By Kelly Robertson, CCA, ISA Soy Envoy

Lime is not just lime. There are differences. The No. 1 thing you can do to help boost yields, improve weed control, improve nitrogen use efficiency and improve soil health isn’t solved by a biological product, foliar feed, cover crop, planter attachment or new, flashy technology. The answer is so simple that we don’t even talk about it anymore. It’s two words represented by two letters that few can clearly define: pH. This stands for “potential of hydrogen” or “power of hydrogen” and is a measurement of acidity or alkalinity. The pH scale runs from one to 14 with seven being neutral. Below seven is acid, above seven is basic or alkaline.

Over the past 10 years, we have been graphing our yearend soil test results to see what percentage of our macro soil elements are low or below the optimum range. What we have learned has now become predictable here in southern Illinois. On average, about 40 percent of the soil samples we take every year have a pH(w) (water pH) of less than 6.2. We have seen this number as low as 30 percent and as high as 42 percent. What is even more shocking is that on average, 12 to 15 percent of soil samples are below a pH of 5.8 and six percent to 10 percent are below a pH of 5.4. It is almost as common to see a pH of 4.8, 4.9 or 5.0 as it is to see a pH of 7.5. This might lead us to conclude that pH is the biggest yield-limiting factor on about 40 percent of our farmed acres.

In corn, soybean and wheat production, we desire our soil pH to be between 6 and 7. The ideal range is between 6.2 and 6.8. This is the pH range in which everything is firing on all cylinders in the soil. Our soil microbes and soil-applied herbicides are happy, and roots can be better at taking up nutrients and water. When we are within the ideal pH range, we are also getting the maximum uptake and use of applied fertilizer. At a soil pH of 6.0, 20 percent of the fertilizer that we apply could be tied up in the soil or not taken up by the plant. At a pH of 5.5, that percentage of fertilizer climbs to 33 percent. We also can have potential yield loss because of what was mentioned above. Weed control, nitrogen use efficiency, water uptake, as well as plant and soil health can suffer.

I made a spreadsheet (see image on this page) that can be used to input the cost of production and yield goals for an acre of corn or beans. This allows us to see what the potential dollar per-acre loss could be if pH is outside of the ideal range. In some instances, it becomes very costly not to correct your pH.

Graph Credit: Kelly Robertson, Precision Crop Services LLC

The only way to correct soil pH is with limestone.

A lime product must contain carbonate (CO3 ), such as calcium carbonate (CaCO3 ) or magnesium carbonate (MgCO3 ). Gypsum is not a lime because it contains no CO3 , only Ca (Gypsum is CaSO4 ). Using gypsum to try to correct soil pH will lead to other problems. Anyone who has advised you to use gypsum to alter soil pH is misguided and does not understand the neutralizing power of the carbonate component of limestone. I will say it again: Gypsum is not a lime and will not change your soil pH.

Often when you take soil samples from the lab, they will generate a limestone recommendation based on the crops to be grown. Unless you specify what limestone you will use, these recommendations are made based on a pure limestone (100 percent) or what is called a “lime 90,” or 90 percent limestone. The problem with these recommendations is that we don’t buy 100 percent or 90 percent limestone; therefore, we or your agronomist must correct this number to the limestone being used.

In Illinois, we have a booklet that is published every year called the Illinois Voluntary Limestone Producers Handbook. This book consists of a compiled list of all participating quarries and the samples they submit show correction factors (CF) compared to a reference limestone. In this book, you can see the differences in one limestone versus the other and then compare their correction factors. The correction factor is used to compare the lime in question to the pure lime or 90 percent lime. The higher the number, the more lime you will need to apply.

Let’s look at an example.

In Figure 1, the lab recommendation shows we need two tons per acre of lime.

However, looking at Lime 2 in Figure 2, we see that Lime 1 has a correction factor (CF) of 1.29. This means that we need to put on about 29 percent more of this lime to equal pure limestone or lime 90. Looking at Lime 2, it has a correction factor of .74. This means we need about 25 percent less lime than a pure lime or lime 90.

Figure 1: Provided by Kelly Robertson, Precision Crop Services LLC

Figure 2: Provided by Kelly Robertson, Precision Crop Services LLC

How is this possible?

Well, it goes back to fineness of the lime grind. The finer the grind, the faster it will break down and correct pH. You can see in the picture on this page the huge difference between these two lime products: Lime 2 is almost like a powder and Lime 1 has chips or almost rocks in it. We need to compare the cost of the lime, the cost of trucking the tons and the spreading cost of those tons to get a true picture of which lime is the better buy. Just because it might be cheaper doesn’t mean it will be the better buy. In this case, even though Lime 2 costs more per ton, we haul less and spread less. In our example, we save $872 on 40 acres or $22 per acre over Lime 1! A ton of lime is not necessarily a ton of lime. Our experience is that in most cases, when we do not “true” the lime source to the recommendation, we under-apply. Fixing pH is the simplest, most cost-effective and highest-ROI activity we can do to increase yields, control weeds, increase fertilizer efficiency and increase soil and plant health. It pays to fix pH, and it pays to do it right. Do the homework and do the math.

Figure 3: Provided by Kelly Robertson, Precision Crop Services LLC

About the Author:

Kelly Robertson, CCA, is a 2024 Soy Envoy. He has been a soil fertility agronomist and precision agriculture consultant since 1989 and also spends time in farm/ agronomy management roles for farms in southern Illinois. In 2012, Kelly and his wife, Lori, started Precision Crop Services in Benton, where they provide agronomic services for their customers including soil testing, crop scouting, data analysis, GPS/GIS services including variable rate seeding and fertility recommendations as well as farm and agronomy management for their customers. He is a Certified Professional Agronomist, Certified Crop Adviser, Certified 4R Nutrient Management Specialist, 2015 Illinois Soybean Association Double-Crop Specialist, 2016 Illinois CCA of the Year and the 2021 Illinois Soybean Association Dave Rahe Excellence in Soils Consulting Award winner. Originally published on the Field Advisor website on Sept. 10, 2024