Republished:
2014 Conference presentation by Matt Norris and Paul Johnstone
The New Zealand Institute for Plant & Food Research Limited

Nitrogen fertiliser is used extensively to maximise productivity across a range of vegetable, arable and forage crops in New Zealand. Matching crop N demand with supply from residual soil mineral N, N mineralisation from organic matter and fertiliser N is central to economic and environmental outcomes in these sectors.

To improve nitrogen use efficiency, effective tools and approaches are required to help guide nutrient management decisions. One potential method is the ‘quick test’ soil nitrate (NO₃-N) approach. This in-field approach uses a ‘test strip’ impregnated with a NO₃-N sensitive alert zone which, with a simple colorimetric scale, may be used to measure soil solution NO₃-N concentrations. Measured NO₃-N concentrations can then be compared with critical threshold limits that have been established for a number of crops.

The quick test strips have already been used for a number of years overseas to support growers in making N fertiliser decisions. Depending on NO₃-N levels at sampling, a test strip reading may indicate the need for fertiliser to be applied, withheld for a period or eliminated entirely. The test can therefore provide more certainty in decision making. In addition to being cost effective and simple to use, the quick test approach provides the user with rapid information thus enabling decisions to be made at short notice.

In 2013–14, Plant & Food Research undertook a series of proof-of-concept trials to examine the ‘quick test’ soil nitrate (NO₃-N) approach under NZ conditions. The aim of the work was to:

1. substantiate the relationship between test strip nitrate values and laboratory-determined mineral N (the ‘gold standard’) and
2. assess the suggested quick test critical thresholds for making N fertiliser decisions in beetroot and carrot crops.

Results from this preliminary work were encouraging. Follow on trials will test further the suitability of the strip in making field-scale N fertiliser decisions.