Em4885 irrigation management practices to protect ground water

3 CHAPTER
The desired conductivity of the soil water solution depends on the sensitivity of the crop to
salt. Some crops are extremely sensitive to salts. Researchers have identified approximate
levels of salt that different crops withstand with no yield decline. They also have been
able to make estimates of how much yield will be lost as the root zone salinity increases.
Growers try to maintain the root zone salinity at or below the point of yield decline
if possible.
A commonly used leaching equation is one developed by Rhodes, et al., and described by
Westcott and Ayers in Water Quality for Agriculture (FAO Irrigation and Drainage Paper
29, rev. 1). It states:
ECiw
LF = .................................
((5 * ECe) - ECiw)
where:
LF = the percentage of applied irrigation water that should become deep percolation
ECiw = electrical conductivity of the irrigation water
ECe = desired electrical conductivity of saturated extract (a standard laboratory test
indicating root zone salinity)
Correcting infiltration and soil structure problems
Correcting an imbalance in the types of salts present may or may not be easy. An
important factor is the amount of free calcium in the soil. If free calcium is not present
a very common technique is to add gypsum to the soil on a regular basis. On the other
hand, there may be plenty of calcium in the soil but it is tied up as calcium carbonate or
calcium bicarbonate. Adding sulphur or sulfuric acid to the soil is common in
these situations.
Gypsum can be added to water to increase the total salt content of the water when
the combination of soil and water chemistry results in very low infiltration rates.
Increasing dissolved salts in irrigation water acts to increase infiltration rates.
Other techniques for salt management include blending of water supplies, deep tillage,
special seed bed configurations, and modified irrigation timing. Salt management can be
complex. Experts should be consulted whenever salinity problems exist. Table 3-1, taken
from Westcott and Ayers in its entirety (used by permission), provides a summary of
potential problems due to different water qualities. The notes to this table are especially
important. They indicate the conditions under which the guidelines in Table 3-1 are valid.
Also, in the notes, references to other Tables or Figures are references to those in FAO
29A, not the Manual.
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