Inputs/Production
40
Liming requirement;
misunderstanding the difference between
efficiency and quantity concepts
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.
Efficiency
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%.
Quality
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 equi-
valent (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.
