A comparison of sap nitrate test and chlorophyll meter for nitrogen status diagnosis in broccoli (Brassica oleracea L. spp. italica)

Vegetable crops, such as broccoli (Brassica oleracea L. spp. italica), are heavily fertilised with nitrogen, often to the detriment of water quality and at the risk of health effects from elevated nitrate levels in our food. Broccoli fertilisation can be made more efficient by diagnosing the nitrogen status of plants, and adjusting the fertiliser accordingly. In this study, two diagnostic tests were compared: a nitrate sap test and a chlorophyll meter. Broccoli plants were given 0, 50 or 100 kg N/ha at planting, and 0, 50, 100 or 150 kg N/ha five weeks post-transplant. Sap nitrate levels were read using the test strips and reflectometer, and non-destructive chlorophyll readings were taken, five and seven weeks post-transplant. Both methods gave values correlated with N application rate, but only the sap test demonstrated a site*treatment interaction. The sap test was more sensitive to changes in application rate than the chlorophyll meter, and is the better choice of diagnostic tool for fine-tuning the second dose of a split application of fertiliser at five weeks post-transplant.