Back Paddock has been providing independent soil testing and interpretation service for over ten years.

One of the most noticeable in recent years has been the increasing % of tests conducted in central, southern NSW, Vic, and SA with pH low enough to consider liming to prevent yield loss and advancing acidity to deeper in the soil. survey of samples conducted through our services indicates up to 40 % of samples take for winter crops and autumn sown pasture across this region from 2015 to 2017 had pH (CaCl2) less than 5.5 and 20 % less than 4.5. As the questions “How to choose lime’ and what rate should I look at usually follow?”

How to choose a lime source?

The capacity of any lime to neutralize soil acidity and raise soil pH is governed by its neutralizing value (NV) and its fineness.  These two factors influence the rate at which is dissolved by acid in soils and the extent to which can neutralize acidity once it has dissolved.

The of any lime depends on NV, fineness, cost of lime, cost of transport and cost of spreading.  The relative costs of different lime sources can be easily compared using these five pieces of information, as will be described in detail later.

Neutralizing value

Limes are natural, and therefore variable products.  NV is a measure of the lime’s ability to neutralize acidity relative to the mass of a standard material, which is usually pure calcium carbonate.  At a given fineness, the higher a lime’s NV the better it will be for neutralizing acidity.  NV varies hugely with the type and source of lime so it is important that growers check and compare NV’s of limes before buying.

Limestone (calcium carbonate) and dolomite (magnesium carbonate) are the main naturally occurring minerals used to neutralize soil acidity.  Less commonly used liming materials include magnesite (magnesium carbonate), quick or burnt lime (calcium oxide), magnesia (magnesium oxide) and hydrated or slaked lime (calcium hydroxide).  Some of these are waste products from manufacturing processes.

Calcium carbonate is the most common liming material.  Western Australia is fortunate in that coastal limestone sands (commonly called ), usually of high NV, can be extracted, transported and spread onto paddocks with minimal, if any processing.  In other calcite tends to be the main form of calcium carbonate, but it needs considerable processing and milling to make it suitable for spreading onto paddocks.

Fineness

At a given NV, the finer a lime the better it will be for neutralizing acidity.  There is more contact between lime and soil with finer particles so reactions are more rapid and there is more neutralization of acidity.

Finer lime is better for two reasons.  Firstly a finer lime has more particles, hence greater coverage when it is spread across the paddock.  Secondly finer particles have a greater surface area so there is more chemical reaction between dissolving lime, hydrogen ions (the acidity) in the soil solution.

Limes usually consist of a range of particle sizes so growers need to check the relative proportions of lime in each particle size range.  Measurements of minimum particle size, and even of average particle size, fail to describe a lime’s potential for chemical reaction or its coverage across a paddock.

Figure 1 emphasizes the importance of particle size by showing how much lime of different particle sizes is required to cover one hectare by topdressing.   Although the figure unrealistically assumes homogenous particle size within lime it highlights the greater coverage and efficiency of limes.

But like NV, fineness alone does not fully describe the effectiveness of to neutralize acidity.  The two measurements need to be considered together to assess efficiency.  The combination of NV and fineness is called the effective neutralizing value (ENV).  This efficiency to neutralize soil acidity then needs to be combined with the ex-pit, transport and application costs to assess the adequacy of any lime.

Cost of limes

The best way to evaluate the real cost of is to calculate its cost per tonne of 100% NV spread on the paddock.  In order to do are required on the NV of each particle size range of the lime to calculate ENV.  Particle size is commonly measured as the percentage by weight of the lime within five particle size ranges (the particle size ranges vary between states).  Costs of lime, transport spreading are also required to estimate cost.

The calculations in Table 1 estimate the cost per spread tonne of 100% NV.  Because lime particles greater than 1 mm take so long to dissolve they are generally considered ineffective and are discounted accordingly.  The effectiveness of particles from 0.5 – 1 mm about as half as effective as finer particles.  It is therefore imperative that for to be it needs to have a large portion of its particles less than 0.5 mm.

Table 1.  Calculations for lime to estimate the cost per tonne of 100% NV spread on the paddock.

By using calculations like those in Table 1 different lime sources can be compared.  For instance Table 2 highlights that Lime A is better value than Lime B despite the on-paddock cost of both lime products being the same.  Lime A is of greater value because it is finer and the NV’s of the finer particles are higher.

Because transport costs are such a large component of cost per spread tonne of 100% NV, an inferior product can be more if it doesn’t have to be transported so far.  In Table 3, Lime A is a better product and cheaper than Lime C but because Lime A has to be carted further it is cheaper to use Lime C.

Every situation is different, especially transport costs, so it is important to do the calculations above (or get someone else to do them for you!) if you are genuinely trying to source the most appropriate lime for your farm.

Table 2.  comparing limes with different specifications and costs.

Always soil test

Equally important as quality and cost knowing where to apply lime.  Highest returns are achieved by applying lime to soils of lowest pH that will respond most to lime applications.  Soil tests, both of topsoils and subsoils (generally 10-20 cm and in some cases 20 – 30 cm), are the best way to determine where to apply lime and at what rate.  The best and cheapest lime is a poor investment if it is applied where it isn’t required.  Similarly lost production by not applying lime where it is required can be very costly.  As with any investment, lime should be applied where it is likely to achieve greatest returns.  Without growers are guessing with their lime investment.

The rate of lime required is a function of soil properties (pH and pH buffering), treatment depth, and target pH and lime quality. See the article about SoilMate online tools in this edition, the LimeMate calculator is a place to go to explore required and value of various lime sources.

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