DR JJ BORNMAN, GENERAL MANAGER STRATEGIC AGRICULTURAL SERVICES, OMNIA FERTILIZER
For many reasons, South African agricultural soils are acidifying at an alarming rate. The rate of acidification (without liming) under intensive grain production in the central areas, is close to one ton of pure calcitic lime per annum.
In recent articles published in SA Graan/Grain, the author has also pointed out the fact of subsoil acidification in these areas that is increasing exponentially. Under current conditions it is most important that soils are appropriately limed to ensure sustainable economic production and to optimise water and nutrient use efficiency.
South African agriculture simply cannot afford further deterioration of soils due to acidification; especially not if it is due to the misuse and misunderstanding of simple concepts such as efficiency and quantity. Unfortunately the spreading of said disinformation is currently causing confusion among producers and could lead to serious financial losses.
In this short article the author hopes to explain the concepts of lime quality, efficiency and required quantity in technical terms, while keeping it as simple as possible to help producers make sound decisions regarding liming material and levels of application.
Pure calcitic lime (calcium carbonate) dissolves and hydrolyses in water to form an alkaline solution able to neutralise detrimental acids, turning it into harmless water. After doing all the scientific calculations, it is a proven fact that 50 grams of pure 100% reactive calcitic lime can neutralise exactly 1 gram of acid (hydrogen ions).
Pure magnesium carbonate or better known as magnesite, reacts similar to calcite in soil, but the amount that is needed to neutralise one gram of acid is much less, as its molecular weight is less. In fact only 42 grams of pure magnesite is needed to neutralise 1 gram of acid. Pure oxides of calcium and magnesium (“burnt lime”) are much lighter than carbonates, but they are also strong alkali. Pure calcium oxide for instance weighs approximately half of calcite per reactive unit and only 28 grams of this material are able to neutralise 1 gram of acid. 20 grams of magnesium oxide neutralises 1 gram of acid.
From the above, different theoretical efficiency values of liming materials may be calculated. Magnesite, calcium oxide and magnesium oxide thus have a relative potential neutralisation capability per unit mass of product relative to pure calcite of 119%, 178% and 250% respectively. Calcium hydroxide (hydrated calcium oxide) has a relative neutralisation efficiency to calcite of 135%.
Unfortunately, absolutely pure forms of the materials mentioned above are seldom encountered in nature. Liming materials are usually found in nature and commerce as impure precipitates and crystals of different hardness, with much lower efficiency levels than the theoretical. These materials are also often mixtures of base forms. For example dolomitic lime that consists of magnesium and calcium carbonate.
Lime inherently has a very low solubility, therefore, in spite of having a large potential neutralisation rate, its reactivity is very slow, if its contact surface with water and soil is not enhanced. It is therefore of absolute importance that liming materials are milled to specified fineness levels to ensure realistic reactivity.
All in all then, the quality (efficiency and reactivity) of lime is determined by its chemical purity, crystal structure and most important, fineness. It must however be realised that fineness cannot improve the efficiency to more than the theoretical values calculated above.
In practice, the quality or reactivity of lime is reported as a percentage relative to pure calcitic lime, the so called calcium carbonate equivalent (CCE) value. Different methods are used to determine the CCE value. Some determine the total potential reactivity regardless of time of reaction (reaction in strong, hot acids) while others simulate relative reactivity in soil over a period of three months (resin suspension method).
Some liming materials are reaching a high level of sophistication. Some companies currently offer finely milled lime down to nanometer level in suspension, claiming very high efficiency levels.
As always, cost follows quality, but real efficiency and quality needs to be carefully taken into account when making final calculations.
A light textured soil with an effective cation exchange capacity (CEC) of 5 centi-mole charge per kg (milli-equivalents per 100 gram soil) and with an acid saturation percentage of 20%, will have one milli-equivalent of acid to neutralise in each 100 g of soil.
In a hectare of light textured soil at a depth of 30 cm deep, there are 4,5 million kg of soil. This means that
45 million milli-equivalents (milligram) of acid need to be neutralised. This translates to 45 kg of acid per hectare. As calculated above, 50 g pure calcitic lime is needed to neutralise one mole or equivalent or gram of acid. So 45 kg of acid will require 2 250 kg pure calcite to neutralise. A soil with a CEC of 10 and the same acid saturation, will need double the amount of pure calcite.
If a 100% calcium oxide nano suspension (absolutely pure and reactive) is used, one will still need 2,25 ton/1,78 (higher efficiency) = 1,26 tons lime per ha or 1 260 kg/1,3 kg/litre (reported density) = 969 litres per ha.
Even if pure magnesium oxide is used, one would still need 0,9 tons or 692 litres. Usually, the suspension products contain low concentrations of actual lime and that will elevate the mass requirement dramatically. A 15% calcium oxide suspension will require 100/15 x 969 litres = 6 460 litres per hectare using the same reported density.
The fact remains, that no matter how pure or reactive (fine) one can get any natural liming material, one cannot exceed an efficiency of 2,5 times that of pure calcitic lime. Furthermore, there simply is a huge mass of pure acid in a hectare of acidic soil and one accordingly needs a pro-rata high mass of lime material to neutralise it.
Movement of lime
Lime in any form, micro or nano fine, powder or suspension, will only move in a coarse textured matrix with no reactive sites. Soil is simply not that. Soil contains reactive clay and minerals that will react with lime on contact, especially when it is fine and reactive. For that reason, one needs to be careful of overemphasized claims of mobility of any liming product in natural soil.
The ability of a lime material to neutralise acid in the soil, depends on the efficiency and quality of the material and the quantity applied must be sufficient to neutralise the actual quantity of acid in the soil. Even when the best product in terms of neutralisation capacity (efficiency and quality) is applied in too small a quantity, the result will be totally insufficient and uneconomical.
The hope is expressed that this article gives some perspective and will save the producer money and reduce associated risk when making a choice of liming material and the quantity of application thereof.
Bornman, J.J., Coetzee, J.E.K. & Van Vuuren, J.A.J. 1988. Inter-laboratory study with regard to the variability of three aglime analysis methods, including a resin suspension method. Comm. Soil Plant Anal. 19 (3) 299 - 310.