Water in the Soil - Agroecology: The Ecology of Sustainable Food Systems - page 116

E E E T E T T

T

P P

A B C D E

Water table P Precipitation

Gravitational water E Evaporation

Capillary water T Transpiration

FIGURE 9.1 Movement of water in the soil of a cropping system. A. Water infiltrates the surface after falling as precipitation,
B. gravitational water percolates downward, leaving the soil above moistened to field capacity with capillary water. At the same time,
evaporation and T begin to remove water from the soil C as gravitational water continues to percolate downward, the soil near the
surface begins to dry out. D. When the gravitational water reaches the water table, most of the soil profile is moistened close to field
capacity. The exception is the upper layer of soil, which has dried out from evaporation. E. Most of the soil above the capillary
fringe, the region kept moist by the water table, has dried out, and the soil once more nears the wilting point. (Adapted from
Daubenmire, R. F. 1974. Plants and Environment. 3rd ed. John Wiley and Sons: New York.)

Even though evaporation occurs at the surface, it can
affect soil moisture deep into the soil profile. As evapo-
ration creates a water deficit at the soil surface, the attrac-
tive forces between water molecules draw water from
below through capillary action. This process continues
until the saturated zone reaches too deep or the upper soil
layer becomes so dry that capillarity is broken. Any kind
of mulch or soil surface cover that slows the heat gain of
the soil surface and presents a barrier between the soil and
the atmosphere will slow the rate of evaporation.

SOIL MOISTURE AVAILABILITY

The attractive forces operating between water and indi-
vidual soil particles play a key role in determining how
soil moisture is retained, lost, and used by plants. Under-
standing these forces means looking at the physical and
chemical properties of the soil solution , the liquid phase
of the soil, and its dissolved solutes that are separate from
the soil particles themselves.
The percentage of moisture available for plant use in
a soil has traditionally been determined by collecting a soil
sample, measuring its weight, drying the soil at 105°C for
24 h, and then measuring its dry weight. The amount of
moisture lost during drying is divided by the sample dry
weight, giving a figure that is expressed as a percentage.
This procedure, however, is not adequate for measur-
ing the amount of water actually available to plants in the
soil because it does not take into account the important
variable of water adhesion to soil particles. As both clay
and organic matter content increase in a soil, water is
attracted more tightly to soil particles and becomes
more difficult for roots to take up. Lettuce may wilt, for

T RANSPIRATION

As described in Chapter 3, plants lose water through sto-
mata in the leaves as transpired moisture, creating a water
deficit in the plant that is balanced by uptake of water by
the plant roots. This biotic removal of water from the soil,
especially by roots that penetrate the soil layers below
those affected by evaporation, constitutes a major avenue
of water movement out of the soil ecosystem. If water is
not added to replace this loss, plants either have to go
dormant or are eliminated from the ecosystem.