Lime: Facts and Fallacies

By Jon Cooner

If you’re a longtime reader of Whitetail News, you already know that almost every issue addresses soil pH and its critical importance to forage growth. That should give you an idea of how important soil pH is. In this article, we’ll touch again on soil pH and what it means to food plotters. Then, we’ll cover a question the Whitetail Institute’s in-house consultants are often asked: “What type lime should I use to raise soil pH in my food plots?”

The Basics of Soil pH What is soil pH?

In simplest terms, soil pH is a number on the pH scale, a set of numbers from 0 to 14. It’s a measurement of whether a soil is acidic (0 to 6.4), neutral (6.5 to 7.5) or alkaline (higher than 7.5). For example, if the soil pH in your plot is 6.8, it’s within neutral range. If the soil pH in your plot is 5.8, it’s acidic.

Why is soil pH important?

Remember I said that soil pH is a measurement? Technically, it’s a measurement of whether soil is acidic, neutral or alkaline. For our purposes, though, there’s another way to define the soil pH measurement that’s easier and more direct: Soil pH is a measurement of how well or poorly forage plants will be able to access and uptake fertilizer and other nutrients in the soil. Soil pH must approach neutral for fertilizer and nutrients already in the soil to be freely available for uptake by high-quality forage plants. Most fallow soils are naturally acidic. If you plant in acidic soils without first increasing pH by adding lime, many of the nutrients you apply as fertilizer can be unavailable to plants. Acidic soils chemically alter important soil nutrients, which makes them unavailable to plants. Making sure that soil pH is within optimum range is so important it can make the difference between the best food plot you can imagine and total failure.

The Importance of Laboratory Soil Testing

As mentioned, soil pH is a measurement — one that’s important to make accurately. It’s just like building a house out of wood. You wouldn’t guess about how much lumber you’d need because that would likely leave you with too little lumber to build a solid structure, or you’d end up buying more wood than you needed, wasting money. Making sure you measure soil pH accurately is important for the same reasons. If your soil pH is acidic, you know exactly how much lime to buy for optimum forage growth, and you won’t waste money buying lime that you don’t need.  Only a qualified soil testing laboratory can make accurate recommendations about how much lime you’ll need to add to the seedbed if your soil is acidic. That’s because soils differ widely in their ability to hold lime activity. And that’s often true even with soils taken from plots close to each other — even seemingly identical soils. The only way to know exactly how much lime you’ll need to add if your soil is acidic is to have a laboratory tell you, based on its analysis of your soil.

A Brief Look at Liming Materials

As mentioned, the rest of this article will discuss things folks consider using to raise (or trying to raise) the soil pH of acidic soil. The list is remarkably long — too long to cover everything here. Some, though, come up more frequently than others, so I’ll hit the high points.

Liming Materials as Used in This Article

Some of the materials folks consider using are not recommended because they might pose pollution hazards or offer unknown actual neutralizing capacity, or they might be too caustic for most folks to safely handle. Even so, I’ll collectively refer to them as liming materials so we’ll have an easy term. Three primary soil amendments neutralize soil acidity: carbonates (calcium or magnesium carbonate), calcium oxide (quick lime) and calcium hydroxide (slaked lime). Each material has advantages and disadvantages.

Natural Liming Materials Carbonates (Aglime and Pelleted Lime)

By far, the most commonly used liming materials are aglime and pelleted lime. Both consist of mined limestone rock that has been crushed to a given particle size. The limestone rock that makes up aglime and pelleted lime can be either of two types: calcitic limestone (calcium carbonate) or dolomitic limestone (a combination of calcium carbonate and magnesium carbonate). Which you’ll find near you will likely be determined by what type is being mined at the closest quarry. The difference in neutralizing capacity is minimal. Aglime is more coarsely ground than pelleted lime. Pelleted lime consists of more finely ground limestone that’s then reformulated into small pellets using a water soluble polymer that quickly dissolves to release the finely ground material. The Whitetail Institute’s consultants are often asked, “Which is better, aglime or pelleted lime?” If you’ll be using a hobbyist-grade spreader to put the lime out, pelleted lime is the better choice. In most other cases, though, aglime is the way to go. One reason is that aglime is much less expensive than pelleted lime. Another reason can be stated in a somewhat oversimplified rule: Smaller lime particles raise soil pH more quickly, and larger lime particles keep soil pH up longer. Did you notice, though, that I didn’t say that pelleted lime works faster than aglime? The reason lies in how aglime is usually stored before sale — in bulk. Because it is usually stored in bulk, it will contain particles of varying sizes, which works to our advantage. The smaller particles in aglime neutralize acidity more quickly, and the larger particles keep soil pH up longer, providing the best of both worlds.

Liquid Lime

Liquid lime also neutralizes soil with limestone. Even so, there are some misconceptions and drawbacks concerning liquid lime that aren’t posed by aglime or pelleted lime. A big misconception about liquid lime is that it works more quickly to raise soil pH than aglime or pelleted lime. That assumption might seem reasonable, given what we said earlier — that smaller lime particles work more quickly than larger ones, and the particles in liquid lime are crushed so finely that they’ll suspend in a spray solution. Actually, though, even though it’s a fluid, the solution does not react any faster than limestone of the same weight ground to the same fineness. One ton of liquid lime solution will not increase soil pH any faster than one ton of limestone ground to the same fineness (100-mesh or finer). In addition, liquid lime has quite a few potential drawbacks not presented by aglime or pelleted lime. Liquid lime is much more expensive and carries a much greater risk of raising pH too high, and its effectiveness is short-lived compared to limestone — so short, in fact, that you must often reapply it every year. Also, it’s almost impossible to apply enough liquid lime through a sprayer to significantly raise soil pH anywhere but at the soil’s surface. Liquid lime can also be tough on equipment. Most sprayers do not have a pump with enough agitation to suspend the slurry, possibly ruining the pump by trying to pump the thick slurry through the sprayer.

Other Natural Liming Materials

A few other natural liming materials will effectively neutralize acidic soil. Few, if any, though are as practical an option for most folks as aglime. Egg shells are one example. Egg shells will neutralize soil pH, but it takes a lot of them, and they have to be crushed to work. Marl is another option that will work, but it’s impractical for most folks. Like calcitic limestone, marl is calcium carbonate. It’s a natural deposit that usually appears muddy when wet or white and crumbly when dry. Moving large amounts of marl from one location to another is time- and labor-intensive, and hardly worth the effort when aglime is so inexpensive to purchase and have delivered.

Synthesized Liming Materials From Mills

When we’re asked about liming materials other than aglime and pelleted lime, it’s usually about a synthesized material, meaning a product of one or more industrial processes. The two most commonly asked about are quick lime and slaked lime.
Calcium oxide (quick lime). Quick lime, which is also referred to as autoclave lime, burned (or burnt) lime, oxide lime, un-hydrated lime and soda lime, has roughly three times the neutralizing capacity of mined limestone rock. However, it’s also a caustic irritant.
Calcium hydroxide (slaked lime). Slaked lime, which is also referred to as hydrated lime and builder’s lime, is basically quick lime that has been exposed to water because it was piled outside. Calcium hydroxide also quickly and efficiently neutralizes soil acidity, with about twice the neutralizing capacity of mined limestone rock.
Handling difficulties. There are quite a few reasons why we don’t recommend folks try to use quick lime or slaked lime in their food plots. The main reason is that they’re difficult for most food plotters to handle safely. The Material Safety Data Sheet for calcium oxide specifies that handlers need to wear goggles, gloves and a chemical resistant apron and avoid breathing calcium oxide dust. Calcium hydroxide is also a caustic irritant and equally miserable to handle.
Pollution concerns. Some synthesized materials that will increase soil pH might damage the environment with varying degrees of certainty. In the past, some mill castoffs have contained heavy-metal pollutants such as lead, zinc, chromium or cadmium. Flue dust (also known as cement kiln dust), for example, is another synthesized liming material that has been identified as sometimes being a source of heavy-metal contamination. These days, though, many mills offer good material for soil enhancement. Be sure you use such materials only if they are specifically labeled as appropriate for use as a soil amendment, such as “For beneficial use.” Although most suppliers of synthesized liming materials these days should guarantee what they offer is pollutant-free, if you’re considering buying from a supplier who won’t offer that guarantee, it would be smart to pass. If you put such pollutants into the soil, it can cause damage to the environment and get you in a mess with the EPA.

Known neutralizing capacity.

Remember, we’re looking at materials to raise soil pH by a specific amount. It stands to reason that knowing a material’s neutralizing capacity is pretty darned important. Some liming materials, though, do not offer any standardized neutralizing capacity. Agricultural slag, for example, has less neutralizing ability than limestone, and it will have to be tested to determine the correct application rate if it isn’t being sold specifically as a liming material.

The Bottom Line Use a laboratory soil test report to test your soil.

Soil pH is the most important thing you can control to ensure food plot success. Accordingly, it makes sense to make sure you know what the soil pH in your food plot is, and if it’s acidic, how much lime to add to increase soil pH to 6.5-7.5. Only a laboratory soil test can tell you these things exactly, allowing you to make sure you add enough lime to raise soil pH to neutral and that you don’t waste money buying additional lime you don’t need.

For most folks, aglime and pelleted are optimum materials for raising soil pH.

 I hope this article has also helped you see that although other options for raising low soil pH exist, you’ll always be well served by following the recommendations in your soil test report, which call for the addition of lime (aglime or pelleted lime) to the seedbed. As Dr. Wiley Johnson, the Whitetail Institute’s first director of forage research, said, “Lime doesn’t cost. It pays!” Thanks to Dr. Carroll Johnson, Whitetail Institute’s weed and herbicide scientist, for his assistance in writing this article.