Water in the Soil - Agroecology: The Ecology of Sustainable Food Systems - page 122

Because plant material decomposes so quickly in the tropics, very little evidence remains to indicate what types
of crops were cultivated on the raised beds. Also impossible to deduce are the planting patterns of the Mayans or
the frequency of cleaning the canals.
However, many traditional farmers in lowland regions of Mexico farm wetlands and their margins today, mainly
in areas with populations of indigenous ancestry, and it is highly possible that the practices they use were handed down
from Mayan ancestors. By looking at these current systems, it may be possible to infer how Mayan agriculture may
have developed. Corn and beans form the basis of the current cropping system, and it is important to note that when
corn is grown in these wetland-adapted agroecosystems, farmers achieve yields over four times higher than those of
nearby fields that have been cleared and drained using modern technology. Researchers are attempting to use the
archeological information from pre-Hispanic systems, combined with knowledge of present-day wetland use, to
reconstruct more sustainable farming systems for present day farmers of the region (Jimenez-Osornio & Rorive, 1999).

SOIL WATER DEFICIENCY THE ECOLOGY OF IRRIGATION

When the rate of moisture loss from a soil through ET is
greater than the input from rainfall or irrigation, plants
begin to suffer. Evaporation depletes the water supply in
the upper 15 to 25 cm of the soil, and depending on the
rooting characteristics and T rates of the plants in the soil,
depletion can extend to a greater depth as plants lose water
to the atmosphere through T. As moisture is depleted from
the soil, soil temperatures near the surface begin to rise,
increasing the rate of evaporation even more. When the
easily available water held to soil particles is depleted
through these processes, levels of soil moisture may reach
the permanent wilting point for plants.
If temporary wilting consistently occurs, leaves begin
to yellow, and growth and development are generally
retarded. Leaves expand more slowly, are smaller, and age
sooner. Photosynthetic rates drop in a stressed leaf, and a
larger amount of assimilated photosynthate is stored in the
plant roots. From a crop production point of view, such
responses are usually negative since they result in a reduc-
tion in harvestable product. But from an ecological perspec-
tive, such responses may provide some adaptive advantage
to the plant. For example, the allocation of more carbon to
the roots of a water-stressed plant may promote more root
growth, allowing the plant to draw moisture from a broader
area. Water stress may force earlier flowering, fruiting, and
seed formation, helping to ensure the survival of the species.
In some cases, farmers can actually take advantage of such
drought responses, as when water is withheld from cotton
plants in late summer to force defoliation and avoid the
need for chemical defoliants before harvest.
Many plants have specific structures or metabolic
pathways that aid in survival under water-stressed condi-
tions. Farmers in an area subject to periodic water stress
would do well to look for crop species and varieties that
demonstrate some of these adaptive traits. Some examples
of drought-tolerant crops are certain cacti species,
garbanzo beans, sesame, nut crops such as pistachio, and
certain deep-rooted perennials such as olives and dates
(Figure 9.6).

In natural ecosystems, vegetation is adapted to the soil
moisture regime set by climate and soil type. Agro-
ecosystems, on the other hand, often introduce plants
with water needs that exceed the ability of the natural
ecosystem to supply those needs. When this is the case,
irrigation is used to provide adequate soil moisture for
crops.
Irrigation represents a major change in ecosystem
function and generates its own particular ecological
problems. At the same time, water supply systems are
costly in terms of both money and energy. Their use must
balance ecological and economic costs if long-term sus-
tainability is to be achieved.
Water harvesting, storage, and delivery systems can
have major impacts on surface and subterranean water
flow. Aquifers can be overdrafted, and the ecology of
riverine, riparian, and wetland ecosystems can be severely
damaged. Since maintaining healthy waterways and water
supplies is as important as maintaining profitable crop
production, the impacts of water-supply systems on local
and regional hydrology must be taken into account (Postel
and Richter, 2003).

S ALT B UILDUP

Nearly all irrigation waters contain salts that can damage
crops if allowed to accumulate. Since irrigation is used
primarily in areas with high ET potential, the deposition
of salts at the soil surface over time is inevitable. If uncon-
trolled, this buildup, called salinization, can reach levels
unfavorable for crop production, especially when the salts
contain toxic trace elements such as boron and selenium
(Figure 9.7). Total salt content is measured as electrical
conductivity in mhos. For each 1.0 millimhos per centi-
meter of applied irrigation water, the salt content of the
water increases by about 640 ppm. Careful monitoring of
salt levels in irrigated soils along with analysis of the salt
content of incoming irrigation water can help avoid exces-
sive buildup.