Boron toxicity in soil solution

Abstract

The relationship between soil solution boron concentration and the dry-matter accumulation and boron concentration in sunflower (Helianthus annuus cv. Hysun 31) was investigated in a 14-day study in a controlled environment. Plants were grown in each of six soils previously equilibrated with various levels of boron applied as H3BO3. The soil, wet to 10 kPa matric suction, was retained in polypropylene containers and the soil solution was extracted by centrifugation from the entire undisturbed soil volume immediately after the top growth had been harvested. Above a threshold of 1.9 µg B mL-1 in soil solution, yield reductions due to boron toxicity occurred. In the toxicity range there was a linear relationship between relative dry matter yield and log10[B] in soil solution. At high levels of boron, the relationship between boron concentration in plant tops and hot 0.01 M CaCl2-extractable boron depended on soil type, whereas the relationship between plant boron concentration and soil solution boron concentration did not depend on soil type. It is suggested that, at high levels of boron, plants were responding to soil solution boron (intensity), while hot 0.01 M CaCl2-extractable boron estimates a quantity factor. The results obtained indicate that the apparatus and technique used in this study could be used to derive toxicity threshold concentrations for a range of plant species.