Em4885 irrigation management practices to protect ground water

13
CHAPTER 3
Retention of water by soil
Soil holds water, retaining it against the pull of gravity. The matrix-like structure of soil
will generate forces that retain water within the voids of the soil structure. The important
aspect of this fact for mankind is that some of the water held is available for use by plants.
There are limits to the amount of water soil will hold and the amount that the plant
can extract.
The upper limit to a soil’s water holding ability is termed “field capacity.” Field capacity
is not saturation. Saturation occurs when most, if not all, soil structure voids are full of
water. The amount of water held in a soil at field capacity is much less than the amount
of water in the same soil at saturation. In general, the amount of water held at field
capacity is about one-half the saturated capacity.
IMPORTANT!! Water will soak into soil that is at field capacity because
there are still open voids in the soil. However, this water will not be held
by the soil. It will drain down through the soil until it reaches an area that
is less than field capacity or reaches a saturated zone.
Soil will not give up all its held water to the plant. The holding forces that retain water
against the pull of gravity can also retain water against the plant. The key fact is that as
the amount of water held by the soil decreases, the holding forces increase. Thus, as the
plant extracts more and more water from the soil, these holding forces increase. At some
point the holding forces of the soil are greater than the plant can overcome and no more
water can be extracted. This is the “permanent wilting point.”
Water in the soil above the permanent wilting point is “available water.” That is, this soil
water is available for the plant to use.
The amount of water held between field capacity and the permanent wilting point is the
“available water holding capacity” of the soil. This is the most water that the soil will
hold available to the plant.
The difference between the total amount of water in the soil at any one time and the soil’s
field capacity is termed the “soil water deficit” or “soil water depletion.” This is a very
important concept for irrigators. The soil water deficit is how much water is needed to
refill a soil to field capacity.
Volumetric soil water measurement
There are two ways of describing the amount of water in soil. One is “volumetric”
measure. This is a direct measure of the water in the soil. The standard of measurement
is “inches of water held per foot of soil” or just “inches per foot.” (Some will use a
standard of measurement of inches of water held per inch of soil, or inches per inch.)
To explain this expression, consider that soil is a porous substance. Both water and air are
contained within the pores. If a cubic foot of soil could be extracted from the soil and all