I have had several conversations over the last three weeks or so with growers that have questions about liming their fields. They are looking for the most efficient use of their dollars this year, and want to know if they need to apply Ag Lime or Pel Lime this year. Usually, those questions are rooted in a confusion that is based on pH, buffer pH, and how Lime actually works. It doesn’t help that when I look at a soil test I see both pH and buffer pH on there, and the numbers are different. It can all be a bit confusing, so today we are going to try to break it down.
Let’s start with pH. pH is a measure of H+ ions. The lower the pH, the more H+ ions there are. When pH begins to go below 6.3 in row crops, we start to see yield reductions, not just from the acidity of the soil and how that affects roots, but also from how nutrient availability of P, K, S, Zn, etc.… is affected by pH. In fact, going from a pH of 6.3 to a pH of 5.7 has the potential to take about 15% off our yield potential, and a drop to 4.7 can steal an additional 50% in Corn. The same thing happens when our soils become too basic, and the pH increases above 7.0. From 7.0 to 7.5 can rob 15% off of our yield potential. So in reality, a soil pH of 6.3 to 7.0 is really the Goldilocks zone. Not too high and not too low. When we make Lime recommendations for row crops, we aim for a pH of 6.3 because there is very little additional yield potential from 6.3 to 7.0.
So then what is the Buffer pH that I see reported on a soil test you might ask. So as we discussed before, pH is the percent Hydrogen ions that we see in the soil. Since pH is a percent of Hydrogen in the soil, we need to understand that soil area changes with soil type. We know that Sand is a very large, coarse soil particle, and clay is a very fine particle. But if you take just one tsp of Sand, did you know that it has the same surface area as the bottom of my boot? Compare that to a Silt soil in which that equal one tsp is about the same as the bottom of a cargo box on a side by side. If you think that is a big difference, one tsp of Clay has about the same surface area as Tom Osborne field in Memorial Stadium. So if we have a pH of 5.5 on a sand or a clay, the pH is the same, but the ppm is vastly different. So a long time ago, they came up with Buffer pH to even the playing field. Think of Buffer pH as a Lime index. If I have a Buffer pH of 6.9, it is going to take about 800 lbs of Ag Lime to correct my pH. If I have a Buffer pH of 6.5, it is going to take about 4000 lbs of Ag Lime to correct my pH. The confusing part is, I can have the same pH on two different soils, but because of the soil texture (the parent material) I can have very different Buffer pH’s, and therefore very different Lime needs.
Then, if all of that isn’t confusing enough, we have the Lime itself. Many people think that we apply Lime because the Calcium helps neutralize pH. In fact, it isn’t the Calcium that does it; it’s the Carbonate that is attached to it. Without getting into too much chemistry, our fertilizer, especially our Nitrogen fertilizers add Hydrogen to our soils. Ammonia (NH3), Urea (CO(NH2)2), and UAN (a mix of (CO(NH2)2) and ammonium nitrate (NH4NO3) all lose those H+ ions during the N cycle, and they attach to the soil. When we add Lime (CaC03), it isn’t the Calcium that gets rid of H+ ions; it’s the Carbonate. H+ ions come into contact with the surface of lime particles. It breaks the bond of calcium carbonate and turns into calcium ions, water, and carbon dioxide.
So the take home for the day is very simply this. Don’t get frustrated that you need to lime again after five years or that it is taking so much lime. Those Clay and Silt particle bring a lot of water holding capacity to your fields, so don’t cuss them too loudly when they need more Lime to correct the pH. And don’t be afraid to ask your FSA to explain pH, Buffer pH, and how that all relates to Lime again next week or next year. This is honestly one of the most confusing parts of a soil test, and I left out a lot of details today. Soil tests and any resulting applications aren’t supposed to be complicated or intimidating. Agronomists answer questions like this every day. If you don’t understand, don’t apply. And if you ever want to ask me about this or anything else, I am available for you.