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Water Quality Data
SALINITY AND DRAINAGE
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Suitability standards for water use
The suitability of water for irrigation is judged on measurement standards that indicate the potential for causing one or more of the four classes of problems. For general yield declines and the necessity for leaching, the standard would be total dissolved solids or the electrical conductivity. For soil structure problems scientists are interested in the sodium absorption ratio (SAR) in conjunction with the total dissolved solids. For specific toxicities, they look at the relative levels of specific salts.
Table SD-1 is the well-accepted Guidelines for Use of Irrigation Water (from FAO 29A, by Ayers/Westcott). Note that the suitability for use is reported in terms of None, Increasing, and Severe problems to be expected with continual use of the water. For example, a TDS reading of 450-2000 ppm is going to indicate an increasing potential problem while with water over 2000 ppm TDS the problem is likely to be severe.
Also, be aware of some of the assumptions that were made in developing the Guidelines. For example, looking at the notes under Site Conditions . . . "Drainage is assumed to be good, with no uncontrolled shallow water table present within 2 meters of the surface." Obviously in some areas of the District this is not true.
Remember, water quality is only one factor in judging the extent of, or potential for, a problem. Depending on the situation, crop selection, soil type, and soil/water management all affect crop yields and quality.
TABLE SD-1: Guidelines For Salinity, ppm TDS
| No Problem | Increasing Problems | Severe Problems | |
| Salinity | < 450 | 450 - 2000 | > 2000 |
Saline Soils and General Yield Declines
As was seen above, there are two ways in which to measure the level of salts. There are also several ways to describe the type of problem, "saline soils", "sodic (or alkali) soils", and "saline-sodic soils".
Saline soils have an excessive level of salts and are associated with poor yields. However they usually have sufficient permeability (water moves freely through the soil). Saline soils can be improved and/or yields maintained if a fairly good quality water supply and sufficient internal drainage are available.
To explain how salts can cause yield declines, remember that previous sections of the Handbook described that soil can hold water. The lower the soil water content, the harder soil holds the remaining water and the harder it is for the plant to extract this water. Thus, lower water levels in the soil put stress on the plant, reducing yields and if allowed to continue, killing the plant.
The water-holding forces of the soil are called "matric forces". But there is another force at work to reduce the amount of water that can be extracted by the plant, "osmotic forces". These osmotic forces increase with an increase in salts. Osmotic forces also act to restrict water extraction by crop roots.
The salts may also interfere with the plant's ability to take up nutrients from the soil. They can do this by affecting some of the chemical reactions inside/outside the plant.
The two forces, osmotics from excess salts and matric from the soil structure, are additive. Thus, in a saline soil, even if it appears wet, the plant can have trouble extracting the amount of water it needs.
For example, use a scale of 1 - 10, with 10 being the highest force for the plant to overcome, to describe the level of osmotic and matric forces. Assume that you are using a furrow irrigation system on a normal soil. The total force working against the plant may peak at . . .
Forcetotal = Forcematric + Force salt
= 7 + 1
= 8
Now assume that instead of a normal soil, you are growing on a saline soil. The forces may be . . .
Forcetotal = Forcematric + Force salt
= 7 + 4
= 11
The matric forces have not changed, but the force holding water back from the crop has increased due to the salt load. Thus, stress appears earlier after an irrigation on saline soils.
One key to managing a saline soil with continued use of a saline water supply, is to maintain a "salt balance". That is, assuming that there is an acceptable level of salts in the soil to begin with (one that will allow acceptable yields), just as much salt must be removed from the soil each year as is added.
Remember that Project water contains about 500 pounds of salt per AF. If you use two feet of Project water a year you are applying 1000 pounds of salt per acre per year. Maintaining a salt balance requires that you remove 1000 pounds of salt per year.
The method of removal is to "leach" a certain amount of water through the root zone. That is we purposely cause some deep percolation. The leaching water will carry excess salts below the root zone, away from the crop. This leaching process will be discussed further below.
Last updated September 2000