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Understanding the Recommendations
When using SWEP soil tests, many people find the recommendations a bit tricky. The notes here should help clear things up. For more information, you should also read about The Mikhail System.
SWEP is 100% independent, so we do not recommend specific products. We also test samples from across Australia and around the World, so even if we wanted to, we could not possibly keep up with all the products available in every area. For this reason, rather than listing fertilisers, we give recommendations in terms of the elemental nutrients as kilograms per hectare. These then need to be converted into appropriate fertiliser applications. Any reputable fertiliser supplier should be able to do this for you, but if you want to learn more about the process and perhaps do it yourself, you should download our Fact Sheet on Converting Nutrients to Fertiliser.
Trace element recommendations are also in this form - as kilograms of the elemental nutrient required per hectare, however, there may be more to consider in terms of applying trace elements so you should also read our notes on Applying Recommendations.
By far the most common causes of confusion, however, relate to the Cation balance recommendations. Importantly, these are for improving soil structure and friability - not plant nutrition - and should be applied separately from fertilisers. The following notes should help make things easier to sort out.
Calcium & Magnesium
The first items listed on the recommendations page are the kilograms of Calcium and Magnesium required per hectare. In most circumstances you will not need to worry about these figures, they are the ones used to calculate the amounts of Lime, Dolomite and Gypsum that may be needed. So long as you have access to good quality materials, you can simply make the recommended applications as given. However, we provide this basic information so that adjusted application rates can be calculated to suit specific materials if necessary.
Gypsum first:
The first step in determining what to apply is to see if the Sodium &/or Magnesium percentages are above their desirable levels. If this is the case we calculate the amount of gypsum that will be required to displace this excess with Calcium. This amount of Calcium is subtracted from the total requirement and the remainder is supplied as Lime.
Dolomite & Lime
If the Magnesium percentage is below the desirable level and exchangeable Hydrogen is high, then the required Magnesium is supplied as Dolomite and the remainder of the Calcium requirement is provided as Lime. In this way, it is possible to have recommendations for all three materials (for example in a soil high in Sodium and Hydrogen, but low in Calcium and Magnesium). The Gypsum is worked out first, then the Dolomite and then the Lime.
Other forms of Magnesium
Some soils can be very high in Calcium and low in Hydrogen, but still have a requirement for Magnesium. In these cases Dolomite is not appropriate, because it contains a significant amount of Calcium (which is not required in such soils). The only other economically feasible materials to use in adding Magnesium to the soil are Magnesium Sulphate and Magnesium Oxide, however, these materials are less efficient at exchanging Magnesium for the 'excess' Calcium. For this reason, the amounts recommended are usually much less than the total requirement. In managing such soils, it is better to apply smaller amounts over several years, than to apply the full requirement and get less than a satisfactory result. For these soil, persistence is the most important requirement.
Since we cannot recommend specific products, our recommendations for Lime Dolomite and Gypsum must be in terms of the generic materials only. However, the quality of these materials will vary from place to place and so our calculations are based on certain quality standards.
Pure Limestone will be 100% Calcium Carbonate (CaCO3), which in turn is 40% Calcium. When choosing a Lime product to use, make sure you do not get these two percentages confused. We often hear people say: "My supplier tells me his Lime is 65% Calcium". This is impossible, it is more likely that is is 65% Calcium Carbonate, which would make it only 26% Calcium! Lets say your recommendation was for one tonne of Lime per hectare. To provide the same amount of Calcium as pure CaCO3 with the material above, you would have to almost double the application rate.
Similarly, the Dolomite recommendation is based on a generic material containing 26% Magnesium Carbonate (which would make it 12% Magnesium).
The Gypsum recommendation is based on 'A' grade Gypsum which is nearly pure Calcium Sulphate (around 20% Calcium).
If you still have questions:
If you are still unsure about anything to do with out soil tests or recommendations, please contact us and we will be only too happy to help.
