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Acidity and Liming

Soil acidity, measured by pH, is the most important chemical and economic property of Mississippi soils. It is often called the master variable because solubility and bioavailability of many essential nutrients is directly affected by pH. Soil microbes which control N and S availability also are affected by acidity. The solubility of aluminum and maganese, detrimental to plants when in excess, increases as pH values decrease.

Successful control of soil acidity depends on the quality and value of the liming materials used to increase pH. Relative neutralizing value (RNV) allows comparison of different lime products using standard product information. Quality of any agricultural liming material depends on:

  1. purity of the material
  2. the fineness of individual particles.

Purity is assessed by laboratory measurement of calcium carbonate equivalent (CCE). Finely ground limestone particles have more surface area available for reaction. The time required for dissolution of individual particles increases dramatically with relatively small increases in particle diameter.

Crushing limestone for the agricultural market is expensive and capital intensive. Some quarries specialize in other rock sizes that produce small by-product material that is coarser than normal agricultural limestone. These materials may handle easier and have better storage properties than finely ground agricultural liming materials. However, handling or storage properties are not quality indicators of liming material and have no function in soil chemistry and fertility.

Research has determined the neutralizing power of various size fractions. Particles that do not pass through 10 mesh sieves do not change soil pH. Half the particles finer than 10 mesh but, coarser than 60 mesh, and all particles finer than 60 mesh dissolve and react in the soil during relatively short time periods.

The CCE and fineness information are used to calculate an index of effectiveness for neutralizing soil acidity. This is called the Relative Neutralizing Value (RNV) or Effective Calcium Carbonate Equivalent (ECCE). Other methods are used by some universities to calculate this property.

Example calculation of relative neutralizing value:

Calcium Carbonate Equivalent, CCE 92
Percentage of material passing 10 mesh screen 94
Percentage of material passing 10 mesh screen 50
  • RNV= 0.92 X (((94 - 50)/2) + 50) = 66.2%

About 66 percent of this sample will be effective in neutralizing acidity in an acceptable time period. In Mississippi's warm and humid climate, this is about 3 to 4 years. Fields should be soil tested at least every 3 years to monitor acidity levels and nutrient status.

Relative neutralizing values can be used to adjust liming recommendations from soil testing laboratories for optimum application. They also allow price comparisons between available materials. Mississippi State University Soil Testing Laboratory recommendations are based on CCE of 100 percent, and have always required adjustment based on the material to be used. Other soil testing laboratories may base lime recommendations on assumed CCE's or RNV's of less than 100 percent. Always check the recommendation basis of any particular laboratory before making lime applications.

Lime is available from many sources in Mississippi. The RNV concept allows comparison before actual purchase. For instance, if two agricultural liming materials are available, one of RNV = 66 percent at $25 per ton, versus one with RNV of 85 percent at $30 per ton, which is the better buy? Dividing the price per ton by the RNV decimal value: (25/0.66) = $38, and (30/0.85) = $35, estimates the agronomic value of the materials. The material with the cheaper initial investment actually costs more to neutralize acidity when the properties of the lime are considered.

The regulations pursuant to the Mississippi Agricultural Liming Materials Act of 1993 were amended in 1997 to define two grades of limestone-based materials. This statute also requires vendors to provide sieve analysis data and CCE of liming materials sold in the state.

Table 1. Standards for Mississippi graded agricultural liming materials (percentages).

  Grade A Grade B
Calcium Carbonate Equivalent over 90 over 80
passing 10 mesh screen 90 80
passing 60 mesh screen over 50 over 30

Grade A lime meets the minimum standards in place before the regulation change. Grade B allows slightly coarser and less pure material to be registered with the Mississippi Department of Agriculture and Commerce.

The properties of these materials will be reflected in their agronomic utility. At the minimum values for each grade, the calculated RNV or ECCE for Mississippi Grade A lime is 63 percent, and for Grade B lime is 44 percent. Obviously, at these minimum levels, more Grade B product is required to neutralize acidity.

Research many years ago found the dissolution of large limestone particles is difficult to predict. Some large particle sizes require more than 20 years to dissolve. It is not good management to apply any material with the hope it will react some unknown day in the future.

The RNV concept emphasizes lime quality factors: CCE and fineness. Ease of transport, storage, or spreading does not indicate quality of agricultural limestone. Producers should obtain and use all legally required and available information about various liming materials in making lime purchase decisions.


Frequently Asked Questions

Why is pH written little p, big H?

The little p, big H is a convention used to show a logarithm is being used. The formal definition of pH is the negative logarithm of the hydrogen ion concentration in a solution. It is much easier to say, and write, pH.

Is there a perfect pH for all soil?

No. Desirable pH's depend on many factors such as texture, clay mineralogy, and the crops to be grown. Some horticultural crops actually thrive best in very acidic conditions! Agronomic crops vary in their ability to grow in acid soils.

When is the best time to apply lime?

When it is required on the basis of a reputable, accurate soil test, and the application equipment will not rut or compact the field. It is true that maximum efficiency is attained by liming 3 to 4 months prior to the next crop and incorporating the material. However, the critical acidic conditions of many Mississippi soils mandates we should not delay application waiting for an "ideal" situation. The very fine particles in liming materials will begin neutralizing acidity soon after soil application.



Soil Testing for the Farmer
For home horticulture: pH and Fertilizers
Soybeans -- Liming and Fertilization
Nitrogen in Mississippi Soils
Phosphorus in Mississippi Soils



Other Soil Information

Soil chemistry of lime application

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Publication Number: IS372
Publication Number: P1466
Publication Number: IS894
Publication Number: IS0871


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