Traditionally in New Zealand, agricultural lime (aglime) has been applied at heavy rates (typically 1-2 tonnes/ha) every 5 years or so, to avoid the soil pH dropping to below 5.5, below which aluminium (Al) toxicity to clover and most ryegrasses becomes increasingly likely. On the non-ash soils at least, it is recognized that an average of 150-250 kg aglime/ha/yr is required to maintain the soil pH over time. Responses to lime itself have rarely been measured above a soil pH of 5.8 on pasture.
In recent years, there has been renewed interest in applying finely ground lime at lower smaller rates (typically 50-200 kg/ha) annually or biennially. The advantages of fine lime include a much faster increase in soil pH. Some have criticised this practice, as the much higher cost of fine lime (typically $80-100/tonne compared to $30/t for aglime), and higher annualised spreading costs, generally means that the farmer is likely to apply less per year on average. Who is right?
They both are, to an extent. To understand the debate, we need to look at why lime came to be applied as it traditionally has been in New Zealand.
Particle Size Effects
It is widely recognised that lime, as well as needing to be good quality (ie, a high calcium carbonate content), needs to be quite fine to be effective in maintaining the soil pH within a desired range (say 5.6 to 5.8 ) over time. Particles above 0.5mm (500 microns) in diameter are unlikely to break down rapidly enough to achieve this. However, fine particles are difficult to spread in solid form from either spreading trucks or aircraft, particularly if not completely dry. In fact they can be very dangerous if they ‘bridge’ in an aircraft hopper when the aircraft is flying close to the ground, especially in hilly terrain. Too many New Zealand pilots have died this way.
The industry fixed the bridging risk by deliberately including a high proportion of coarse material, particularly for fixed-wing aircraft application, ensuring a reliable rate of discharge. However, this meant that the lime was typically only 50-75% as effective as it could be.
The traditional lime application to hill country meant that soil pH would rise slowly for 2-3 years after application, then start falling again, over a cycle of say 5.5 up to 5.8 and slowly back to 5.5 again.
It began to be noticed by some farmers that very small applications of fine lime appeared to be giving not just a much faster response to soil pH, but also a more significant boost to pasture production. The jury is still out on this one, but there is increasing anecdotal evidence that fine lime may be up to twice as effective as aglime per tonne in maintaining the soil pH. Even more importantly perhaps, is may be capable of boosting and maintaining soil biological activity at a higher level in the long-term. The initial kick is quite likely to be some sort of biological ‘priming’ due to stimulation of soil microbial activity. Annual reapplication can maintain the soil pH in a much ‘tighter’ and more desirable range, thereby benefiting both soil microbes and plant nutrition.
Methods to overcome the significant problem of the extreme dustiness of fine lime and associated difficulty in spreading include (a) applying it as a heavy fluid or ‘suspension’, (b) granulating it in a readily dispersable form, and (c) wetting the lime with a water spray as it leaves the aircraft. The later seems likely to be the most cost-effective, in our current view.