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 three functions:
1) It is attached to other critical minerals. Potassium, manganese, copper, zinc, iron, and some sources of magnesium and calcium are all sold as sulfates. If we are applying these critical minerals, 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.
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, but the elemental form of sulfur does set them back for a short time. So, we limit Ag Sulfur to less than 100 lbs/ac.
2) 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). Let’s use Bill’s numbers, 45 or 70 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).
3) Sulfur is a powerful tool for removing excess cations. Sulfur in the sulfate form never stays around in the soil for long because all sulfates are readily soluble so excess cations attached to sulfate leach out easily. Excess sulfate, being an anion, combines with the base cations sodium, potassium, magnesium, and calcium; it also combines with manganese, copper, zinc, and iron. Then, these sulfates are leached by rain or irrigation. This can be a good thing, if you need to remove excess cations 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 your soil.
So, in figuring how much sulfur our soil needs, we will consider the following three parameters:
We will apply whatever sulfur is required to carry the sulfate forms of important minerals into the soil. We will supply at least enough sulfur to grow our next crop, and apply additional sulfur when there are excess minerals in the soil.
To determine if the soil needs extra sulfur to remove excess cations, look at the major base cations, Ca, Mg, K, and Na, to see if any of them exceed our ideal percentages, e.g. Ca > 70% and Mg > 18% or K > 120% of target K or Na > 2.5%. Another way to look at whether the soil is balanced is to look at the pH. If our pH is high, we want to apply more sulfur to acidify the soil. Conversely, if the pH is low, we don’t want to apply more than is necessary — we don’t want to further acidify an already acid soil. So, we can define high pH and low pH as: High pH > 7.2 and Low pH < 6.0. If you have a high pH, you have excess cations. If your pH is low, you are lacking some cations. This pH test is useful in finding a middle ground for our sulfur application target, as we shall see.
We are going to establish three targets for sulfur, depending on how many excess cations we want to strip, if any. Some of these numbers are linked to the TCEC, so they appear as ratios of other elements. But, they don’t have any intrinsic relationship to these other elements. The ratios are just a convenient way of getting the sulfur target into the right ballpark. Sometimes it is necessary to have a maximum, to keep from applying too much to high TCEC soils, as in the 432 lbs/ac limit below. Also below, you will recognize McKibben’s number as the minimum target, necessary to grow a crop. No Excess Cations: If pH is less than 6.0 or (Ca < 70% and Mg < 18% and K < 8%and Na < 2.5%): Then if TCEC <10, TARGET S=45 lbs/ac. TCEC>= 10, TARGET S=70 lbs/ac.
If pH > 7.2
Then TARGET S=Mg/2, or 432 lbs/ac, whichever is less
(This is a range of 101 to 432 lbs/ac, depending on TCEC)
Some Excess Cations:
If pH is between 6.0 and 7.2 and the soil does have excess cations (NOT Ca < 70% and/or Mg < 18% and/or K < 120% of target K and/or Na < 2.5%): Then S=P/3 (This is a range of 101- 132 lbs/ac, depending on TCEC)
Here’s a shorthand version of what we have done to establish the sulfur target:
Target S = Target Mg/2 or 432 lbs/acre, whichever is smaller, if the pH is >7.2. If the pH is <6 or there are no excess cations (Ca<72%, Mg< 23%, K<120% of target and Na < 3%), then for TCEC’s less than 10 the target is 45 lbs/ac and 70 lbs/ac for TCEC’s >= 10. All other cases target P/3 lbs/ac.
Those are the sulfur targets. Except for one more thing. We have another source of sulfur we haven’t talked about yet. Gypsum. And, whether to use lime or gypsum to supply any calcium the soil needs. This is pretty simple, really. Here’s what we want to do:
No Excess Cations:
Use lime, not gypsum, to get the pH up
Excess Cations but calcium is deficient:
Use gypsum, not lime, to strip the excess cations
Some Excess Cations:
Use gypsum, not lime, to strip the excess cations.
For reasons described elsewhere we always want to use lime to reach 60% calcium saturation. Then we can use gypsum, if it is called for.