SoilMate NutriFact CEL-08

Transcription

1 SoilMate NutriFact CEL-08 STEPS TO SELECT COST-EFFECTIVE LIME The cost-effectiveness of any lime depends on neutralising value, fineness, lime price, transport charges and spreading costs. The relative costs of different lime sources can be easily compared using these five pieces of information. The capacity of a lime to neutralise soil acidity and raise soil ph is governed by its neutralising value (or in simple terms, its purity). The rate of chemical reaction of a lime increases with the fineness of its particles. and particle size are the two main factors that influence the rate at which a lime is dissolved by acid in soils and the extent to which a lime can neutralise acidity once it has dissolved. Relative to neutralising value and particle size, the type of carbonate mineral in a lime has a minor influence on effectiveness. Although aragonite is slightly more soluble than calcite, any difference is negligible compared to the effects of neutralising value and fineness. Limes are natural, variable products. is a measure of the lime s ability to neutralise acidity relative to the mass of a standard material, which is usually pure calcium carbonate that has a designated neutralising value of 100%. At a given fineness, the higher a lime s neutralising value the better it will be for neutralising acidity. varies significantly with the type and source of lime so it is important that growers check and compare neutralising values of limes before buying. Limestone (calcium carbonate) and dolomite (calcium carbonate and magnesium carbonate) are the main naturally occurring minerals used to neutralise 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. In Western Australia, coastal limestone sands (commonly called limesands), usually of high neutralising value, can be extracted, transported and spread onto paddocks with minimal, if any, processing. Outside WA, calcite limestones tends to be the main form of calcium carbonate but unlike limesands, it needs considerable processing and milling to a fine particle size to make it suitable for spreading onto paddocks. Fineness At a given neutralising value, the finer a lime the faster it will be for neutralising acidity. There is more contact between lime and soil with finer particles so reactions are more rapid.

2 Finer lime is better for two reasons. First, a finer lime has more particles, hence larger coverage when it is spread across the paddock. Second, finer particles have a larger surface area so there is more chemical reaction between dissolving lime and 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. This is emphasised with the figure below showing how much lime of different particle sizes is required to cover one hectare by topdressing. Although the figure unrealistically assumes homogeneous particle size within lime it highlights the greater coverage and efficiency of finer limes. But like neutralising value, fineness alone does not fully describe the effectiveness of a lime to neutralise acidity. The amount of lime required to cover one hectare increases with lime particle size. Both neutralising value and fineness need to be considered to assess the agronomic efficiency of limes and in combination are often referred to as Effective Neutralising Value (ENV). ENV is combined with the costs associated with buying and applying lime to determine the cost-effectiveness of any lime, and therefore allow comparisons between lime sources. Cost of limes The best way to evaluate the real cost of a lime is to calculate its cost per tonne of 100 per cent neutralising value spread on the paddock. For this, specifications are required on the neutralising value of each particle size range of the lime. In WA particle size is commonly measured as the percentage by weight of the lime within five particle size ranges. Costs of lime, transport and spreading are also required to estimate overall cost.

3 The calculations shown below estimate the cost per spread tonne of 100% neutralising value. Because lime particles greater than one half millimetre take so long to dissolve they are generally considered less effective and are discounted accordingly. within range Less than = 8 / 100 x 88 = = 38 / 100 x 82 = = 36 / 100 x 87 = = 0.5 x 15 / 100 x 74 = 5.6 Greater than = 0.25 x 3 / 100 x 80 = 0.6 Sum of neutralizing value within particle size ranges = = 75.7 Cost of lime ($/t) 8.50 Cost of transport ($/t) Cost of lime spread on paddock ($/t) = Cost of 100% neutralizing value spread on paddock ($/t) = Cost of lime spread on paddock / Sum of neutralizing value within particle size ranges x 100 = / 75.7 x 100 = The effectiveness of particles from 0.5 1mm are about as half as effective as particles less than 0.5mm. Similarly particles greater than 1mm are only half as effective as those between 0.5mm and 1mm. For lime to be cost-effective, it needs to have a large portion of its particles less than 0.5mm. By using calculations like those above different lime sources can be compared. For instance the comparison on the following page 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 higher value because it is finer and the neutralising values of the finer particles are higher. Because transport costs are such a large component of cost per spread tonne of 100% neutralising value, an inferior product can be more cost-effective if it does not have to be transported so far. In the example on the following page, Lime A is a better product and cheaper than Lime C but because Lime A has to be transported further it is cheaper to use Lime C. Every situation is different, especially transport costs, so it is important to do the calculations to source the most appropriate lime. Always soil test Just as important as quality and cost is knowing where to apply lime. Highest returns are achieved by applying lime to soils of lowest ph that will respond most to lime applications.

4 Soil tests, both of topsoils and subsoils, are the best way to determine where to apply lime and at what rate. The best and cheapest lime is a poor investment if applied where it is not required. Similarly, lost production by not applying lime where it is required can be very costly. Without soil tests growers are guessing. Lime A within range Less than Greater than Sum of neutralizing value within particle size ranges 86.3 Cost of lime ($/t) 6.00 Cost of transport ($/t) Cost of lime spread on paddock ($/t) Cost of 100% neutralizing value spread on paddock ($/t) = Lime B within range Less than Greater than Sum of neutralizing value within particle size ranges 50.6 Cost of lime ($/t) Cost of transport ($/t) Cost of lime spread on paddock ($/t) Cost of 100% neutralizing value spread on paddock ($/t) = 67.19

5 Lime C within range Less than Greater than Sum of neutralizing value within particle size ranges 71.6 Cost of lime ($/t) 7.00 Cost of transport ($/t) Cost of lime spread on paddock ($/t) Cost of 100% neutralizing value spread on paddock ($/t) = 34.92