Soil testing benefits returns from fertiliser investments

06 February 18

Soil testing technology is improving and researchers are reminding farmers that knowing what is going on in their soil can benefit their bottom line.

Farmers are being reminded of the benefits soil testing can have on their bottom line as recent GRDC surveys show soil testing rates have experienced a 50 per cent reduction compared to eight years ago.  This is despite the fact that fertilisers make up about a third of crop production costs for many growers, and new soil test methods on the market offering more accurate and more comprehensive information than ever before.

GRDC Manager Nutrition and Soils (South) Dr Stephen Loss says there was no one reason the rates of soil testing were decreasing, but rather a number of factors contributing to the decline.  GRDC Farm Practices surveys show that in 2008 around 40 per cent of cropped area in the southern region was tested for nutrient status, while in 2016 this was less than 15 per cent.  “There appears to be many reasons for this decline including issues with collecting soil samples in summer, confusion over different sampling and analysis methods, and a poor perception of the usefulness the results,” Dr Loss says.  “We know soil analysis is the best way to understand how much nutrient might be supplied to the crop from the soil and how much fertiliser is required to supplement the soil supply and achieve a yield target.

“In Victoria for example, we know from growers who are currently soil testing that about 15-20 per cent of samples had Colwell P levels above 80 milligrams of phosphorous per kilogram where soil supplies are high and savings in fertiliser are likely. In contrast, about 15 per cent of samples were below 20 mg P/kg where increased returns are possible from extra fertiliser applications.”  Improved soil testing methods have been released in the past decade including the DGT method for phosphorous, giving another option to the longstanding Colwell P test.  DGT is particularly helpful for the large parts of the southern region with highly calcareous soils, delivering more accurate results in terms of the soil P that is available to plants.

International Plant Nutrition Institute regional director Dr Rob Norton says many farmers have moved towards a phosphorus budget rather than relying on soil testing, ignoring the need for long term monitoring of soil phosphorus and other nutrient levels.  “Farmers think they know from previous testing, perhaps from a long time ago and from the paddock history, where their phosphorus levels are sitting,” he says.  “However, they still need to soil test for P to monitor the soil reserves. With phosphorus, there’s a bit of leeway in what isn’t used by the crop in the year the fertiliser is applied might be picked up in subsequent years, so there is a bit of flexibility with it. But this is rarely the case with nitrogen.  “The benefit from ongoing soil testing is by monitoring phosphorus inputs and removals to get a longer-term view of whether the nutrient status is heading up, down or static.”

Dr Norton says by budgeting for phosphorus and other nutrients, farmers are dismissing proactive measures and instead choosing to test “if something goes wrong”. “The research is pretty definite that Colwell P is recommended as it provides a reasonable assessment of response to phosphorus fertilisers for everything except high carbonate soils such as those on the Eyre Peninsula in South Australia,” he says.

Interpreting Colwell P values should also consider the soil’s phosphorus buffering index (PBI). The PBI is a measure of the soil’s ability to bind and release P for plant uptake. Soils with a high PBI require more applied P to achieve an increase in soil P test value because much of the applied nutrient is bound to the soil. Soils with a very low PBI need regular P applications because it readily leaches.

University of Adelaide professorial research fellow and GRDC Southern Panel deputy chair Dr Mike McLaughlin says soil testing should be thought of as having a fuel gauge in your car, in that only once you know what’s in your tank can you go for a smooth drive.  “It may be that our fuel gauge isn’t the best fuel gauge in the world, but it’s certainly better than just hopping in the car and without knowing what’s in the tank,” he says.  “You don’t need to soil test every paddock every year, but once every three to five years to make sure you’re maintaining P levels, while monitoring other soil characteristics such as levels of acidity and organic matter.  Dr McLaughlin says in many cases he would suggest both Colwell P and DGT testing on farm, as this would give a wider range of information to assess against critical values, particularly on calcareous soils.  “For nitrogen and other nutrients that change each season, you can’t just go on a budgeting approach, as you don’t know what’s lost and how much may be mineralised over summer and during the season.” 

“I don’t know the underlying reason why southern region farmers are turning away from having any soil testing done,” he says.

“Like having a fuel gauge, it is money well spent considering the magnitude of fertilizer investment by most growers.”

Source: GRDC | Author: Rachael Oxborrow | Date: 29 Jan 2018