Sodium Adsorption
Ratios: With or Without Adjustments
Here’s another one of those notes which tries to put another one of those fancy ‘soil science’ terms into some kind of understandable form. Sodium adsorption ratio - SAR!. This past winter I had more calls about SAR and sodium in soil and water than I care to recall. It was all prompted by CBM - coal bed methane - development issues in southeastern Montana and northeastern Wyoming. A significant concern was raised about the sodium in CBM water and what impact this might have on soil quality - especially permeability. So, in response to a question by Mike Foate, Rocking Horse Ranch, Arvada, WY, here’s what I wrote.
What is the distinction between SAR and adjusted SAR?
First - if you have an irrigation water that is of questionable quality or you are concerned about the impact of irrigating or spreading salty water - or if you think the infiltration rate of your soil has decreased, or you seem to be having more surface crusting problems than you remember in the past, this is one of the measurements that you want to have run on your soil test. Let’s start with the definition of SAR - SAR is the sodium adsorption ratio, which is an index used to estimate what the changeable sodium percentage of a soil (or water) is, or what it is likely to become if the water of known SAR is used for years on that soil. (Miller and Donohue, 1995, pg 325). The SAR has a good correlation to the exchangeable sodium percentage (ESP) and is much easier to calculate exactly or to estimate.
About SAR - Exchangeable sodium in concentrations above about 15% (or SAR = 13) exerts its greatest effect on plant growth by dispersing the soil. As low as 10% exchangeable sodium in fine-textured (clayey) soils and 20% in sandier soils have caused dispersion damage. Colloid dispersal makes the soil less permeable, or even impermeable, and causes it to form hard surface crusts when dry (Miller and Donohue, 1995, pg 323). .... some clayey soils disperse with only 9-10% exchangeable sodium. ... Dispersion is destruction of soil structure....
The SAR is calculated as a modified ratio of the concentration of sodium relative to the combined concentrations of calcium and magnesium.
The adjusted SAR is a value (expression of sodium impact on soil or sodium concentration of water) corrected to account for the removal of calcium and magnesium by their precipitation with bicarbonate or carbonate ions in the water added, giving higher values for adjusted SAR than for SAR and a truer picture of the sodicity of the soil (or water). (Miller and Donohue, 1995, pg 484.) What that essentially says is that in an irrigation water which has calcium, magnesium and sodium, in some cases (in the absence of carbonate or bicarbonate - both of which are present in CBM discharge water), the calcium and magnesium can remain in solution and off-set the effects of the sodium, i.e., replace it. However, in a water which has a high concentration of carbonate and bicarbonate, along with calcium and magnesium (such as the case with CBM discharge water), the calcium and magnesium in the water will precipitate as carbonates/bicarbonates when exposed to air.
For example, water with an SAR or 2.4 and a high concentration of bicarbonate/carbonate could potentially calculated to an adjusted SAR of 5.6 when accounting for this removal of calcium and magnesium due to precipitation. Or an SAR of 12 might calculate to an adjusted SAR of 40 or more. In essence, what that adjusts SAR is is the more accurate reflection of the SAR that the soil will be exposed to. Hence, if the SAR of the water (without adjustment) is 8.0, but the adjusted SAR is 40, the decision to irrigate should be based on the adjusted SAR. In other words, the SAR calculated on the water without accounting for the carbonates/bicarbonates is a very conservative value. To get a true reflection of the SAR that the soil will be exposed to, you need to consider the presence of carbonates and bicarbonates and use the adjusted SAR.
It’s kind of like having a tail wind. If you are flying at 80 miles per hour, but you have a 40 mile per hour tail wind, you are actually traveling at 120 miles per hour. Or, the cumulative effect of two vehicles colliding head on at 60 miles per hour each is the consequence similar to crashing into a cement wall at 120 miles per hour.
*This note is part of the Agronomy Notes series, a weekly news release and fact sheet series prepared by Dr. Jim Bauder, Extension Soil and Water Quality Specialist at Montana State University. Past Agronomy Notes can be viewed at http://agnotes.org/. For additional information on this subject, for subscription (no charge) to this Agronomy Notes series, or for questions about Agronomy Notes, Jim Bauder can be contacted by e-mail at jbauder@montana.edu, or by calling (406) 994-5685 at MSU.
Category: Soil Quality
Date: 2000