Water in the Soil - Agroecology: The Ecology of Sustainable Food Systems - page 118

PLANTS' UPTAKE OF SOIL MOISTURE

1% of the total soil-particle surface area within the volume
of soil occupied by a plant's roots is actually in contact
with root surfaces. This fact underlines the importance of
capillary movement of water and the complementarity of
water movement and root extension.
Most annual plants distribute most of their roots in
the upper 25 to 30 cm of the soil, and as a result, absorb
most of their water from that horizon. Many perennial
plants such as grapes and fruit trees have roots that extend
much more deeply and are able to pull moisture from
deeper in the soil profile. But even these plants probably
rely heavily on water that is absorbed by roots in the upper
horizons when it is available — the usual situation during
the cropping cycle. When water is not sufficient, even
annual plants such as squash and corn will rely on their
deeper roots in an attempt to replace transpirational losses.
The relationship between soil moisture and plants'
water needs is the result of a complex interaction between
soil conditions, rainfall, or irrigation regimes, and the
needs of the crop. Farmers try to maintain a balance
between these components during the cropping season,
but many times events or conditions occur, which shift the
balance toward an excess of soil moisture or a deficiency.

While they are transpiring, plants must continually replace
the significant amount of water they lose through their
stomata. At any one time, however, only a small propor-
tion of available soil water is close enough to the root
surfaces that actually absorb the water. Two processes
compensate for this limitation. First, water is drawn pas-
sively through the soil to root surfaces through capillary
movement of water, and second, plant roots actively grow
into the soil toward areas with sufficient moisture for
uptake.

C APILLARY M OVEMENT OF W ATER

As a plant takes in water through its roots to replace that
which it loses through T, the soil moisture content of the
area immediately surrounding the root is reduced. This
increases the energy of suction in that region, creating a
gradient of lower water potential that tends to draw mois-
ture in all directions from the surrounding soil. Most water
is probably drawn from deeper in the soil profile, espe-
cially when the water table is close to the surface. Capil-
lary movement is due partly to the attraction of water
molecules to soil particle surfaces, and partly to the attrac-
tion of water molecules to each other. The speed at which
capillary movement occurs depends on the intensity of the
water deficit and the type of soil. In most sandy soils,
movement is fairly rapid because the larger-sized particles
hold water less tightly. In soils with more clay, especially
those with poor crumb structure, movement is much
slower.
It has been shown that water can move only a few
centimeters a day through capillary action. But due to the
extensive volume of soil occupied by most root systems,
movement of any greater distance is probably not needed.
Plants can obtain a large proportion of their water needs
through capillary movement even when T rates are very
high. The increased suction pressure created in the imme-
diate root zone during the day is replaced by water move-
ment from areas of lower suction during the night. It is at
times when soil moisture content has been severely
depleted and plant growth has slowed that such movement
is of greatest significance. Otherwise, the plant reaches
permanent wilting point.

EXCESS WATER IN THE SOIL

When excess water is present in an agroecosystem for an
extended period of time, or movement of excess water out
of the system is impeded, the condition known as water-
logging can occur. High rainfall, poor irrigation manage-
ment, unfavorable topography, and poor surface drainage
can bring about waterlogging and associated changes in
the soil ecosystem. Waterlogged soils occur throughout
the world, ranging from riverbank sediments to marshes,
swamps, and peat bogs. Even well drained soils can expe-
rience periods of waterlogging if they are subject to sea-
sonal flooding.
Waterlogging occurs frequently and broadly enough
that agricultural systems around the world have developed
ways of dealing with excess water. More recently, this has
involved the construction of costly draining and damming
infrastructures. Simpler and traditional techniques, in con-
trast, have the goal of working with the condition of excess
water rather than getting rid of it. In many wet areas of
the world, for example, rice is cultivated as a crop ideally
suited to wetland agriculture.

E XTENSION OF R OOTS I NTO THE S OIL

Plants are continually extending roots into the soil, ensur-
ing that new sites of root contact with the soil are being
established. Roots, rootlets, and root hairs all combine to
produce an extensive network of soil-root interface.
Despite continued root penetration and the large volume
of the root network, the total amount of any particular soil
volume that is in contact with a plant's roots at any one
time is very small. According to most estimates, less than

N EGATIVE E FFECTS OF E XCESS W ATER

In a soil where air fills the pore spaces between soil
particles, oxygen diffusion is rapid and there is rarely a
deficiency of O 2 for ecological process (i.e., root metabo-
lism, decomposer activity, etc.). But when the pores are
filled or saturated with water, the diffusion rate of O 2 is
greatly reduced. Oxygen movement in saturated soil can