Plant and soil boron in relation to boron deficiency in lucerne

Plant and soil samples were collected from 91 stands of lucerne ( Medicago sativa L.) in the main growing areas of New Zealand. In 39 samples of lucerne with B deficiency symptoms in the field, B concentration in whole shoots ranged from 5 to 17 ppm with lesser concentration in upper than in lower portions of the plants. B concentration in non‐deficient plants ranged from 17 to 64 ppm with equal distribution or greater concentration in upper than in lower portions. Available soil B was determined by hot water (HWS B) and ammonium acetate (NH 4 Ac B) extractions. HWS B was a better indicator of available B than NH 4 Ac B because (i) average HWS B values were lower in soil under deficient than under non‐deficient plants but NH 4 Ac B did not distinguish clearly between the 2 groups, and (ii) HWS B was correlated with B concentration in plants but NH 4 Ac B was not. It was not possible to define a critical value for available soil B nor was there any improvement in correlation between plant and soil B by using soil from below the surface rather than topsoil. Plant analysis was more reliable than soil analysis in diagnosing B deficiency in lucerne but there was no advantage in using only the apical portion rather than whole shoots.