The WEED DOCTOR Our Dozen-a-Day Lesson: Soil Acidity and Liming

By W. Carroll Johnson, III, PhD., Weed Scientist and Agronomist

As a 10-year-old, piano lessons were obligatory and torture. Part of this inhumane rite of passage were the warm-up lessons “Dozen a Day” — repetitious monotony. I hope readers do not judge repeated discussions of soil acidity and the importance of liming with the same degree of contempt. The fact is, soil acidity is a universally persistent problem in food plots and limits productivity. Further, soil acidity is easily corrected with limestone, making the continual problem even more perplexing.

What is Soil Acidity, and Why is it Relevant?

Soil acidity is the quantity of hydrogen ions in the soil. In the eastern and central United States, soils tend to be acidic, even in a natural and undisturbed state. The problem comes with our land-use patterns, which exacerbate the natural acidic tendency. The mineral structure of most soils has limited buffering capacity, and any form of soil disturbance — such as fire, timber harvest or long-term tillage — removes the limited buffering capacity. As a result, soils quickly become more acidic. Acidity is expressed as pH, with values ranging from 0 (very acidic) to 14 (very alkaline). The pH value is a negative logarithm, which mathematically means that soil acidity differs by a power of 10. The target soil pH varies slightly among soil types and crops but is about 6.5-7.0. A soil with a pH value of 4.5 is 100 times more acidic than a soil with the ideal pH of 6.5. Acidic soils are an indication of soil infertility. Chemically, acidic conditions alter the chemical form of important soil nutrients, making those elements unavailable to crops; that is, nutrient starvation. Acidic soils also alter other naturally occurring elements in the soil, making them toxic to crops, with aluminum being the major culprit. I distinctly remember a classmate asking an instructor about the major reason why acidic soils affect crop growth, and he quickly responded that aluminum toxicity was the major reason — that is, poisoning. Soil acidity has an additional insidious effect on forage legumes such as clover and alfalfa. Legumes have the unique ability to capture and use nitrogen from the atmosphere (actually in air spaces in the soil) for plant growth. This is done with a symbiotic relationship between nitrogen-fixing bacteria in the soil and legume roots. Acidic soils inhibit the survival and efficiency of the symbiotic bacteria, affecting growth of forage legumes. How much growth reduction? I have seen in the agricultural scientific literature a report of a 28-time yield increase in limed alfalfa compared to alfalfa grown in a non-limed, highly acidic soil. This is why properly limed soils are essential for food plots planted to forage legumes such as Imperial Whitetail Clover and Alfa-Rack Plus.

Correcting Soil Acidity

Neutralizing soil acidity is fairly straightforward. Common neutralizing soil amendments are carbonates (limestone), oxides and hydroxides. These materials are combined with calcium or magnesium, which are often incorrectly thought to neutralize soil acidity. Calcium  and magnesium are essential elements for plant growth, but do not directly neutralize soil acidity. Carbonates, oxides and hydroxides are the chemical agents that neutralize soil acidity. Consider calcium and magnesium to be carriers of the actual neutralizing agent. Which is best to neutralize soil acidity in food plots: carbonates, oxides or hydroxides? Carbonates offer the best combination of cost, chemical reactivity, widespread availability and ease of use. Carbonates include calcitic limestone (calcium carbonate) and dolomitic limestone (a mixture of calcium carbonate and magnesium carbonate). In the context of neutralizing soil acidity, there is no real difference between calcitic limestone and dolomitic limestone. Both are naturally occurring, and the material locally available is often based on which type is mined in the closest limestone quarry. In commercial agricultural regions, limestone is sold in bulk and stored at dealers in massive outdoor piles. Bulk limestone is sold as a blend of particles of varying size. Smaller particles quickly react to neutralize soil acidity. Larger particles take longer to react, extending the length of neutralization. Consider the particle size blend to be an ad-hoc form of time release. This is why farmers typically lime about every three years. Limestone is also sold in bags at home-improvement stores, mainly for use in home gardens, lawns or landscape areas. Bagged products contain finely ground limestone that has been pelleted for ease of homeowner application. The coating is water soluble, releasing the finely ground limestone that quickly neutralizes acidic soils. This sounds good at face value, but the uniform fine texture equates to short-term neutralization, and bagged limestone costs significantly more than bulk limestone. However, I can think of food plot scenarios where bagged, pelleted limestone is a wise choice, assuming the user is willing to accept the higher cost. Pound for pound, oxides and hydroxides are the most efficient and quick-acting materials to neutralize soil acidity. However, they are potential irritants to handlers and overall difficult to apply. Further, their quick-acting properties mean very short-term neutralization. These disadvantages largely outweigh the advantages of neutralization efficiency, but perhaps oxides and hydroxides are viable options in some extreme situations. From time to time, I’m included in discussions on using liquid or sprayable formulations of limestone on food plots. Sprayable limestone is a slurry of ultra-fine limestone particles in water. It will neutralize acidic soils quickly but no quicker than applying dry limestone of the same particle size. Because of uniform small particle size, the benefits of sprayable limestone are short-lived, just like dry fine limestone. However, I do not recommend sprayable limestone because of limitations of commonly used sprayers. Sprayable limestone is often used for hydromulching/seeding operations that have trash pumps to spray dense slurries of solids (seeds, fertilizer, mulching materials) in large volumes of water (thousands of gallons per acre) with aggressive mechanical agitation in the tank. A typical ATV- or tractor-mounted sprayer cannot generate the necessary output for sprayable limestone and does not have adequate agitation to maintain suspension in the spray tank. Additionally, spraying heavy slurries using an ATV sprayer can destroy the pump.
For food plots, dry liming materials are the practical choice. Despite the straightforward solutions to soil acidity, liming acidic soils in food plots remains a monumental obstacle. Why? One reason might be the rapid turnover of hunting property and reluctance to invest in a long-term solution. But, there are quick-acting liming materials for those situations. I believe the major reason why liming acidic soils remains challenging is the sheer volume of liming materials needed to adequately neutralize soil acidity. Limestone rates are usually expressed in tons per acre. It’s difficult to procure and transport bulk materials to remote food plot sites. Additionally, small-scale food plot equipment is not suited to apply large amounts of bulk limestone. As a result, food plots are often under-limed and over-fertilized in an attempt to compensate for soil acidity causing nutrient starvation. This is wasteful in terms of unfulfilled food plot potential and overuse of expensive fertilizer. The solution to these logistical obstacles is smart management and planning. Bulk limestone is affordable and can be stockpiled at remote sites for future use. Small-scale equipment can be used to successfully apply liming materials, but it will be slow-going because of the volume of liming materials applied. Plan on extra time to apply liming materials. Even better, hire a commercial applicator to spread limestone. When I was an undergraduate student at Auburn University in the mid-1970s, all students in every major in the College of Agriculture (this includes pre-med and pre-vet students) were required to take an introductory soils class taught by a distinguished professor, Dr. Joe Hood. Much of the lecture was spent on soil acidity. Every biological system from crop production to wildlife to orchard crops is affected by soil acidity. This illustrates the fundamental importance of managing soil acidity, including in food plots.