A critical evaluation of on-farm rapid tests for measuring nitrate in leafy vegetables

Nitrogen (N) supply to crops impacts on the synthesis of the green photosynthetic pigment, chlorophyll, and on plant nitrate concentration. Insufficient N supply decreases nitrate concentrations, chlorophyll synthesis and plant productivity. Excess N supply causes undesirably high nitrate concentrations in leafy vegetables and does not increase chlorophyll concentration. Consequently, analysis tools have been developed to determine the nitrogen status of crops. This paper reviews three tools that have been recommended for on-farm monitoring of crop nitrogen status: nitrate-selective electrodes, nitrate-sensitive test strips, and chlorophyll meters. The electrodes and test strips analyse plant sap, which in addition to nitrate, also contains other substances, including chloride. These substances decrease the accuracy of nitrate measurements made using ion selective electrodes more than those made using test strips. Chlorophyll meters indicate whether sufficient N has been assimilated to maximise plant production, but provide no information about current supply or whether excessive nitrate has accumulated in the leaves. Consequently, chlorophyll meters may be useful for on-farm monitoring of crop N status; however, for managing both the sufficiency of N supply and nitrate levels, only nitrate test strips can be recommended. Greater general awareness of the weaknesses of field N testing methods would lead to better guidelines for their use, and perhaps, improvements in the technologies.

► Chloride and other substances in plant sap decrease the accuracy of nitrate measurements, particularly when using a nitrate-selective electrode.
► Chlorophyll meters indicate the sufficiency of assimilated nitrogen for maximum growth, but not the excess nitrate accumulated in leaves.
► For managing both the sufficiency of supply and nitrate levels of leafy vegetables, nitrate test strips are recommended.
► Greater awareness of the weaknesses of field N testing methods would lead to better guidelines for their use.