One of many visits in a recent Groups Together tour took us to Clogrennane Lime Ltd in Co Carlow. This is a limestone quarry with more than 200 years of activity but is different to a standard limestone quarry and services many markets outside of agriculture.

Lime from Clogrennane is processed or burnt limestone and it trades under the White Rhino brand. This converts it from calcium carbonate (limestone) to calcium oxide (quicklime). This conversion results in a more concentrated and fast-acting product.

Clogrennane starts with a relatively pure source of limestone rock which is then quarried.

Standard ground limestone involves taking the rock and grinding it mechanically into particle sizes that can be weathered by climate and acidity to free up the calcium from the CaCO3.

The finer the limestone rock is ground, the quicker the calcium can be freed up to neutralise the negative ions in the soil and increase pH.

The specification for ground limestone requires one-third of the sample to be ground to a level capable of working in the first year of application.

The remainder of the ground limestone is of a size that will degrade to do its job in years two to four following application.

Particle size is the main way ground limestone has a long-term effect on soil pH.

Based on the specification provided for burnt or white lime, it is about 1.7 times more effective at increasing soil pH than finely ground limestone. Clogrennane burnt lime is sold as Growmax.

This is a fast-acting quicklime, which is mainly calcium oxide (CaO rather than CaCO3). It contains 68% calcium rather than 38% in a typical ground limestone and it reacts with moisture to help increase soil pH.

So what is white lime?

Clogrennane, a CRH company, has produced kiln or burnt limestone for the past 200 plus years.

It was always regarded as a highly effective product because it produces rapid improvements in soil pH and this helps the efficacy of all other nutrients, whether applied or in the soil.

Soil pH is important for growth because it directly affects availability and uptake of nutrients, whether they be in the soil or applied through fertiliser.

Essential nutrients are poorly available at lower pH levels (see Table 1) but even at pH6, expensive phosphorous is said to be only 52% available to support plant growth.

Growmax

It is useful for farmers to understand what exactly Growmax is and how it is produced.

It starts with the same limestone rock source which is blasted and crushed to kiln-feed size (40mm-110mm) and screened to remove large lumps (which are re-crushed) and the undersize, which are sold for other uses.

The kiln feed is then put into a modern-style kiln which heats it to about 1,000°C.

This splits the CaCO3 to produce CaO and carbon dioxide (CO2). This process is fuelled by natural gas and petcoke.

The removal of the CO2 leaves a more concentrated form of calcium which has a higher total neutralising value (TNV) or a higher calcium carbonate equivalent.

The smaller active molecule means it has a higher calcium density per tonne, with more molecules per unit.

The kilning process increases the calcium concentration in the lime by a factor of about 1.7. So 1kg of CaO is equivalent to about 1.7 times the amount of calcium in CaCO3.

Total neutralising value

TNV is a measure of the potential of a material to neutralise soil acidity and raise pH level to optimise plant growth.

It is expressed as calcium carbonate equivalent and standard ground limestone is said to have a TNV of about 35%.

This is because 65% of the product is not yet fine enough to have neutralising value but this becomes available in time.

Granulated limestone products have a TNV of 95-98% because they are ground finely enough to be available post application. But this is still CaCO3 and the TNV of Growmax (calcium oxide CaO) is 160-170%, hence the 1.7 times higher calcium content.

Growmax might be described as having an angular to dusty consistency following the kilning process. It is not a fine product and so it is said to be spreadable accurately at up to 24m with suitable spreaders.

Its rapid activity means it is much faster acting than ground limestone, which means it delivers visible growth improvements on low pH soils in the year of application.

This is the reason why burnt lime was often referred to in the past as quicklime.

Cost

While there is less grinding cost compared with ground limestone, the energy requirement makes quicklime a more expensive product per tonne to produce.

But this cost, adjusted for liming ability, works out at roughly the same cost as ground limestone spread.

But the longevity of quicklime is said to be around two rather than 3-4 years for ground limestone and so it needs to be applied every two years or so.

Clogrennane agronomist Colm Dempsey told us the pH of burnt lime is around 12.5 compared with 8.1 for ground limestone products.

He said the activation of ground limestone to increase soil pH is more efficient where soils are more acidic. But as levels get closer to pH 7 (neutral) ground limestone may be less efficient at getting all the way up to this level for crops such as beans.

In such situations, Growmax may have a particular advantage for pH improvement.

Colm warned that Growmax contains dust and so the spreaders should be washed immediately after application. It can be spread through a standard fertiliser spreader so farmers have control over application timing.

He also warned that it should not be spread during damp/wet conditions as the dust will stick to and consolidate on machinery.

He also advised that it should not be spread in a mix with other fertilisers.

The rough cost of Growmax is €180/t and its concentrated formulation means that much less is required on a per acre basis.

It is a bagged product and generally sold through the merchant trade. Colm suggested the following rates for different soil pH levels to provide a guide on cost. It is supplied in 375kg bags with four per pallet, making 1.5t.

At these cost levels it compares well with ground limestone, especially for anyone who needs an immediate pH correction and for those who need to keep soil pH levels close to 7.

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