On-farm measurement of nitrates in leafy vegetables
Optimising nitrogen application in intensive vegetable production relies on being able to adjust N supply during crop growth, so monitoring needs to be quick, cheap, accurate.
Monitoring would best be done on-farm and three Industry and Investment NSW researchers are trialling a field test that uses the sap to measure N circulating in the plant.
The sap can be obtained using something as simple as a garlic press.
Whichever way nitrogen is applied, such as in urea or animal manures, plants take up most of it as nitrate.
Excess nitrate can accumulate in leafy vegetable crops, a potential concern for human health, and now being recognised internationally as a negative food quality factor.
Other undesirable consequences include contamination of water bodies and the emission of greenhouse gas into the atmosphere.
This is driving farmers to optimise N inputs and minimise unwanted effects from excessive applications.
"Optimisation is not simple," said Dr Sophie Parks, a plant physiologist with Industry and Investment NSW, based at the Gosford Primary Industries Institute.
"Although the amount of nitrogen removed by a crop is readily calculated, the calculated amount is an underestimate of the amount of fertiliser needed to grow the crop."
"This discrepancy can be large, and occurs because various factors affect how efficiently the applied nitrogen is taken up."
Dr Parks and her colleagues, Donald Irving at Yanco and Paul Milham at Richmond, are focusing on the fact that the nitrate content of sap squeezed from leaf stems or leaves is a well established index of nitrogen supply.
The nitrate is readily measured in the laboratory, however the lab methods are unsuited for use on-farm.
The methods tested most widely to optimise N supply on-farm are ion-selective electrodes and test strips.
Reviewing available literature, the researchers found the most pertinent studies compared the results of measuring nitrate in the field using test strips and/or an ion-selective electrode with results obtained in the laboratory.
Test strips consistently gave accurate measurements, whereas ion-selective electrodes tended to give high results.
The difference between the results from the on-farm electrode and laboratory methods varied between studies and was large in some, possibly due to the effects of organics and chloride in the sap.
The researchers aim initially to quantify the interference by chloride and so increase the accuracy of the nitrate measurement.
Contact Sophie Parks, Gosford, (02) 4348 1914, sophie.parks@dpi.nsw.gov.au