Ag Lime

The primary purpose of Ag lime is to increase to soil pH so before we talk ag lime, we need to talk soil pH. pH, regardless of what is being measured pH is the – log H⁺ ions. So how many followed that! Basically liquids, we are measuring the soil solution, are made up of a ratio of hydrogen (H⁺) cations and hydroxide (OH-) anions. When the two are at a even balance pH = 7. When OH^– out number H⁺ pH is greater than 7 (basic) and when H⁺ out numbers OH^– , the pH is less than 7 (acidic). When you send a soil sample off to the lab the most basic test preformed is pH. There are a few methods to do this but at Oklahoma State we take 10 grams of your soil and add 10 mls of water, stir it and record the pH with a probe. This pH reading is measuring the amount of H⁺ that is present in the soil solution, basically the H+ not attached to the soil, this is called active acidity. However, the amount of H⁺ in the soil solution is 1/100th the amount adsorbed on cation exchange sites. And if we want to change the pH we have to account for the H⁺ in the solution and the H⁺ that is on the exchanges sites. These H⁺ on the exchange sites will quickly pop off the CEC when the concentration of H⁺ in the soil solution is decreased. That is why when the pH is below a critical point the lab will run a buffer analysis. Think of it in this way, the lab adds what is equivalent to one ton of pure lime per acre to the cup, mixes well and re-measures the pH. The lab is looking to see how much the pH changes. Say there are two soils which both started at a pH of 4.3 and after the buffer solution was added with the first soil the pH changed to 6.7 while the second changed to 5.1. This tells us the second soil had a significantly greater amount of H stored on the CEC and therefore will take a lot more lime to change the pH in the field. Just a side note more than likely soil 1 was sandy, low CEC soil while soil 2 and fair amount of clay and significantly higher CEC. Therefore, we use the buffer index not the soil pH to make a lime rate recommendation.

Now that you have had a crash course in the chemistry of pH lets jump off the deep end of chemistry and talk Ag Lime. Ag lime is the short name for ground limestone used to change the soil pH of a field, garden, or lawn. We work heavily with calcitic (CaCO₃) and dolomitic (MgCO₃) lime. Both limes are basically a cation, calcium or magnesium, and a carbonate. The combo of a cation and carbonate is the winning ticket for pH change. However, many tend to think the cation is doing the work. Which is not true, if it was, we would be using gypsum (CaSO₄) as the primary source of Ag lime, because it is slightly more soluble than limestone. {Side note sulfate (SO₄) has already been oxidized and has no effect on pH, decreasing or increasing.} The cation does play an important role as its purpose is to kick the H⁺ off the soil particle. But removing H⁺ is easier said than done. Below is the lyotropic series, this represents the strength that cations are held to the CEC. Note the Al³⁺ and H⁺ is at the top of the list, meaning if all cations are present in equal amounts the Al³⁺ and H⁺ will bind to the CEC before any other.

Al ³⁺ = H⁺ > Ca ²⁺ = Mg ²⁺ > K⁺ = NH4⁺ > Na⁺

The one way the lyotropic series can be beat is by overwhelming the soil system with a cation lower on the list. In other words, we can put large quantities of Ca²⁺ and Mg²⁺ into the soil and they kick the H⁺ off the CEC. This is critical because the pH can not be changed unless the H⁺ enters the soil solution. This is where the CO₃ does the heavily lifting of raising the pH. The CO₃ reacts with H⁺ in the soil to form H₂O and CO₂.

2H⁺ + CO₂ -> H₂O + CO₂

So, the amount of lime that the soil test recommends you apply is based upon the amount of H+ in the soil solution and on the soil CEC that needs to be neutralized to increase the pH to the desired level. Typically, these recommendations are made assuming a 6” incorporation depth, so that means we need to apply enough CO₃ to neutralize the H⁺ in and on the two million pounds of soil.

Below are two Sun-up videos where I discuss soil pH and lime.

If you have any questions or comments please feel free to reach out. b.arnall@okstate.edu