Does Elemental Sulfur Lower Soil pH?

Some farmers have been calling and asking about something they heard on TV recently. They heard that applying large amounts of elemental sulfur (for example, 1000 lb/acre) would quickly lower soil pH.

Soil pH is a soil chemical property that measures soil acidity or alkalinity, and it affects many soil chemical and biological activities. Soils with high pH can reduce the availability of certain nutrients, such as phosphorus and zinc. Soils of the Northern Plains and Canadian Prairies frequently have high soil pH (greater than 7.3). Most of these soils have calcium carbonate (free lime) in the topsoil. Calcium carbonate is relatively insoluble and buffers soil pH around 8.0. Carbonates in topsoils of this region originate from soil formation processes since the latest glacial period.

So what about this suggestion that applying 1000 lb/acre elemental sulfur will lower pH? Elemental sulfur is oxidized by soil bacteria to create sulfuric acid, which certainly will lower soil pH. However, a high rate of elemental sulfur is required to permanently lower pH on soils with carbonates (soils with pH higher than about 7.3).

To lower pH in soils containing carbonates, the naturally-occurring carbonate must first be neutralized by the sulfuric acid produced by the breakdown of elemental sulfur. You can visualize the fizz that takes place when you pour acid on a soil with carbonates. That fizz is the acid neutralizing (reacting with) the carbonates in soil. Once all calcium carbonate in the soil has been neutralized by sulfuric acid, only then can the soil pH be lowered permanently. It is important to note that sulfate sources, such as gypsum (CaSO4), do not create sulfuric acid when they react with soil, so they cannot neutralize the calcium carbonate.

In 2005, AGVISE started a soil amendment demonstration project on a soil with pH 8.0 and 2.5% calcium carbonate equivalent (CCE). One of the treatments was 10,000 lb/acre elemental sulfur. We figured if 1000 lb/acre elemental sulfur is suggested to lower pH, then 10,000 lb/acre should be even better! Good science tells us that this treatment would not significantly lower soil pH, but sometimes people must see the results for themselves. The figure below shows the soil pH decreased markedly in the first year, but the pH increased over the next few years because calcium carbonate in the soil was too much for the sulfuric acid generated from the elemental sulfur to neutralize. The soil pH increased back into the original pH range because of the remaining carbonates in the soil. The 10,000 lb/acre elemental sulfur rate was too low to neutralize the carbonate in the soil, and the lower pH was only temporary.

A rough calculation showed that this soil with 2.5% CCE would require about 16,000 lb/acre elemental sulfur to neutralize all carbonates in the topsoil. Such high rates of elemental sulfur are both impractical and expensive. The suggestion that adding a large amount of elemental sulfur, say 1000 lb/acre, to lower soil pH is probably not going work for most high pH soils in our region. We hope farmers do not spend their hard-earned money applying high rates of elemental sulfur in hope of lowering soil pH.

Posted: 10/5/17