FACTORS DETERMINING THE EFFICACY OF AGRICULTURAL LIME

South Africa has an abundance of agricultural lime sources distributed throughout the country which should enable farmers to source lime as economically as possible. Lime is classified as a Group 2 fertilizer and regulated by The Fertilizer, Farm Feeds and Agricultural Remedies Act of 1947 (Act 36 of 1947). Dolomitic and calcitic lime sources are used to ameliorate soil acidity, Al³⁺ and Mn²⁺ toxicities, raise soil pH and manage calcium and magnesium levels in the soil. The sources of lime may be of both natural and industrial origin and vary significantly in their chemical and physical properties which in turn will determine the efficacy the product being used.

Factors affecting efficacy of lime

In the article, Soil Acidity and its Management in Crop and Pasture Production; Miles and Farina indicate that the effectiveness of various liming materials varies widely, with the following factors being of particular importance:

1. Chemical purity ─ the presence or otherwise of non-reactive materials such as sand and clay greatly affects the neutralizing value of the lime (importantly, the colour of the liming material is not a reliable indicator of its quality!).
2. Chemical composition ─ the nature of the calcium and magnesium compounds present.
3. Fineness ─ the finer the lime particles, the faster will be their reaction in the soil. Lime particles larger than 0.84 mm in diameter (about the size of a match head) are of little value. Very coarse liming materials are completely ineffective.
4. Hardness ─ the solubility, and hence neutralizing value, of lime depends on whether it is derived from hard crystalline material or from softer relatively unconsolidated material.

Where uncertainty exists as to the quality of a particular liming material, they advise that a sample should be submitted for analysis.

The rate of chemical reaction

When lime is applied to the soil it reacts with the acidic components of the soil, H⁺, Al³⁺ and Mn²⁺ , the rate of chemical reaction is determined by temperature, surface area for reaction, relative concentration of the reactants and the presence of soil moisture.

Four factors determine the efficiency of lime:

• Rate of application
• Purity (CCE)
• Particle size distribution
• Degree of incorporation into soil

Chemical purity – Calcium Carbonate Equivalent (CCE)

The chemical composition of lime varies according to its geological or industrial origin. The term calcium carbonate equivalent (CCE) is a measure used to distinguish the neutralizing capacity of a lime source relative to the mass of pure calcium carbonate required to neutralize hydrochloric acid (HCl). The CCE of pure calcium carbonate is rated as 100%, pure magnesium carbonate has a lower molecular mass and as such less magnesium carbonate is required to neutralize the equivalent amount of HCl, the CCE of magnesium carbonate is 119%.

The CCE of lime will vary according to the calcium (Ca) and magnesium (Mg) content as well as other impurities such as sand and clay; large variances in CCE may exist between different sources of lime. The minimum CCE of lime in terms of Act 36 is 70%.

Fineness – Particle size

The CCE of a lime does not indicate how quickly it will react, particle size is a key factor in determining the rate of reaction within an acid soil; the finer the particles the greater the surface area for reaction.

The surface area for reaction of lime increases proportionately with decreasing particle size. Consider that the exposed surface area of a 1cm cube is 6cm², the corresponding comparative surface area of a 1.7 mm particle (maximum particle size in terms of Act 36 of 1947) is 35cm²cm^-3 compared to that of a 0.25 mm particle which is 240cm² cm^-3 ; and a 0.106 mm particle (a micro fine lime) which has a surface area of 566cm²cm^-3.

Consider that in terms of Act 36 of 1947:

• 100 % of a lime sample must pass through a 1.7 mm sieve, and
• 50% thereof must pass through a 0.25mm sieve.
• For a micro fine lime, 95% of the sample must pass through a 0.25mm sieve, and
• 80% thereof must pass through a 0.106mm sieve.

Now refer figure 1 below; research has shown that:

• Particles finer than 0.149 mm react within six months or less.
• Particles coarser than 0.149 mm but finer than 0.25 mm react within the first year of application.
• Particles coarser than 0.25 mm but finer than 0.841 mm, will achieve 50% reaction in first year of application.
• Particles coarser than 0.841 mm have limited liming value, will persist longer in the soil and continue to react with the acidity over a longer time period which could be several years.
  

  Figure 1: The effect of particle size on soil pH over a period of three years.

Figure 1 adapted from: The Pennsylvania State University: https://extension-ssl-45413.nexcesscdn.net/media/wysiwyg/extensions/catalog_product/9/f/24bc3614354dcdb5e9302f76844437/effect-of-aglime-fineness-on-speed-of-reaction.jpeg

Granulated micro fine lime & liquid lime

In recent years the use of granulated lime (also referred to as pelletized) and liquid lime have been recommended as an alternative to conventional lime, coupled with claims that these products are an effective economic alternative to conventional lime.

Granulated lime

As discussed particle size is of paramount importance in determining the efficacy of lime; the granulation of micro fine lime effectively reduces the surface area available for reaction to the surface area of the granule until it is broken down. Granule size ranges from between 0.841mm – 4.75 mm, considering that the granules are made of micro fine particles one would expect the lime to react very quickly once the granule has broken down.

The effective rate of reaction in the soil is determined by the rate at which the granule breaks down. Lignosulfonate, molasses and other agents are used to bind the micro fine particles into a granule; breakdown of the granule will be determined by dissolution of the binding agent by water and microbial activity before the full reactive surface area of the micro fine particles are exposed and available to react.

When lime granules are incorporated in the soil they are held firmly in place by the surrounding soil (as shown in photo 1 below) and the micro fine particles are restricted to a smaller volume of soil. The fourth factor determining efficacy of lime is the degree of incorporation in the soil. Greater efficiency would be achieved if micro fine lime was spread and thoroughly mixed in the soil.

Photo 1: J.G. Dreyer, Department of Geology and Soil Science, North-West University, Potchefstroom.http://www.grasland.co.za/.cm4all/iproc.php/Downloads/Report%20for%20Grasland%20on%20powder%20lime%20vs%20granular%20lime_Mrt17.pdf?cdp=a

Some benefits of granulated lime include:

• Ease and accuracy of application with fertilizer spreaders.
• It can be blended with fertilizers for row or broadcast application.
• Lower application rates are possible e.g. 250kg ha^-1.
• It may be useful for spot treatments of localized areas.
• It may be effective as surface application on pastures and horticultural crops.

However these benefits need to considered in light of the following limitations:

• The significantly higher cost compared to standard or micro fine lime.
• The reduced surface area for reaction.
• Granulated lime is best suited for maintenance application of the top few cm of soil.
• Low applications will not be effective through the entire soil profile

Liquid Lime

Liquid lime is sold as a suspension; typically the particles are finer than 0.149mm and mixed in water using clay to hold them in suspension. The benefits of using liquid lime include ease of handling, precise application and a fast reaction time due to the fineness of the lime particles relative to a standard lime.

Liquid lime has been recommended as at highly effective source of lime due to the reactivity of the micro fine particles, so much so that claims have been made that 10l of liquid lime will provide the same benefit as 2 tons of standard lime!

The same factors that determine the efficiency of standard lime determine apply to liquid lime; rate of application, CCE, particle size and degree of incorporation. Referring to Figure 1 above, particles finer than 0.149mm will completely react within six months of application, the increase in pH is short-lived due to the faster reaction time.

Empirical calculations that a low volume of liquid lime will significantly raise the soil pH are flawed. pH is a measure of H+ in the soil solution and doesn’t indicate the presence of Al³⁺ or Mn²⁺ or the exchangeable acidity of the soil. Any perceived increase in pH will be short lived due to chemical equilibria reactions within the soil as H⁺, Al³⁺ and Mn²⁺ cations will be released by the clay particles to take the place of those neutralized by the lime.

Movement of micro fine liquid lime down the soil profile will be determined by the soil texture and is only likely to take place in sandy soils with a clay content less than 15%.

Evaluating the effective relative cost of lime

Having taken consideration of the factors determining the efficacy of lime, consideration should be given to assessing the effective relative cost (ERC) and calculate which available sources will be most cost effective. The cost of lime includes, product price, transport, and handling / incorporation; the cheapest source of lime isn’t necessarily the best option.

HCl is a strong acid and doesn’t represent the acidity of the soil, as such CCE (HCl) is not necessarily the best indicator of the neutralizing capacity of a source of lime. An accepted alternative measure of the CCE of a lime is the Resin Method. The regulations of Act 36 prescribe that both the CCE (HCl) and CCE (resin) must be provided by the supplier.

The ERC cost can be calculated accordingly:

ERC = (Cost of Lime + Transport + Handling) ×(1/(R/100)) ; (R=CCE resin value)

The product with the lowest ERC is the most cost effective source.

An alternative method is to adjust the CCE (HCl) using an efficiency rating for the lime based on particle size distribution. Lime particle efficiency may be calculated accordingly:

% particles passing through a 0.25mm sieve: 100% efficient – A

% passing 0.841mm but not 0.25mm screen: 60% efficient – B

% passing 2.8mm but not 0.841 mm sieve: 20% efficient – C

% not passing 2.38 mm sieve 0% efficient – D

% Efficiency = (A X 1.0) + (B X 0.6) + (C X 0.2) + (D X 0)

R=CCE (HCl) x % Efficiency of particles

ERC = (Cost of Lime + Transport + Handling) × (1/(R/100))

Soil acidification is a natural process which is accelerated through the use of fertilizers and crop removal of base cations. The depletion of organic matter through crop removal and losses due to tillage contribute further to soil acidification, creating an overall hostile environment for crop roots which ultimately effects soil productivity.

Regular soil sampling enables the formulation of a strategic fertilizer and liming program which combined a soil organic matter conservation plan will ensure long term sustainable crop production and soil productivity.