In Part 1 and Part 2 of this series of articles in the Agronomy eUpdate we discussed the basic concept of pH and acidity in soils, why pH control is important in crop production, and how to make lime recommendations. This third and final article will discuss what reactions must occur in soil to change pH and selecting an appropriate and cost-effective liming material.
The “lime reaction”
The process of adding a material to raise soil pH, commonly referred to as liming, removes hydrogen (H+) from the soil cation exchange capacity and replaces the H+ with a basic cation such as calcium (Ca+2) and/or magnesium (Mg+2). But to complete the liming process, the hydrogen removed from the CEC must be neutralized and removed from the soil solution. The most commonly used liming product which both replaces exchangeable H+ with Ca+2 and neutralizes the replaced H+ is aglime. The chemical reaction explaining this reaction is given below:
Lime, calcium carbonate (CaCO3), reacts with an acid soil replacing some of the exchangeable H+ with Ca+2, and forming carbonic acid, H2CO3 in the soil solution. Carbonic acid is what gives a soda the fizz, and is very unstable. The carbonic acid breaks down quickly to CO2 and water. The CO2 bubbles off to the atmosphere, and the H + is neutralized to form water. So the soil pH is raised by reducing the H+ in the soil solution.
Compare this reaction to what happens when gypsum, calcium sulfate CaSO4, is added to soil:
Gypsum is the salt of a strong acid, and dissolves to form Ca+2 and sulfate SO4-2 in the soil solution. The Ca+2 replaces some of the H+ on the CEC, and sulfuric acid, H2SO4, a strong acid, is formed in the soil solution. There is no way to neutralize that acid. The net effect is that little or no change in pH occurs when the salt of a strong acid, such as CaSO4 or KCl (potash), is added to the soil.
The bottom line is that there must be some reaction to neutralize the replaced exchangeable H+, for a material to be an effective liming tool and increase soil pH.
Liming sources available
The most commonly used liming material is ground limestone. Several liming materials are marketed, and it is important to recognize differences among them.
Ground aglime (dry). Limestone rock is crushed and ground into a material known by several names, including aglime, agrilime, ag stone, ground agricultural limestone, and lime. Ground aglime is the most widely used liming material in the U.S., as it is easy to transport and apply. The calcium in limestone is in the carbonate form and most Kansas limestones also contain some magnesium carbonate. Calcium content ranges from 15 to 40% and magnesium content normally ranges from 0 to 15%. Limestone with less than 5% magnesium is designated as “calcitic limestone.” Limestone with more than 5% magnesium is referred to as “dolomitic limestone.”
Fluid lime (liquid lime). In recent years a product called fluid lime or liquid lime has been marketed in some areas. This product is prepared by mixing very finely ground aglime (100% passing a 100 mesh sieve and 80-90% passing a 200 mesh sieve) with water along with a suspending agent (attapulgite clay). The material is then applied with a liquid fertilizer applicator. The aglime content of liquid lime is normally around 50% by weight.
The main advantage for liquid lime is that a more uniform application can be made in the suspension form. Claims are also made that liquid lime will raise the soil pH faster than standard dry aglime and that much less material is needed because it is very finely ground. Both statements are partially true. For the first few months after application, the soil pH will rise faster where liquid lime is applied due to the extremely fine size of the lime particles. But, within a year or so, changes in soil pH will normally be equal for liquid and dry sources applied at equivalent rates of ECC.
The relative speed at which a lime particle reacts is a function of its surface area. The smaller the individual particles, the greater the relative surface area, and the faster the particle will react and change pH. Particles that are smaller than 60 mesh are considered to be 100% available and will be effective in changing soil pH within a year of application. Grinding particles finer than 60 mesh may speed the rate of reaction, but it will not alter the overall effectiveness of the lime.
Higher cost is the main disadvantage of liquid lime. This is primarily due to the additional cost of grinding the material fine enough to keep it in suspension, the cost of the suspending agent, and transportation costs for the water used in the suspension. A common application rate is 1,000 pounds per acre -- 500 pounds water and 500 pounds aglime. This rate would be adequate for a maintenance program to offset acidity caused by N fertilization, but it would not normally be economical as a corrective remedy where lime recommendations are 2 tons per acre or greater.
Pelletized lime (pel lime). To avoid the dust and distribution problems associated with spreading very fine particles, finely ground aglime can be compressed into pellets or granulated using a binding agent. The resulting product can then be spread similar to dry fertilizers. Individual pellets are readily dispersible in water and like liquid lime, will react very quickly in soil. Application rates can also be reduced one-third to one-half because of the fineness of the particles used to make the granule. Claims that a few hundred pounds can substitute for a ton of aglime have not been substantiated by land grant university research. Likewise, the practice of banding 200 to 400 pounds per acre of pelletized lime in the row as a starter fertilizer may be a means of supplying calcium or magnesium to the crop, but would have no effect on other key processes controlled by soil pH, and has only a limited potential for creating a yield increase capable of paying for the product.
While pelletized lime is an excellent product and has a high ECC, often approaching 1,800 to 2,000 pounds ECC per ton, like liquid lime, cost is often a use-limiting factor.
Marl. This is soft, unconsolidated lime material made up of marine shell fragments and calcium carbonate and found under many shallow organic soils in the Great Lakes region of the U.S. Marl also commonly contains clay and organic matter as impurities and is mined wet and sold by the cubic yard because of the high moisture content. As a general rule, two cubic yards of marl has a neutralizing value equivalent to one ton of aglime. Uniform spreading is difficult unless the material is dried and ground, which increases the cost of the product. Use is generally confined to local areas very near marl deposits. Since marl contains no magnesium, repeated applications of marl may result in soils deficient in magnesium. Deficiencies can be prevented by occasional reliming using dolomitic limestone.
Burned lime (quicklime). Limestone rock is heated at high temperatures to drive off carbon dioxide to produce calcium oxide, CaO. It is a fast-acting liming material, but is also corrosive, disagreeable to handle and more expensive than aglime. It is usually used for special non-ag purposes.
Slacked lime (hydrated lime). This is produced by adding water to burned lime to produce calcium hydroxide, Ca(OH)2. It has many of the same characteristics and limitations as burned lime.
Evaluating the relative value of liming materials
How effective a liming material will be in correcting soil acidity depends on two primary factors: chemical purity and fineness. Many states have lime laws which vary from labeling laws, like Kansas, specifying strict minimum purity and fineness standards to qualify for sale.
Chemical purity. Liming materials vary in their composition and thus in their capacity to neutralize acidity. Calcium carbonate equivalence (CCE) is the standard for measuring purity. Pure calcium carbonate has a CCE of 100%, while high magnesium dolomite has a CCE of 108%. Most aglime contains both calcium and magnesium carbonates along with varying levels of other impurities. The CCE of most midwestern limestone generally ranges between 85 and 107%. A suggested minimum CCE in many states is 80%.
Fineness. This refers to particle size and is important as mentioned earlier, because it governs how quickly the lime will dissolve and react to neutralize acidity. Most liming materials contain a mixture of particle sizes, from dust to fine-gravel. Small particles dissolve rapidly and react quickly due to their high surface area. Coarse particles react very slowly and are of little value in correcting an acidity problem due to low surface area.
Fineness of aglime is determined by passing the material over a set of sieves (screens) of different sizes. Sieve size is expressed in terms of the number of openings per linear inch; an 8 mesh sieve has 8 openings per inch (64 per square inch). The required screen sizes used and the number of screens required to quantify neutralizing value will vary across states. In Kansas, 8 and 60 mesh sieves are used to determine fineness. Table 1 shows the relative effectiveness of different size particles at changing pH over an extended period. It is suggested as a general recommendation in some states that a minimum of 80% pass an 8 mesh sieve and 25% pass a 60 mesh sieve for good lime performance and economics, since transportation and spreading costs are often greater than the cost of the lime at a quarry.
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Table 1. Relative effectiveness of different size limestone particles |
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Percent dissolving in |
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Particle size |
1 year |
4 years |
8 years |
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Larger than 8 mesh |
5 |
15 |
15 |
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Pass 8 mesh, held on 30 mesh |
20 |
45 |
75 |
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Pass 30 mesh, held on 60 mesh |
50 |
100 |
100 |
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Smaller than 60 mesh |
100 |
100 |
100 |
A series of efficiency factors based on the relationship between partical size and rate of reaction have been developed. An example of these factors as used in Kansas are listed in Table 2 below:
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Table 2. Efficiency factors used to determine the effect of lime particle size on Effective Calcium Carbonate (ECC) content of lime |
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Particle size |
Efficiency factor |
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> 8 mesh |
0 |
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8-60 mesh |
0.5 |
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< 60 mesh |
1.0 |
The concept of ECC. The ultimate effectiveness of aglime is determined by the interaction of chemical purity and particle fineness. Since both fineness and purity vary from one lime producer to another, there have been several systems devised to compare limes for the purpose of economics and adjustment of recommended rates for quality. In Kansas, lime quality is determined by measuring the chemical neutralizing capacity of the lime product, CCE, and by determining the fineness of the product by measuring the percent of the product which passes through both an 8 mesh and a 60 mesh sieve. A typical lime analysis report will give the sieving analysis as the weight or percent of lime retained on the 8 mesh sieve, the percent retained on the 60 mesh sieve, and the percent passing through the 60 mesh sieve. This is then used to calculate the fineness score or fineness factor. The fineness score or factor is calculated as follows using the efficiency factors given in Table 2 above:
Percent retained on 8 mesh sieve x 0
+ Percent passing 8 mesh sieve but retained on 60 mesh sieve x 0.5
+ Percent passing through a 60 mesh sieve x 1.0
= Fineness factor or score
An example for a typical average quality aglime would be:
Weight or percent on 8 mesh sieve = 19% x 0 = 0
Weight passing through 8 mess but retained on 60 mesh sieve = 40% x 0.5 = 20
Weight or percent passing through a 60 mesh sieve = 41% x 1.0 = 41
= Fineness score (FF) = 61
Using the chemical purity expressed as a Calcium Carbonate Equivalent, CCE, together with the fineness score, the percent Effective Calcium Carbonate, sometimes referred to as the percent lime, is calculated as follows:
(Fineness factor x CCE)/100 = percent effective calcium carbonate, % ECC, or % lime
An example of the calculation for a typical aglime would be:
[(FF) 61 x (CCE) 90]/100 = 54.9% ECC
To calculate the pounds of ECC per ton of lime, one simply multiplies the percent ECC x 2000. For example using the values above:
(54.9% ECC x 2000)/100 = 1,098 pounds ECC per ton
Impact of moisture. Lime is often stored in piles outside and is impacted by weather. Thus to standardize the value of a ton of lime, results of lime analysis are often reported on a dry weight basis, much like grain moisture can vary and is adjusted to a standard moisture content. Lime moisture content will vary from 3 to 10% moisture depending on recent rainfall patterns.
Industrial byproducts as lime sources
A number of industrial byproducts or coproducts have been used successfully as liming materials. Key factors to evaluate when considering the use of these products include cost, transportation, easy of spreading, and neutralizing value. In many states the state EPA or Department of Environmental Management regulates the land application of these products. Always check that the material is properly registered or permitted before using these products.
Lime sludge. Some water treatment plants produce a soft lime sludge containing fine lime particles. Lime sludges vary in their calcium carbonate equivalent (CCE) and water content, which will influence the amount of sludge needed to equal dry aglime. Since the particles are very fine, it reacts quickly in soils. Lime sludge is often used to produce liquid lime products. It also can be stacked for extended periods and “dewatered” to allow for spreading with manure spreaders. Unfortunately, distribution can be an issue if the material is not well dried and chunks broken up with the spreading equipment.
Fluid-bed ash, fly ash, and stack dust. Modern electrical generating plants mix limestone into ground coal as a means of controlling burn rates and enhancing efficiency of operation. Much of the ash produced in these fluidized or plasma bed generators contains large quantities of calcium oxide and has neutralizing value. Ashes and dust collected from smoke stacks and cement kilns also can have lime value, though the neutralizing value can vary widely. Many of these products can also contain heavy metals and other environmental contaminants. Thus, these products are closely regulated and application rates can be limited by metal content. Make sure to check with your state department of environmental management before using one of these products to ensure the product is safe and legal for use.
Agricultural slags. One of the steel industry by-products is a magnesium silicate or slag. Air-cooled slag must be ground the same as limestone when used as a liming material. Water-cooled slag is a porous granular material produced by applying water to the hot slag. Usually it is screened and the fines are used as a liming material.
Summary
Soil acidity and low soil pH are a growing problem in many parts of Kansas. The widespread use of nitrogen fertilizers has lowered soil pH statewide. Appropriate soil testing programs to monitor pH and liming practices to maintain pH in the appropriate range for the crops you grow is critical for high yields, effective chemical weed control, and long-term soil health and sustainability.
Dave Mengel, Soil Fertility Specialist
dmengel@ksu.edu
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