The saturated soils this year has folks thinking about applying gypsum to the compacted areas to reduce sodium (Na) levels and improve the soil structure long term. This is an interesting idea and generated a lot of discussion. Will it work and where? How much to apply? Can farmers substitute elemental sulphur for gypsum? To answer these questions, we need some soil analyses and quite a few calculations.
How do you estimate the gypsum requirement? Published tables are available that list the amount of gypsum or sulphur needed to replace exchangeable sodium. Basically, it takes 0.86 tonnes of gypsum per acre per six-inch depth to replace 230 ppm of Na (1meq/100g).
In the zero-to six-inch depth there is 189 ppm/230 = 0.82 meq of Na/100g of soil multiplied by 0.86 t/ac = 0.7 tonnes per acre. In the six to 12 inch depth, there is 348 ppm/230 = 1.51 meq/100g of soil multiplied by 0.86 t/ac = 1.3 tonnes per acre. Therefore, to replace all the Na in the first 12 inches of soil would require two tonnes per acre of gypsum. Ideally, you would want to replace most of the Na in the root zone (three to four feet) so you would need at least six tonnes per acre. That’s a lot of gypsum.
Unless you have access to a low-cost gypsum by-product, given the cost of both the gypsum and handling more material, you might want to look at elemental sulphur to accomplish the same effect. Elemental S oxidizes to sulphuric acid, the sulphuric acid reacts with the calcium carbonate in the soil to produce calcium sulphate (gypsum). It would take 186 pounds of elemental S to produce 1,000 lbs. of gypsum. Therefore, if you are going to apply elemental S in place of gypsum to replace Na, you need to know two things:
Is there enough calcium carbonate in the soil to react with the elemental S? Without calcium carbonate, you won’t have any gypsum produced.
Are the soil carbonates (free lime) mainly calcium carbonate or are they high in magnesium carbonate? The sulphuric acid produced by the oxidation of elemental S will also react with magnesium (Mg) carbonate to produce magnesium sulfate. Magnesium sulfate is a soluble salt, so it will increase soil salinity. The important characteristic of gypsum is that it is soluble enough to release enough Ca to replace Na and Mg but not soluble enough to create a salinity issue.
Based on the pH and Ca and Mg values in the example, there is likely some free lime in both depths but it would be useful to measure it and determine the actual Ca and Mg content. If there is quite bit of Mg carbonate, increasing salinity with an elemental S application is a concern.
Which brings us to the final question (which should probably be the first question) — will the replaced Na and Mg leach? In the above example, the soluble salts (SS) are already elevated indicating that salts are accumulating, not leaching. Soluble salts levels, one of the most important soil tests, are missing on the six to 12-inch and 12-to 24-inch depths. SS is 0.7 in the zero to six-inch depth which is equivalent to about 1.5 to 2.0 EC. Na and S are much higher in the six to 12-inch and 12 to 24-inch depths, so SS is likely quite a bit higher.
The same issue of leaching salts is also relevant to the application of gypsum. The Ca from gypsum will replace Na and Mg but there has to be somewhere for them to go (down). Therefore, if there is no net downward movement of salts, the benefits of applying gypsum or elemental S will be limited. However, on sodic soils (high Na) without salinity issues, application of gypsum will increase the Ca to Na ratio and improve soil structure.
In this example, it would be useful to determine if the water table is within three to four feet of the surface at least part of the time. Alternatively, have changes in cropping practices (i. e. elimination of summerfallow, growing deep rooted/ high water use crops like alfalfa, and/or drainage) corrected the water table issues? When the water table is within three to four feet of the surface, water and salts will wick up (move up by capillary rise).
To summarize the questions that need to be answered before a farmer can confidently proceed on this field:
What is the soluble salt levels at the lower depths? We need to get the lab to do this analysis.
How much free lime is available and what proportion is Ca carbonate? The lab can do this too.
How deep is the water table?
Are there soil structural issues that make the downward movement of Na (and Mg) difficult? Are there heavy clay subsoils or compaction layers? If so other partial remedies, like deep ripping perhaps with Ca injected into the rip zone, need to be contemplated.
Answering these questions will make the difference between success and an expensive failure. Soil analyses are essential for determining the best course of action.
This Is The Soil Analysis From The Original Question That Started This Discussion: Ph Cec %K %Mg %Ca %H %Na 7.9 35.9 0 244 1165 7200 8.0 47.8
S NZnMnFe 120 4 4.3 150 59
PP1P2 K Mg
16 19 0 313 825 3 3
3 2 0 206 1180
283 2 0 176 2 0
Ca 5490 7200
Al K/Mg Cl Na 91 0.12 20 189 12 0.06 0 348