Understanding Industry Lime Standards
Current industry assessments focus primarily on chemical purity of the lime source, expressed as NV or neutralising value and particle sizing. This is determined by an arbitrary 5 sieve range performed on oven dried samples.
What This Tells Us
Neutralising value is an excellent measure of the acid neutralising potential of a lime source. However, it tells us very little about the reactivity of a lime source when applied to the soil, or the capacity for the lime to move into contact with sub-soil acid.
If a lime is slow acting or unable to come into contact with acid, the return on investment for the grower is slow and limited – unless they make a costly mechanical intervention to access more of the neutralising capacity of their applied lime.
WA Lime Standards Approach
Particle size testing, in Western Australia, is intended to provide information to consumers regarding speed of action. Modelling in lime evaluation tools commonly used in WA ascribe an ‘efficiency’ value of 100% to lime particles under 500 microns (0.5mm).
The Nationally Recognised Approach
Compared to the WA standards, there is a large discrepancy in assessing the value of lime (as described by Western Australian Soil Acidity Research and Development Update 2001).
CSIRO only ascribe a 100% efficiency value to lime particles below 75um (0.075mm) with particles larger than this dropping to 58% efficiency and continuing to fall.
Why are only these very fine fractions of lime particles considered 100% efficient by CSIRO? They believe a) the extremely high surface area of these small particles is necessary for 100% efficient neutralisation to take place and b) the ultra-fine particle sizes will leach more readily into the soil profile and reach acidity deeper in the soil profile, even if it lays under an alkaline surface soil.
What To Look Out For in WA Lime Sand?
Investigation of various lime sand resources from Dongara to Albany reveals a very low percentage of particles under 75um (typically around 2.5% or less).
The WA standard doesn’t adequately define particles in this range as the smallest sieve size is 125um. So, it’s important that your lime supplier can identify this critical measure for you if you are requiring your lime to leach to depth with only the action of rainfall.
If permeation through the soil and speed of action are important to you, consideration should be given alternative liming products.
So, Is WA Lime Sand Effective?
Let’s be very clear. WA lime sand is an excellent pH adjustment tool when it is used in the appropriate way. It has great power to change the soil pH, but it is not necessarily the best lime product to use in every situation.
So What Else Should You Consider In Your Choice of Lime Product?
The decision to lime your soil to reduce soil acidity is not taken in isolation. You should be conscious of the location of the acid in your soil profile; the impact of spreading and incorporating other soil ameliorants; and thinking about mechanical intervention for structural soil problems.
Considering all factors allows for the appropriate weighting to be applied to the physical properties of the lime source under assessment – NV, particle sizing and solubility.
Making a decision within this framework will allow you to determine the most cost-effective lime source to be utilised for the current application.
Changes Over Time
It is important to understand that liming causes changes in the soil and that subsequent application of limes may call for a different approach.
What works over one period of time, might not work as well over the next period of time. In addition, your objectives, systems and requirements might change too.
