Soil - Agroecology: The Ecology of Sustainable Food Systems - page 111

Other Soil Amendments organic matter content, and cause the soil to lose some

of the elements of productivity. For this reason, paying
attention to how the soil is cultivated must be an integral
part of soil organic matter management.
Many different patterns of soil tillage exist, but the
main pattern employed in conventional agriculture is a
three-stage process involving a deep plowing that turns
the soil, a secondary tilling for preparation of a seed bed,
and finally postplanting cultivations (often combined with
herbicide use) for controlling weeds. Soil erosion, loss of
good soil structure, and nutrient leaching are well-known
problems associated with this pattern of tillage. Despite
these problems, most conventional farming systems,
especially those producing annual grains and vegetables,
are dependent on extensive and repeated tillage.
At the other extreme, there are many traditional farm-
ing systems in which no tillage is used at all. In swidden
agriculture, traditional farmers clear land using slash and
burn techniques and then poke the soil with a planting
stick to sow seeds. Such systems, which have the longest
history of sustained management, respect the need for a
fallow period to control weedy vegetation and to allow
natural soil building processes to replace removed nutri-
ents. Many agroforestry systems, such as coffee or cacao
under shade, depend on the tree component of the system
to provide soil cover and nutrient cycling, and only receive
occasional surface weeding. Permanent pasture is rarely
cultivated either.
Alternative tillage techniques, many of them borrowed
from traditional farming practices, have been developed
for and tested in conventional annual crop systems. These
have demonstrated that annual crop systems do not have
to remain dependent on extensive and repeated tillage and
that reduced tillage can help improve soil quality and
fertility (Franzluebbers, 2004).
Using the technique of zero tillage , soil cultivation is
limited to the actual seedbed and is done at the time of
seed planting. In some cases, special equipment is used
that allows planting directly into the crop residue left from
the previous crop. Other steps, such as fertilization and
weed control, can be completed at the same time as
planting. Unfortunately, many zero tillage systems have
developed a great dependence on herbicides, which may
create other ecological problems.
In order to reduce herbicide use, a number of reduced
tillage systems have been developed. One in particular
that has been quite successful for corn and soybean pro-
duction is ridge tillage . After an initial plowing and for-
mation of planting beds or ridges, the only cultivation that
occurs is seed planting and weed management with spe-
cially designed tillers that cultivate the surface of the soil
only. Some ridge till systems can go through many years
of repeated planting without deep tillage, and the reduced
soil disturbance helps preserve soil organic matter and
structure. The Thompson Farm in Boone County, Iowa,

A range of other types of organic soil amendments can be
used as well. Humates, kelp, fishmeal, animal by-products,
mined guano, and others are on the market. Each one has
specific applications, advantages and disadvantages, and
optimal scales of use. Each organic matter source needs to
be examined for short-term crop response, but more impor-
tantly for possible long-term contributions to soil organic
matter development and maintenance.

Sewage

A final source of organic matter — underutilized except in
a few parts of the world — is sewage. To complete nutrient
cycles, nutrients that leave the farm should ultimately come
back to the farm. If they can come back in an organic form,
then they will also add to the soil-building process.
Solid material removed from wastewater during treat-
ment, known as sewage sludge, has been spread on the
land for decades. As a percentage of dry weight, sewage
sludge can contain 6 to 9% nitrogen, 3 to 7% phosphorus,
and up to 1% potassium. It can be applied as dried cake
or granules, with water content of 40 to 70%, or as a liquid
slurry that is 80-90% water. Sewage sludge is widely used
on turf grass, degraded rangeland, and even on the ground
below fruit trees. The liquid portion of treated sewage,
known as effluent, has been applied to land for a long time
in Europe and selected sites in the U.S. Some cities oper-
ate, what are called, sewage farms where effluent is used
to produce crops, usually animal feeds and forages, that
partially offset the cost of disposal, where in other cases,
it is used for irrigating golf courses, highway landscaping,
and even forests.
There is much to learn, however, about how to treat
sewage so that pathogens are dealt with properly. Collec-
tion, treatment, and transport all need to be examined with
an eye toward the goal of linking waste management with
sustainable agriculture. The fact that many sewage sys-
tems around the world do not separate human from indus-
trial wastes, contaminating the resultant sludge with toxic
amounts of heavy metals, complicates the process
immensely.
Nevertheless, sewage will undoubtedly become a more
important resource in the future as a source of organic
matter, nutrients, and water for crop production. Many
small-scale and traditional practices for turning sewage
into a useful resource can serve as an important basis for
future research on this important link to sustainability.

T ILLAGE S YSTEMS

The conventional wisdom in agriculture is that soil must
be cultivated to control weeds, incorporate organic matter,
and allow root growth. Despite its potential benefits,
however, cultivation can degrade soil structure and