Humidity and Rainfall - Agroecology: The Ecology of Sustainable Food Systems - page 77

Several examples of how agroecosystems function
within the constraints of local rainfall regimes are pre-
sented below, providing another way of examining the
aspects of sustainability inherent in farming approaches
that work with ecological conditions rather than striving
for their alteration or control. These examples were chosen
to cover the range from very wet to very dry rainfed
agriculture. The aspects of managing moisture once it gets
into the soil will be described in more detail in Chapter 9.

than the purchase of hybrid or “improved” seed produced
at distant locations. The name of one corn variety —
mejen , from a Maya word meaning “precocious” or “early
maturing” — shows the link to the past that this system
may have (Figure 6.3).
The corn grows very quickly in this system, and when
fire is not used excessively and flooding is allowed to
occur every year, weeding is usually not necessary. After
about two and a half months of growth, the mature corn
stalks are “doubled-over” just below the corn ear, facili-
tating final drying of the grain for another 2 to 4 weeks
before harvest. Yields of 4 to 5 t/ha of dry grain are
common, with some yields reaching 10 t/ha. This is many
times the average yield of 1 to 1.5 t/ha for mechanized
production on lands that have been cleared and drained in
the same region. These greater yields are obtained at a
fraction of the input costs and labor invested in mecha-
nized production systems (Amador, 1980).
Following the harvest, all crop and noncrop residues
end up on the soil surface. This contributes to a key
element in the productivity of the system — maintenance
of organic matter in the soil. Soil profiles demonstrate
the presence of a thick, organic-rich soil to a depth of
30 to 40 cm below the surface. During the 9-month inun-
dation, organic matter produced by the marsh plants or
left by the previous cropping cycle is incorporated into
the soil and conserved in the anoxic conditions under
water. In addition, nutrient minerals that enter the system
with surface drainage are captured by the highly produc-
tive aquatic sector of the ecosystem. These factors result
in the formation of a soil that has organic matter levels
over 30%, total nitrogen as high as 3%, and high levels
of other important plant nutrients. The key element in the
management of this system, then, is the way in which
inundation during the wet season is taken advantage of.
When the system is drained artificially in an attempt to
extend the cropping season, the organic layer in the soil
can be reduced to 5 cm in less than 2 years, and yields
drop dramatically.

A GROECOSYSTEMS A DAPTED TO A L ONG W ET S EASON

In very humid regions with extended rainfall, farmers are
concerned more with excess water than with water defi-
cits. Frequent and heavy rainfall creates problems of
waterlogging, root diseases, nutrient leaching, abundant
weed growth, and complications for most farming opera-
tions. Even wetland-adapted crops such as rice or taro are
difficult to manage in regions with a long wet season.
Conventional approaches to excess precipitation most
often look to some type of major habitat modification such
as drainage projects and flood control. An agroecological
approach to an extended wet season, in contrast, looks for
ways to accommodate the system to the excess moisture.
A very interesting and productive use of land that is
flooded for the entire wet season is seen in Tabasco,
Mexico (Gliessman, 1992a). This region receives more
than 3000 mm of rainfall distributed over a long wet
season that extends from May until February of the next
year. The staple local crop of corn is planted on higher
ground around wetlands that are shallowly flooded dur-
ing most of the year. In March, however, the drop in
rainfall permits the planting of another corn crop. Low-
lying areas dry out enough for the soil surface to become
exposed. Farmers follow the receding water line with
this special corn planting, known locally as the March
planting or marceño .
During much of the year, constant rainfall keeps the
low areas inundated to a depth that ranges from a few
centimeters to as much as a meter. The marsh vegetation
that densely covers these areas during the wet season is
felled quickly with machetes as the water level recedes.
A very dense, 10 to 20 cm mat of organic matter is pro-
duced by this process. Seed is planted into holes made
with a pointed stick driven into the mat. About a week
after the sowing, fire is used to burn part of the organic
mat, as well as to kill back any weed seedlings or sprouts
of the marsh plants. The burning must be timed so as to
burn only the dry leaves on top of the mat and not the
moist lower layers or the soil. The corn seed, planted 10
to 15 cm below the surface of the soil, is not harmed by
the fire. Local short-cycle varieties of corn (2 to 3 months
from planting to harvest) are most frequently used. The
practice of using seed from the previous harvest for the
subsequent planting favors the use of local varieties, rather

A GROECOSYSTEMS A DAPTED TO A LTERNATING
W ET -D RY S EASONS IN THE T ROPICS

Many parts of the world have a monsoon-type climate in
which average annual rainfall is relatively high, but nearly
all the rain falls during a wet season of medium length.
Farmers in these areas have to deal with excess rainfall
at one time, and a lack of rainfall at another.
A very interesting and productive agroecosystem in
such an alternating rainfall regime has been observed
in the state of Tlaxcala, Mexico (González, 1986; Anaya,
et al., 1987; Crews & Gliessman, 1991; Wilken, 1969).
In an area known as the Puebla Basin, a triangular flood
plain of about 290 km 2 is formed where the Atoyac and
Zahuapan Rivers meet in the southern part of the state.