Fertiliser, often a farms largest annual expense, is usually based around a soil test and a Fertiliser company’s recommendation. Over the coming weeks I’ll try to make soil tests understandable. Most farmers, that I speak to don’t understand their soil test results and rely on someone to tell them how to spend their money.
Because it is such a large part of farm expenditure, I believe that farmers should have a basic understanding of their soil tests. Remember its “your soil test”, “your farm” and “your money”, you should be able to look at any fertiliser recommendation and decide “Does it makes sense”? I’ll cover the most common tests and not the dozens that are available on request. Detailed scientific explanations are available on Google etc.
First it’s important to realise that because of the variables that are involved in sampling, seasonal changes and Laboratory accuracies, soil test results are perhaps closer to an indication rather than an exact result. If possible it’s important to look at test results over a number of years rather than just a single test result in isolation. As soil conditions have a significant effect on soil test results, taking samples at the same time of year is one way you can help reduce the variations. If possible avoid cold, very wet or very hot and dry conditions. I suggest that you have one of your soil test reports to refer to as we proceed.
Laboratory reports will usually show the test name, the level found, an optimum level as well as bar graphs. The bar graph is the “level found” graphed against the “optimum level”. This optimum level is “someone’s” opinion and therefore determines the length of the bar graph. This opinion may be that of the Laboratory or it may be a Fertiliser companies. I’m not suggesting that either is wrong. Over time you may develop your own opinions and ignore the bar graphs.
- pH is an abbreviation. The small ‘p’ is for ‘potential’ and although not absolutely correct you can think of this as ‘percentage’, and the capital ‘H’ is for Hydrogen which is acidic. When plants remove alkaline minerals such as Calcium and Magnesium from the soil,they swap, (exchange), them for acidic Hydrogen. The more Hydrogen that plants leave in the soil the more acidic it becomes. Laboratories report pH on a scale where 1 is very acidic, 14 is very alkaline and 7 is neutral. Different crops prefer different pH levels. I believe that pastoral farmers should be aiming towards a pH of 6.0 or above.
- Olsen P is one of many available soil tests that measure soil Phosphorous, (P). It measures P that is present in the soil solution and that is weakly bonded to soil particles and is often regarded as the readily plant available portion of P and is usually less than 5% of the soils total P.
- Resin P is sometimes regarded as a more relevant test than Olsen P where there has been a history of slow release Phosphate applications such as RPR. Because the Resin P test extracts the Phosphorous at the same pH as the soil sample, it is also considered to have a better correlation to pasture growth than Olsen P that extracts the Phosphate at pH 8.5, which is much higher than any of our NZ soils.
- Total P, this includes the P in, (a) the soil solution, (b) the soil organic matter, plus, (c) the P that is bonded to other minerals. It is not unusual to see total P levels of more than 1.5 tons/ha in just the top 150mm of the soil. While there is debate, and many strong opinions, as to how much of this total P is accessed by plants, there is no argument that P moves between the various forms. Olsen and Resin P test results are a snapshot at the moment the sample was taken and results can vary by up to 20%.
- Anion Storage Capacity (ASC) previously known as P Retention. This is an indication of the soils ability to immobilise Phosphate. High ASC soils require higher application rates of Phosphorous to move Olsen and Resin P test results than with low ASC soils. As this is a soil characteristic and doesn’t change over time, this test is usually only required once.
To be continued…
Andrew de Lautour. PFP Fertilisers Ltd.©
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