Soil Balancing, Part 2 (Soil pH and Sulfur)

October 23, 2023

Soil pH is often talked about as important (and it is). The reality, though, is that soil pH is merely a reflection of overall soil balance, especially base saturation balance of calcium, magnesium, potassium, and sodium. So it is really the balance of those nutrients that we care about, for when they are balanced, the soil pH will also be balanced.

Desired values for each of these nutrients are as follows:

• 68% calcium
• 12% magnesium
• 4-7% potassium
• .5% to 1.5% sodium

When one or more of these nutrients is excessive enough, and nothing else is low enough to counterbalance, the soil pH will register as high.

On the flip side, a soil pH that is too low essentially means that there is a nutrient deficiency in the soil. We need to bring the above-named nutrients to proper levels.

Why does pH really matter?

Ideal soil pH is generally 6.5 to 6.8, and is where you will get the best nutrient uptake from a chemistry standpoint. It is also more optimal so that fungi can thrive. High pH encourages bacteria activity while low pH encourages fungal activity.

Next let’s look at what pH stands for. While there are sometimes arguments about what pH stands for, the common theme is that pH stands for Potential of Hydrogen. Hydrogen is an important element in our soils, for without it, nutrients are released to plants much slower than normal.

What is important to understand is that hydrogen is the lightest element in the soil, having an atomic weight of 1, while the atomic weight of calcium is 40, potassium 39, magnesium 24. This means that when these minerals are above adequate levels in the soil, they will push out exchangeable hydrogen.

So why should we care about pH? It is a quick glance into whether our soil is overloaded or underloaded as a whole. When soil pH is 7 or above, it is overloaded and the soil becomes alkaline, meaning that it has no more exchangeable hydrogen, which by extension means that we need to introduce acidity (or hydrogen) in some other way in order to grow good crops.  In a conventional system, there are lots of options to introduce hydrogen and acidity, including ammonium sulfate, lots of nitrogen, and lots of manure. The challenge is that some of these options disappear once you are certified organic. Organic options are based much more on having aggressive biology, but also include the use of sulfur – the elemental form in particular.

Sulfur we want at 35 ppm, although it is common to see lower numbers. Sulfur is an important element for life and living systems. It is needed for the production of several amino acids such as cysteine and methionine. It is also one of the minerals that are needed so that complete proteins can be formed, which allows plants to become more resistant to insect pests.

Sulfur, just like all our nutrition, should ideally be coming from the ground up. The challenge is that sulfur, being an anion, is almost always deficient because it leaches so easily in low organic matter soils. In other words, because we live in a high-rainfall environment and generally are low in organic matter, sulfur will probably need to be added annually due to its tendency to leach out of the soil.

Sulfur is needed to stabilize nitrogen. If sulfur is deficient, the nitrogen that is already in the soil is very easily lost, either through leaching or through changing forms and then gassing off into the atmosphere.

Sulfur is also very helpful for balancing soil. It is a remediator for excesses of soil calcium, magnesium, potassium, and sodium, especially when combined with the minerals that are deficient. An example of this would be when we have high magnesium soils that are low in calcium. In this case, if the pH is slightly high, we would probably add gypsum, which is calcium sulfate. So we are basically adding calcium, which helps remove the magnesium from the soil colloid, then the sulfur combines with the magnesium and forms Epsom salts, which are leachable. In such a way, soil can be balanced.

It is important to understand that there are several forms of sulfur; in particular, the two forms that I want to talk about today are sulfur and sulfate. A plant absorbs sulfur in the form of sulfate. In other words, sulfur forms such as “elemental sulfur” need to be converted into sulfate before the plant will absorb it. This conversion process takes energy; therefore, it is generally helpful to apply sulfate instead of sulfur. Sulfate is usually combined with another nutrient, such as calcium sulfate (commonly called gypsum), or magnesium sulfate (commonly called Epsom salt), or potassium sulfate (commonly called 0-0-50 or sulfate of potash).

There is one situation, however, where elemental sulfur may be preferred. That is in high pH situations – what we talking about above. Why? Because sulfate does not create acidity like sulfur does. Here is a quote from The Ideal Soil book:

Elemental Sulfur must be converted to the sulfate form before it can be used by plants. This is done by Sulfur converting bacteria which occur naturally in soils. The soil temperature must be above 55⁰ for the bacteria to do their work. During the conversion, 4 atoms of oxygen will be taken from H20 (water) to form SO4, leaving 8 atoms of free hydrogen (H+), which is acidic and will lower the soil pH. When sulfur is applied as calcium, magnesium, or potassium sulfate, it generally will not affect the soil pH much.

So this quote is indicating that if we want to lower soil pH, generally we need to apply elemental sulfur, which will then release hydrogen, which will then release nutrients to plants more easily. Ideally, this would be applied in the fall to help reduce soil pH, but it can also be applied in spring – up to 200 pounds per acre – as long as nitrogen is properly accounted for. Otherwise, you will have sluggish, slow growing crops.

Generally, for pH correction, we would prefer that the sulfur be applied in the fall at rates of 400 pounds or more per acre. Some growers have gone as high as 1200+ pounds per acre in extremely high pH situations, but this should always be done with humus. It makes the sulfur more effective and helps prevent the burnup of soil organic matter in the conversion process.

Summary for today: if you balance your soil minerals, soil pH will take care of itself. And, never forget sulfur. It should be applied annually in most soils, and when pH is high (7 or higher), it should be applied in the form of elemental sulfur to release hydrogen, which then creates more available minerals for your plants.