The chemical assessment of the boron status of soils

Abstract

Extraction of soils by shaking for 1 h at 20° with 0.01 M CaCl2, + 0.05 M mannitol was found to be a more convenient soil test for plant-available boron than the standard hot water soluble (HWS) method, and to be as good in predicting the response in boron uptake by plants. Wheat plants were grown in a pot experiment involving 18 different surface soils having a range of HWS-boron concentrations of 0.22-2.52µg g-1. Two treatments applied were with and without the addition of 1µg g-1 H3BO3 to the soils. The relative boron uptake by the plants in response to the treatment was correlated with extractable boron in the untreated soils, as measured by the HWS method and six alternative extractants. Boron determined in the low availability range using a CaCl2- mannitol extractant was significantly related to the HWS boron, although less boron was extracted. At potentially toxic concentrations the mannitol method extracted more boron than the HWS method. Optimization of the mannitol procedure is described, and the advantages of the extractant are discussed. The clear, colourless extract obtained was suitable for analysis by inductively coupled plasma spectrometry and by spectrophotometry.