Changing Ph with elemental sulfur

Asked April 13, 2020, 8:47 AM EDT

I have 3 cu yards of compost with a pH of 7.4_ how much elemental sulfur is needed to lower the pH to 6.5?

Livingston County Michigan

1 Response

Hello!

Here is some advice from a professor in the MSU Horticulture Department:

"A first question to ask is whether it is necessary to lower the pH. The pH of most compost naturally declines as it ages. A pH of 7.4 suggests to be that it is fairly well aged (weeks or possibly months) already. It likely will continue to drop to around 7 to 7.2 in the following weeks. It is unusual for the composts I have made to go much lower, although some have.

I would not be concerned with compost pH of 7.4 to be adding to a garden. I have grown plants in 100% compost made specifically for that purpose, and the pH has been in the 7 to 7.4 range. Above 7.5 is where I have seen some limitations on growth. I would expect a pH of 6.5 to work well for growing plants in containers, perhaps better than at pH 7.

Based on my experience, to lower the pH as requested, from 7.4 to 6.5, or approximately 1 pH unit will likely take in the range of 2 to 4 lbs per cubic yard of elemental sulfur. This is not much different than the rates used for soil. The amount depends in part on the feedstocks used to make the compost and the bulk density (weight per unit volume), which is influenced by how much soil is in the compost. Heavier, with more soil included, will generally take a higher rate. You may have seen that recommendations for rates of sulfur to change soil pH are dependent on whether the soil is more sand or clay, with clay requiring higher rates, for the same reason."