The test has been used overseas for several years, providing valuable information to guide nutrient management decisions. The technology is “rapid, cheap and very simple to use” says research associate Matt Norris.
“These attributes make it really useful for making informed decisions on the spot and for reducing some of the risk associated with applying nitrogen.”
Under cropping conditions, soil testing is traditionally done at or around cultivation time whereby organic and inorganic N levels are determined in the lab. Inorganic N is readily available for uptake by plants and where plant demands are expected to exceed the supply in the system, nitrogen fertiliser can be applied.
“The risk is that N in any system is very dynamic; there are a lot of variables involved and a risk that the initial soil test will not hold true prior to the N application,” Norris said.
This could lead to under applying N, therefore preventing crops reaching optimum yields, or over applying and adversely affecting the environment via nitrate leaching as well as reducing the economic efficiency of the system.
These issues can be mitigated by carrying out more soil testing at sowing or emergence as well as over the duration of the crops’ growth period.
This is not always practical and the time needed for soil testing and lab analysis may delay decisions. This is where the N quick test may provide a solution.
Norris said the N quick test uses a “test strip” impregnated with a nitrate sensitive alert zone which can be compared on a colourimetric scale to indicate the concentration of nitrate in soil solution.
“The efficiency of N use could be improved by having a tool to guide immediate application decisions.”
The process involves mixing a 10ml sample of soil with 30ml of calcium chloride solution and leaving it to stand for 30 minutes. A “test strip” is then dipped into the solution and left for one minute to develop in colour.
“The colour can be read against a scale and a correction factor used to convert the value to a soil nitrate basis by taking into account soil texture and moisture status.”
Settled soil mixed with the calcium chloride solution. The test strip is dipped into the solution to measure the nitrate concentration.
In the field, farmers or growers would be able to take a series of samples, and as long as the sediment was well-settled after mixing – usually after about 30 minutes – they would have an “on the spot” indication of their paddock’s plant-available nitrate status.
The resultant value can be compared against critical threshold values which indicate whether an N fertiliser application is needed or not.
“The risks involved are that the colour presented on the test strip and the soil texture are both interpreted by the user,” Norris said.
He advised that it might be wise for unfamiliar users to seek support. Also “the test only measures nitrate, therefore if there is a lot of ammonia in the solution there is potential that the test will underestimate the level of plant-available N”.
“However, generally ammonia concentrations are low and not an issue.”
In 2013-14, Plant & Food Research undertook a series of “proof of concept” trials under NZ conditions with results indicating a good initial relationship between the quick test and the standard laboratory method.
“More data will improve the relationship,” Norris said – he will continue to work on the test’s suitability for making field-scale N fertiliser decisions.
“A key area of work for the coming years will be assessing the suitability of the test for making predictions close to the critical threshold level.
“A high or low nitrate concentration makes it easy to determine the need for N fertiliser, however if the test comes with a value close to the critical level, more care needs to be taken before making decisions around N fertiliser use.”
