Determination of the nitrogen status of irrigated potato crops: II. A simple on farm quick test for nitrate‐nitrogen in petiole sap

Abstract

This paper describes the development and calibration of a simple on farm quick test for sap nitrate‐N in potatoes (Solanum tuberosum L). A description is given of the plant tissue to sample, bulk sampling procedures, sap extraction, dilution and measurement of sap nitrate‐N. Thirty petioles of the youngest fully expanded leaves per plot were collected and bulked. Segments 10–20 mm long were excised from near the centre of each petiole, crushed and the sap collected. One mL of this sap was diluted a hundred‐fold. The concentration of nitrate‐N in sap was determined by immersing a test strip into the 1: 100 diluted sap extract and then reading the intensity of colour in the strip by reflectometric analysis with a simple, hand held meter. Three strips were read for each sample tested. Standard solutions were used to calibrate test strips. Linear regression equations were used to investigate the relationships between nitrate‐N concentration determined in sap and dry matter. A highly significant linear relationship was found between nitrate‐N concentration as determined in sap (by the quick test method) and that in dry matter (by a laboratory method), (R² = 0.89∗∗∗). Effects of site, cultivar, sampling time and certain interactions although significant were of low magnitude and in total accounted for only 7.9% of the total variation in the above relationship. Depending on sampling times, the estimated critical nutrient ranges (CNR) of sap nitrate‐N for 95 and 100% of maximum tuber yield varied from 1366.1–1443.7 mg/L, at the first sampling time to 530.0–826.5 mg/L at the Gnal sampling time. These CNR can be used as ‘target’ levels in crop monitoring programs aimed at efficient N management. The quick test for sap nitrate‐N can be used to monitor the N status of crops and recorded nitrate levels compared to our CNR and N applied if needed before significant reductions in yield occur. Each test on collected petioles took only 10 minutes. Costs of equipment and materials for a run of 500 samples (with 3 test strips per sample) were approximately $A2 per sample. This sap test offers a rapid, accurate method for plant tissue nitrate‐N analysis that permits the grower or consultant to sample, analyse and monitor potato crops on‐farm at a modest cost and could easily be extended to many other crops.