Again, another great question came over our Agri- Trend Network chat wondering, “How much elemental sulphur (S) would it take to change the soil pH any appreciable amount?”
For soils with a pH under 7.5, the amount of elemental S required to decrease soil pH a given amount is fairly well related to cation exchange capacity (CEC), which is related to soil texture and organic matter. Adding acid to lower pH is somewhat like determining the amount of lime required to increase pH, only backwards. The guideline that Dr. Ieuan Evans uses for gardens is useful.
In sandy soils it takes a quaterpound of sulphur per square yard to change the soil pH from 7.5 to 6.5 and a half-pound per square yard if it’s a heavy clay. For loam soils, it takes six ounces. Since there are 4,840 square yards per acre, then a heavy clay would need 2,420 lbs. of S per acre; a sandy soil 1,210 lbs. and you can do the math for the loam.
On high clay soils (60 per cent plus), I get a requirement of 3,000 lbs./ac. or higher.
For soils above pH 7.5 with free lime, the estimates based on CEC are not valid. The acidifying agent must neutralize all the free lime (carbonates) before soil pH will decrease. It takes a lot of an acidifying agent to neutralize the free lime. One percent free lime in a soil is equal to 20,000 lbs./ac. (one per cent of 2,000,000 lbs.). Soils with free lime often have 5 per cent or more free lime. If the free lime is pure calcium carbonate, you need 6,400 lbs./ac. of S (20,000 X 32/100) to oxidize to sulfuric acid and then react with the calcium carbonate to produce calcium sulfate, or gypsum (32 is the atomic weight of sulfur and 100 is the molecular weight of calcium carbonate, in case you were wondering about the 32/100). Then, after you add enough S to react with all the carbonate, you need to add S at a rate based on the CEC or guidelines Ieuan provided to lower the pH.
THE RISK OF SALINITY
Another caution is that, if the free lime has a significant amount of magnesium carbonate, the process generates magnesium sulfate, which is a soluble salt. In the example above, if the free lime were 20 per cent magnesium carbonate, you would generate enough magnesium sulfate to create a salinity concern.
WHY CHANGE PH?
Apart from creating a more plant friendly environment, another advantage of reducing soil pH is that in the process many nutrients become more available. There are some reports of yield responses to the addition of elemental S at rates that would not result in an appreciable reduction in soil pH (this is elemental S added as a soil amendment, not as a nutrient source). It is hypothesized that acidification at the microsites where the sulphur particles are oxidizing, results in increased availability of phosphorus and some micronutrients. This use of elemental S needs further evaluation in this circumstance.
Elston Solberg, senior agri-coach with Agri-Trend Agrology says, “Farmers have been using elemental sulphur for hundreds of years to manipulate soil pH. In Western Canada on high pH soil, depending on soil texture, it can take anywhere from hundreds to thousands of pounds of sulphur to reduce pH appreciably. Despite this, many farmers have used this strategy successfully. Variability of pH across a field is usually quite significant, so one solution would be to treat the high pH zones, reducing the amount used or redistributing it to where it is really needed.”
The bottom line is that elemental sulphur is a good acidifying agent for soils under pH 7.5 but if the soil contains free lime, the amount of an acidifying agent needed to react with all the carbonates is usually too high to be practical. Therefore, before applying elemental S to reduce pH, you should have soil analyses to assess CEC (buffering capacity), presence of free lime and an estimate of the Ca and Mg content of the free lime.
ELEMENTAL SULPHER (ES) REQUIRED TO REDUCE SOIL PH (INITIAL PH IS 7.5)