Not too long ago there were a rash of fertiliser schemes that offered a "liming alternative" consisting of a few kilograms - say 20 or 30kg/ha - of product that was supposed to be as effective as 2,500 kg/ha of ordinary limestone. This seemed too good to be true! If it worked, all our present theories about soil acidity and liming are due for a massive revision.
I obtained a sample of one of these materials. A simple test to see what it might be was to mix it with water. Limestone maintains a pH of 8.5 in contact with water, while slaked lime will raise the pH to 13. A quick pH test soon showed that the pH was 8.5 - so the material was basically limestone, although very finely ground.
Could this work? I went and looked at the data left us by the late Dr Alan Sinclair, who, before he died left us with some excellent data on the relationships between liming and soil acidity.
Acidity is caused by the presence in soils of hydrogen ions . The amount of acidity present is measured by the pH index which runs from 0 to 14. pH levels less than 7 indicate acidity; the lower the number, the stronger the acid. A figure of 7 indicates a neutral condition - neither acid nor alkaline (alkaline is the opposite of acid, but beware - strong alkaline solutions can burn you just as effectively as strong acid solutions, so don't be fooled into thinking that strong alkaline solutions are safe). Once pH levels rise above 7, alkaline conditions are present, and the closer to 14 the pH measurement gets, the more alkaline conditions have become.
To grow good pasture we need a soil pH close to 6.0, or slightly acid. At lower or higher pH levels, pasture growth will be poorer than at pH 6.0.
If we use the weight of a hydrogen ion - one - as a starting point, we can rate a material like limestone by the weight needed to use up one unit of hydrogen. For pure limestone, we find that we need 50 units to use up, or neutralise, one unit of hydrogen. If we use a 90% pure material, we will need 56 units to do the same job, and for the lowest acceptable grade in New Zealand, 70% pure, we will need 71 units, or 1 1/4 times more than the 90% pure product.
Now, back to Dr Sinclair's work. To put these relationships into practical terms, he used a kilogram of hydrogen as the basis, so the amount of pure limestone needed to neutralise it would be 50kg.
When a soil becomes acid, most of the acid is absorbed and very little remains in the soil water. If the acid in the water is neutralised by adding limestone, the soil releases acid to replace it. Only when all the limestone has been used up by this process will the soil pH stop changing. Dr Sinclair's data showed that to change the pH of a soil from 5 to 6 (that is, to make the soil less acid) you need to put on enough lime to neutralise the equivalent of 100 to 150 kg of hydrogen per hectare.
This would mean the application of 5,000 to 7,500 kg of pure lime per hectare, 5,500 to 8,300 kg of a 90% pure product, or 7,100 to 10,700kg/ha of a 70% pure product.
Soil pH does not change quickly. The soil only releases acid from its reserves once the acid in the soil water has been used up. This acid is released from the soil organic matter and aluminium and iron oxides which release hydrogen ions to replace the ones neutralised in the soil water. As the process continues, more and more acidity is removed, so the soil pH increases. (Remember, the pH scale is "back to front", so a higher number means less acid than a lower number).
Now, if we put a lime into the soil that reacts very quickly, we will quickly use up all the acidity in the soil water, which will make the pH rise very quickly. Later, as the organic molecules and the iron and aluminium oxides adjust to the loss of acid from the soil water, and release acidity to replace the losses, the soil pH will go down again. If we use a limestone that reacts more slowly, we will not see the rapid rise in soil pH, but any rise we do see later on should last.
Another factor we have to take on board is that lime is insoluble. At 200C, only 0.0014 grams of pure limestone will dissolve in 100 ml of water. This means that all the reaction between the acid in the soil and the limestone takes place on the surface of the lime granules. If we grind our limestone really finely, we will get more surface area than if we grind it coarsely.
For example, if we had a gram of particles 1mm in diameter, the total surface area would be 11 square cm. If the particles were 0.1mm in diameter, the surface area of our gram of limestone would be 222 square cm. This is a more than 20-fold increase in area for reaction with the soil acids for a 10-fold decrease in particle diameter.
If we use finely-ground material, we will remove acidity much more quickly than if we use coarsely-ground limestone.
With very finely-ground material, and here we're talking about lime particles of less than 1/2 mm in diameter, the initial removal of acid from the soil water may happen faster than the soil can replace it. This could show up (as we discussed above) as a very quick rise in soil pH following the addition of the lime. Later on, measurements of soil pH will show it falling again, as the soil released acid to replace what was neutralised by the lime in the soil solution.
If coarser material is used, with particles ranging from 2mm down to less than 1/2 mm, the initial rise in pH will be slower, but it will not be followed by a fall in pH. If we were to wait five or six months after application before measuring pH, it is likely that, if the same weight of lime had been applied, there would be very little difference in soil pH between areas topdressed with coarser or finer lime particles. This would be because the same amount of acid had been removed from the soil by both forms of lime.
What it comes down to is that the same amount of acid is removed from the soil by the same weight of lime, whether it is finely or coarsely ground.
We can't use just any particle sizes, though. We do need to use a reasonably fine material. The minimum fineness standard is that less than 5% (by weight) of the material will fail to pass a 2 mm sieve, while more than 50% will pass through a 1/2 mm (500 micron) sieve. The lime should also be at least 70% pure. This standard was set in the 1950's by the then Department of Agriculture to make sure that all material sold as lime and eligible for a transport subsidy would react quickly enough to be of practical use.
Armed with all this knowledge, lets return to our "lime alternative".
We have found out that it is a very finely ground form of limestone.
We have also discovered that finely-ground limestone can remove acid from the soil water faster than the soil can release it into the water, but coarser lime will do the same job a little more slowly. After a few months the overall result will be the same.
From here, the conclusion is obvious: the small weights of very finely-ground material may APPEAR to do more than heavier weights of ordinary limestone if we measure soil pH very soon after application, but the differences will disappear soon after. Not only that, but the heavier rate of coarser material (provided it is at least as fine as the old NZ standard) will continue to neutralise soil acid and keep the soil pH going up. However, the small amount of finer material will have no more effect and the soil pH will very quickly regain its old, low, value.
