Water in the Soil - Agroecology: The Ecology of Sustainable Food Systems - page 115

9 Water in the Soil

Water is continually flowing through the body of a plant:
leaving the stomata via transpiration (T) and entering
through the roots. For this reason, plants depend on having
a certain amount of water available to their roots in the
soil. Without adequate soil moisture, they quickly wilt and
die. Thus, maintaining sufficient moisture in the soil is a
crucial part of agroecosystem management.
Yet, soil moisture management is not simply a matter
of there being adequate inputs of water into the soil from
precipitation or irrigation. Soil moisture is part of the
ecology of the soil and of the whole agroecosystem. Water
availability and retention is affected by a myriad of factors
and water itself plays many roles. It carries soluble nutri-
ents, affects soil aeration and temperature, and impacts
soil biotic processes. A farmer, therefore, must be aware
of how water acts in the soil, how water levels in the soil
are affected by weather conditions and cropping practices,
how inputs of water affect soil moisture, and what the
water needs of the crop are.
Rarely is the moisture availability of a soil exactly
optimum for a crop for a very long period of time. Water
supply varies between deficiency and surplus from day to
day and throughout the season. The actual optimum is
hard to determine, since it is affected by a range of other
factors, and conditions are constantly changing. But we
do know a lot about the range of moisture conditions that
promote highest yields for most crops. The challenge is
to manage water in the soil in ways that keep conditions
within this range.

surface runoff or even evaporation if it cannot penetrate
the soil surface easily. Infiltration is affected by soil type,
slope, vegetative cover, and characteristics of the precipi-
tation itself. Soils with greater porosity such as sandy soils
or those with high organic matter content are more open
to easy infiltration of water. Flat terrain is more apt to
allow better infiltration than sloping ground, and a smooth
slope loses more water to runoff than one that is broken
by microtopographic variation caused by rocks, soil
clumps, slight depressions, or other obstructions on the
surface. Vegetative cover, both alive and as litter on the
surface, greatly aids initial water entry. In general, assum-
ing optimal conditions, the greater the intensity of rainfall,
the greater the infiltration rate until saturation is achieved.
However, with excessively intense rainfall, increased run-
off will occur.

P ERCOLATION

Once saturation of the upper layers of the soil occurs,
gravitational forces begin to pull the excess water more
deeply into the soil profile. This process, known as per-
colation , is shown in Figure 9.1. The rate of percolation
is determined by soil structure, texture, and porosity. A
soil with good crumb structure and aggregate stability
will allow water to move freely between soil particles.
Sandy-textured soils have larger pore spaces and less
soil-particle surface area to hold water than more finely
textured soils, and will therefore allow the most rapid
movement of water. A soil that is very high in clay
content may allow rapid percolation initially, but once
the clay micelles swell with water, they may close
the pore spaces and impede movement. Root channels
and animal burrows, especially those of earthworms, are
important pathways for percolation, but soil texture and
structure are probably of greater importance, especially
in frequently cultivated agroecosystems.

MOVEMENT OF WATER INTO AND OUT
OF THE SOIL

In natural ecosystems, water enters the system as rainfall
or snowmelt at the surface of the soil. In agroecosystems,
water enters from the same sources, as described in
Chapter 6, or is added as irrigation. Sustainable manage-
ment of soil moisture depends greatly on understanding
the fate of this applied water, with a goal of maximizing
efficiency of water use by the system.

E VAPORATION

Once moisture enters the soil, it can be lost to the atmo-
sphere through evaporation. The rate of evaporation from
the soil surface depends on the moisture content and tem-
perature of the atmosphere above the surface, as well as
the temperature of the soil surface itself. Wind greatly
accelerates the evaporation process, especially at higher
temperatures.

I NFILTRATION

For the water falling on or applied to the soil surface to
become available to plants, it must infiltrate into the soil.
Infiltration is by no means a given: water can be lost to

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