4% in an adjacent area of uncut tropical forest
(Gliessman and Amador, 1980). A tree-covered cacao
plantation on the same soil type was able to maintain the
soil organic matter in the same layer at 3.5%, demonstrat-
ing the negative impact of soil disturbance on organic
matter in cropping systems and the role of vegetative
cover in retaining it.
A study comparing soils after 75 years of organic and
conventional wheat production in eastern Washington
found that organic matter was not only maintained in the
organic system, but also actually increased over time,
while production levels for the organic farmer were near
equal to the conventional (Reganold et al., 1987). We can
see from these three examples that crop type, input man-
agement, local environment, and cultivation practices all
help determine the long-term impacts of farming on soil
organic matter.

SOIL MANAGEMENT

In present-day farming systems, soil is treated as if it were
mainly a medium for holding the plant up. When soil is
managed for sustainable production and emphasis is
placed on the role of soil organic matter, however, the role
of soil is greatly expanded.
Many farmers feel that if a high yield is obtained
from the land, then this is evidence of a productive soil.
However, if the perspective is agroecological and the goal
is to maintain and promote all of the soil-forming and
soil-protecting processes involving organic matter, then
a productive soil is not necessarily a fertile soil. The
processes in the soil that enable us to produce a crop take
on greater importance in sustainable agriculture. Fertiliz-
ers can be added to raise production, but only through an
understanding of nutrient cycles and soil ecological pro-
cesses — especially soil organic matter dynamics — can
soil fertility be maintained or restored.

M ANAGEMENT OF S OIL O RGANIC M ATTER

The first step in developing soil organic matter is to main-
tain constant inputs of new organic matter to replace that,
which is lost through harvest and decomposition. If the
agroecosystem were more similar to a natural ecosystem,
a diversity of plant species would be present in addition
to the crop or crops being grown for harvest. Many
agroforestry systems (Chapter 17), especially in tropical
agriculture, have a large number of plants, many of them