Tests show some limes are lemons

Particle size was the main concern, as it affects a lime’s impact on reactivity time to remove soil acidity constraints. Four samples had more than 10 per cent of their lime above 5mm particle sizes. Two of these had high moisture contents, so particles had clumped together after sampling in winter. The other two were dry and reflected poor screening. Particles of this size could take 20 years to breakdown and result in poor pH change.

In comparison, NSW superfine lime where the majority of the lime is less than 0.07mm reportedly breaks down within eight weeks.

Course limes are slowed down, as only the outside dissolves which is in contact with the acid soil. The rate of lime breakdown slows considerably once surrounding soil pH increases and stops when the soil becomes alkaline. The remaining lime will only break down further once the soil acidifies.

This has repercussions, especially for surface applied lime. It doesn’t create as much alkalinity to leach down the soil profile and so acid layers can form around 10cm. Also, farmers applying lime in the year before sowing an acid sensitive crop like lucerne or faba bean can have reduced yields caused by untreated acidity at lower depths. Farmers who need their lime to create maximum pH change quickly need to check particle size distribution. Ideally, the lime should contain 50 per cent less than 0.3mm and very little above 1mm.

An Effective Neutralising Value calculation historically used in Victoria to compare limes was found to be ineffective. An ENV calculation would rate particle sizes of 5mm, the same reactiveness as particle sizes of 1mm.

Softer rock limes have increased solubility compared to hard rock – creating 20 per cent more pH change in the first 12 months compared to hard rock limes with the same particle size distributions. However, this is no reason to ignore particle size of softer limes. The degree of fineness is important for all lime types.