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Water Quality Data
SALINITY AND DRAINAGE
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Sodic Soils and Soil Structure Problems
Sodic or alkali soils have a salt imbalance. Typically the ratio of sodium to calcium and magnesium is too high. A measure of this potential imbalance is the "sodium absorption ratio". The potential for a problem is also dependent on the amount and type of clay in the soil.
It is a highly technical process to explain but in the wrong situation, excess sodium "attaches" itself to clay particles and weakens the soil structure. This leads to "dispersion" (commonly called puddling) of the soil, which can clog soil pores. The permeability is lowered and it becomes harder to get water into the soil.
Some clays/clay-loams are more susceptible than
others. Many people will use the term "a shrinking/swelling" type of clay
or clay-loam soil. These types of clay soils have the most potential for
structural problems
(montmorillite, 2:1).
Sodic soils are improved by changing the chemistry of the soil. Commonly a chemical amendment, such as gypsum, is applied. The gypsum may be broadcast or mixed with irrigation water. The calcium that is in the gypsum will replace the sodium that has attached to the soil. Improving sodic soils can take some time as the infiltration rate has been reduced. This makes it harder to get the improving amendment into the soil. (It seems strange to say but when fixing a permeability problem, some Growers will purposely add salts to the water as saltier water will penetrate faster.)
Sulfuric acid can be used if there is already sufficient calcium in the soil. Acids work quicker than gypsum but must be carefully handled.
Always consult a qualified soil scientist when working with sodic soils. Take several samples of the field at different depths and have them analyzed to determine the proper amending chemical and required application rates.
Saline-Sodic Soils
Saline-sodic soils have both excess salts and the imbalance problem. They are improved by first treating the structural problem (the "sodic" problem) with chemical amendments. With the soil structure, and thus, infiltration rates, improved, the soil than can be reclaimed with leaching to remove the excess salts. Leaching Requirements When treating salt-affected soils or maintaining a salt balance, two things are required, a fairly good quality and sufficient water supply, and sufficient internal drainage. The only way to maintain yields with a salty water supply is to continually leach the excess salts applied with the irrigation water through the root zone. That is, a certain amount of applied water is meant to drain through the root zone. This internal drainage will carry excess salts out of the root zone.
There are two equations used to determine how much leaching water is required. The derivation of the first is somewhat technical and depends on some assumptions about water extraction patterns by plants from different depths in the root zone. The one presented here is in widespread use. It says that . . .
(1) LR = ECw / 5 * ECe - ECw
where: LR is the decimal fraction of irrigation water that must be leaching water in order to maintain the root zone salinity at the desired level ECe.
ECw is the electrical conductivity of the irrigation water.
ECe is the average electrical conductivity in the root zone that will result in a satisfactory yield.
ECe, a measure of the average root zone salinity, is a management-chosen salinity level that will result in a satisfactory yield. You choose the ECe you want to maintain. In most normal situations this would be an ECe that allows 100% yields with the most salt-sensitive crop in the rotation.
This is an important point. Cotton is more salt-tolerant than fresh vegetables. The ECe could be higher when growing cotton than green peppers. However, if you operated the irrigation system on a field to maintain an ECe for cotton, and then decided to grow peppers in that field, the ECe would be too high for maximum tomato yields. Soil salinities must be managed for the most salt-sensitive crop in the rotation, "manage the soil, not the crop".
With the leaching requirement determined, the depth of water to apply can be calculated by . . .
(2) AW = ETc / (1 - LR)
where: AW = total net irrigation requirements (you will need to factor in your application efficiency to calculate the gross amount of water to apply)
ETc = net crop evapotranspiration
LR = the leaching requirement as determined above.
Recommendations for annual leaching requirements are contained in the Management Techniques section below (Table 6-2). Always check with qualified agronomists when designing salt management programs.
Last updated September 2000