Every once in a while someone calls me with a question that is worth repeating, sharing, or expanding on. One example is the following. I received a call from a lady asking me how much water she should be irrigating with. Well.... the answer is sort of long and drawn out and complicated, you know, that depends on a lot of different factors. But, before we finished our conversation, I offered her what turned out to be one good piece of advice.
The question turned into an answer like the following: The available water holding capacity of a soil is a function of the texture. How much water the soil can actually hold (or how much of the water which is being applied) depends on both the water holding capacity and just how dry the soil is when the water is added. As an example, the following table illustrates the Available Water Holding Capacity of Soils. This is the amount of water that would and could be made available to plants after the soil had been irrigated:
|
Soil texture |
Inches of Water Per Foot of Moist Soil |
|
Sands and fine sands |
0.75 |
|
Very fine sands, loamy sand |
1.00 |
|
Sandy loam |
1.50 |
|
Loam |
1.90 |
|
Silt loam, silt |
2.20 |
|
Silty clay loam |
1.90 |
|
Clay loam, sandy clay loam |
1.70 |
She then started asking questions about all the terms that appeared on her water test report. Like, what is EC, SAR, ESP, and Conductivity. So.....
Conductivity (also referred to as EC, electrical conductivity): an index of the dissolved solids concentration. Usually presented in either micromhos/cm or millimhos/cm. Low salinity water is water with a conductivity between 0 and 250 micromhos/cm (0.25 mmhos/cm). Low salinity water can be used for irrigation with most crops on most soils with little likelihood that soil salinity will develop, as long as there is good drainage. Some leaching is required (rainfall will generally be enough), but this occurs under normal irrigation practices.
Medium salinity water has a conductivity between 250 and 750 micromhos/cm (0.25 to 0.75 mmhos/cm). This water can be used if a moderate amount of leaching occurs as a result of the combined effects of irrigation and rainfall. Plants with moderate salt tolerance can be grown (grasses do well).
High salinity water has a conductivity between 750 and 2250 micromhos/cm (0.75 to 2.25 mmhos/cm). This water should not be used on any soils with restricted drainage or where excessive water is not available for continuous leaching. Special management for salinity control is necessary with this water.
Needless to say, water with salinity above 2250 micromhos/cm is very saline and should not be used for irrigation. This water is only occasionally suitable, where excess leaching with good quality water will follow. As for SAR(sodium adsorption ratio): this is (generally speaking) the ratio of the amount of sodium to the amount of calcium and magnesium. Ideally, this number should be small. Low sodium water is water with an SAR less than 10. This water can be used for irrigation on almost all soils with little danger of development of harmful levels of exchangeable sodium. However, sodium sensitive crops such as stone fruit trees may accumulate injurious concentrations of sodium.
Medium Sodium water is water with an SAR between 10 and 18, and this water can present appreciable sodium hazard in fine textured soils having high cation exchange capacity, especially under low leaching conditions, unless gypsum (calcium carbonate, which is common in most Montana soils) is present. This water may be used on coarse textured or organic soils with good permeability.
High sodium water is water with an SAR between 18 and 26. This water may produce harmful levels of exchangeable sodium in most soils and will require special soil management such as good drainage, high leaching, organic matter additions, gypsum additions.
The following table helps categorize each of the water qualities:
|
Salinity hazard: EC (micromhos/cm) |
|||
|
Low |
Medium |
High |
Very High |
|
100-250 |
250-750 |
750-2250 |
>2250 |
|
Sodium hazard: SAR |
|||
|
Low |
Medium |
High |
Very High |
|
0-10 |
10-18 |
18-26 |
>26 |
Categories: Salinity, Soil Quality, Water Quality
Date: 1995-1996