One of Nature's Gifts to AgricultureNatural processes and agricultural practices tend to make Missouri soils that are used for crops and pastures become acidic. Acidic soil limits grain production, plant growth and forage quality. The cure is agricultural limestone (aglime), which is readily available throughout the state. It is often one of the most cost effective products a farmer can apply to fields.
The use of nitrogen-containing fertilizers (ammonia nitrate and anhydrous ammonia) is extensive in agriculture, and this practice is a leading contributor to soils becoming acidic. That's a primary culprit, but other contributors are decomposition of organic matter, leaching of soil components, rainwater and plant uptake of calcium and magnesium.
Acidity, as well as alkalinity, is expressed as pH. Values less than 7 are acid and greater than 7 are alkaline. The "sweet spot" between 6 and 7 contributes to highest yields for most crops. This is a logarithmic scale, so a pH value of 5 is ten times more acidic than a value of 6. A value of 4 is one hundred times more acidic than a value of 6. As a soil becomes more acid, there is an increasing reduction of crop yields and forage quality.
Aglime is a natural substance since it is merely finely ground limestone that can be purchased at quarries throughout the state. Because limestone is abundant in Missouri, aglime is available throughout the state at a reasonable cost. It is usually broadcast dry on the surface of fields using a spreader truck. Other application methods have been used, such as "fluid lime" and pelletized lime. Such application methods are typically appropriate in specialized situations.
One history of liming says that the practice dates as far back as 200 B.C. It has been widely used as a soil treatment in the United States since the 1920's.
The acid neutralizing ability of aglime is dependent on a couple of factors. One is its purity. The chemical makeup of calcitic limestone is CaCO3. Dolomitic limestone is CaMg(CO3)2. Both are the carbonates of calcium and magnesium, an identity gained from the remains of animal life that settled to the bottom of an ancient sea that once covered this part of the continent. The amount of acid that a certain amount of aglime can chemically react with determines its calcium carbonate equivalent (CCE). This value can vary from one source of limestone to another, so a Missouri law that regulates the sale of aglime requires that it must have at least 65 percent CCE.
The second factor is the size of aglime particles that's applied on the soil. Small particles react more quickly with acid than larger particles. Soil scientists have developed a rating system that reflects the effect of particle size on acid-neutralizing ability. The ratio is based on the amount of aglime that would be expected to react in soils in one year. Aglime particles that cannot pass through an 8-mesh screen (8 openings per square inch of screen) are too large to have any short-term value. Aglime particles that pass through a 60-mesh screen are considered to be 100 percent effective. The aglime law requires that no more than 10 percent of the particles in aglime can be too large to pass through the 8-mesh screen.
The CCE value of limestone deposits throughout Missouri commonly varies from 70 to 105. In addition, the particle size of aglime also varies from source to source. As a way for farmers to compare the cost value of one source of aglime to another, they can use what's called Effective Neutralizing Value (ENM). This value incorporates the variables of CCE and fineness of particles. It is calculated by using the following formula: ENM=CCE x Fineness Factor x 800. The 800 number is a constant that refers to the pounds of effective calcium in one ton of pure lime. Aglime with an ENM value of 600 will neutralize twice as much acidity as aglime with an ENM value of 300. If a farmer can buy 600 ENM aglime at a cost of just 50 percent higher than 300 ENM aglime, this represents a relative bargain.
The aglime law requires producers of this product to pay 6 cents for every ton sold to fund a testing program administered by the University of Missouri Agricultural Experiment Station. Aglime samples are tested in the Experiment Station's chemical laboratory and are given an ENM value. Purchasers of aglime must be informed of this value.
Farmers can find out precisely how much aglime is needed on their fields, if any, by sending soil samples to a lab for analysis. The University's soil testing lab or private labs will determine the soil's deficiencies for all essential plant nutrients, as well as the amount of aglime required to achieve the desired pH. The aglime requirement from Missouri labs will usually be expressed as pounds of ENM to be applied per acre.
Achieving the proper soil pH benefits crops in a number of ways. At the top of the list is maximum availability of plant nutrients. Other benefits include: (1) Reduced likelihood of aluminum and manganese toxicity; (2) Promotion of microbe activity in the soil; (3) Improved soil structure and tilth; and (4) Improved activity of certain herbicides.
An often-overlooked benefit of aglime is its value as a source of calcium and magnesium in the soil. These minerals are not easily available from other sources, but are essential for plant growth. They also help prevent forage plant species (as well as the animals that graze on them) from acquiring certain toxic reactions caused by mineral imbalances.
With so many benefits of achieving the proper soil pH, as well as determining the need for other nutrients, it seems logical that every farmer would routinely send soil samples to a lab for analysis. It's one of the mysteries of agriculture that only 5-10 percent of Missouri's farmers have soil tests done on a regular basis. Simply comparing the use of nitrogen for crop fertilizer in the state (which tends to make soils more acidic) with the volume of aglime sold indicates that many soils are probably acquiring pH values well below 6. In fact, one of every three samples analyzed by the University of Missouri soil testing lab reveals excess acidity. This indicates that most farmers aren't aware that one-third of their cropland could have significantly increased yields by the application of low-cost aglime.
The evidence is clear that aglime is truly one of nature's gifts to agriculture.