Monitoring sap nitrate in vegetable crops: Comparison of test strips with electrode methods, and effects of time of day and leaf position

Abstract

A field crop of 3‐month old cabbages was sampled every 2 h from 0600 h until 2000 h. At each sampling, an upper, middle and lower leaf were taken from four plants, and their petiole nitrate status measured by three methods (a) “Merckoquant”; test strips (b) specific ion electrode on a fresh macerated extract and (c) the same electrode on an extract of oven dry tissue. All three methods led to similar conclusions: The test strips are capable of giving satisfactory results provided that sufficient plants are sampled, but values thus obtained for sap may be lower than for macerated fresh tissue, especially at the low end of the concentration range.