Understanding how much of the major base cations (Ca, Mg, K, Na) to apply is relatively straightforward. Understanding how much sulfur to apply is a bit more complex.
Sulfur has four functions:
1) Sulfur is attached to other critical minerals.
Potassium, manganese, copper, zinc, iron, magnesium and calcium can all be obtained as sulfates. If we are applying these critical minerals in their sulfate form, we are applying sulfur too. Let’s make this our first principle: Always apply as much sulfur in sulfate form as needed to carry these important elements. We really need potassium, manganese, copper, zinc, iron, magnesium and calcium in their proper proportions in the soil.
Always keep track of how much sulfur is going into the soil, but never limit these important elements because of their sulfate component. The sulfate form of sulfur doesn’t do any harm to our friends the microbes.
2) Sulfate is an essential plant nutrient. A certain amount of sulfur is necessary to grow a crop.
How much? Good question. Gary Zimmer says at least 25 lbs/ac, but he likes to see 30 to 40 lbs/ac. Bill McKibben, who works with the same Logan Labs tests we use, targets 40-50 (TCEC < 10) or 60-80 lbs/ac (TCEC > 10). OrganiCalc is generous with sulfur, we use 50 or 80 lbs/ac, depending on TCEC. Sulfur is a necessary nutrient; it works with nitrogen and enables plants to form complete proteins (some amino acids contain sulfur). Sulfur deficiency looks a lot like a nitrogen deficiency except that it affects the new leaves first.
3) Sulfur is a powerful soil corrective tool for removing excess Ca, Mg, K, Na cations.
Sulfates will combine with Na first, then K, then Mg, then Ca. They can then leach below the root horizon under the influence of water and gravity.
As an example of how we might use sulfates to balance a soil, suppose we have low calcium and high potassium in a soil. If we apply calcium sulfate (gypsum) we get calcium and sulfate separating in the soil. The sulfate can then combine with the potassium, forming potassium sulfate, which is subject to leaching. As a result we have added calcium to the soil and removed potassium.
Sulfur in the sulfate form never stays around in the soil for long because all sulfates are readily soluble. This can be a good thing, if cations need to be removed in order to balance the soil . But if balance has been achieved, excess sulfur will remove cations you would rather keep, and thereby decrease the fertility of the soil.
When we apply deficient potassium, manganese, copper, zinc, iron, magnesium, or calcium as sulfates, we expect the sulfate molecule (SO4) to break away from the elements.
It is important to realize this reaction can go both ways.
When we have excess potassium, manganese, copper, zinc, iron, magnesium, or calcium (+1 or +2 charge) we expect the sulfate ion (-2 charge) to combine with the positively charged cations. The sulfate forms of the elements can then be leached away.
The solubility of minerals in their sulfate form has important implications for no-till situations. As long as the minerals remain dissolved in their sulfate form, they will penetrate to the depth of the water. It can be an advantage to apply minerals when rain (or a major irrigation event) is expected. However, excess rain or irrigation can wash sulfates out of the root zone.
4) Elemental sulfur is a powerful tool for lowering pH
So far we have considered sulfur in the sulfate form which does not change pH. Sulfur in its elemental form is one of the few options available to organic growers to lower pH. Maybe we had better explain why we consider pH important. pH is a backwards and logarithmic measurement of the H+ cations. H+ is a mobile cation. We want mobility; movement fosters life.
In order to lower the soil pH, elemental sulfur must be consumed by certain soil microbes which in turn produce sulfuric acid, the compound that lowers soil pH. These microbes are active when the soil is warm and moist and are less active in cold, hot, dry or waterlogged conditions. Not much activity occurs outside the range 50° F (10° C.) to 85° F. (30° C.). Therefore the amount of time it takes for elemental sulfur to lower soil pH depends on the temperature and moisture conditions.
These microbes are likely to be missing from potting (and Living Cannabis) soils. For these soils the preferred method of lowering pH is to mix in more low pH Canadian Sphagnum Peat. The peat is added to the mix and pH measured on-site. When the pH gets to be 6, we send the soil to the lab, then apply what is missing.
Soil pH affects the availability of the minerals due to the major forms the minerals take as a function of the relative quantities of H+ ions, OH- (hydroxyl) ions and electrons in the soil.
There is a narrative that elemental sulfur harms soil biology. Consequently, for some time we limited elemental sulfur recommendations to 100 lbs/ac. We have since consulted with agronomists and soil scientists who have not found this narrative not to be true; rather the opposite – elemental sulfur does foster soil biology. Really, phosphorus, iron, and other minerals are unavailable at higher pH, and these minerals are necessary for any life to flourish. So, we have changed the elemental sulfur application limit to 300 lbs/ac once (rarely twice) per year. We have seen excellent results at this rate in our own garden.
We have developed a calculator for determination of the amount of sulfur needed to lower soil pH in non-calcaeous soils. See this page.
Elemental sulfur is slow to act in the soil. For that reason we often will often add gypsum too, providing nutrients for the current crop.