Plant & Food Research has been assessing the suitability of just such a test and last month relayed its findings at the Foundation for Arable Research conference, Palmerston North.
The problem with current tests is that typically 90% or more of the total soil nitrogen isn’t immediately available because it is bound in organic matter, and what is plant available, as indicated by a mineralised nitrogen test, can change rapidly because leaching reduces plant-available nitrogen (mineralisation) from the organic nitrogen pool adding to it, explained Plant & Food’s Matthew Norris.
“The numbers you get from the test are not the same as they are [at the time of] side dressing.”
Current best practice is to sample and test for total soil nitrogen and mineralised nitrogen at the start of the growing season and then plan fertiliser requirements and timings from those using experience and crop modelling packages.
However, a growing number of nitrogen quick tests are available which use a colour response on a test strip card to indicate the nitrogen concentration of a soil and water solution.
Once soil samples have been taken from the field following usual protocols according to crop or pasture situation, a sub-sample is mixed with a calcium chloride solution, then allowed to settle, which typically takes 10-30 minutes. When the solution is clear, the test strip is dipped into it and allowed a set time – 30 seconds or a minute depending on the brand – to develop colour before reading against a chart converting the colour to a nitrate reading.
Colour can continue to develop over the set time, hence it is important the duration is the same for every test strip. Samples should also be tested soon after removal from the field. “Same day analysis should be quick enough,” says Norris.
Plant & Food’s work has been with Merck Millipore’s Mquant nitrate test but Norris says there are several brands available online. “They all use much the same chemistry so it doesn’t really matter which you go for,” he told Rural News.
They’re also relatively cheap, at most $1/test strip in some cases so if time permits many samples can be taken and tested. However consistency in approach and sample preparation is important to get reliable results and those nitrogen concentration results then need correlating to crop needs.
Work with such tests on paddocks growing maize in the US has shown that results of over 20 parts per million of nitrate mean fertiliser isn’t needed. “Further work is required within our New Zealand context to validate these results.”
Another point he made at the FAR conference was that if there is a large amount of ammonial nitrogen in the soil the test will underestimate plant-available nitrogen.
