Prevention of aluminium toxicity with supplemental boron. II. Stimulation of root growth in an acidic, high‐aluminium subsoil

Root growth inhibition is an early symptom of Al toxicity and B deficiency. Our hypothesis is that Al toxicity may induce B deficiency, and it was our objective to determine if incorporation of supplemental B would promote root penetration into an acidic, high‐Al subsoil. Alfalfa (Medicago sativa L. cv. Hy‐Phy) was grown in slanted tubes with a Plexiglas window along their length. The top half of the tub contained silt loam soil and the bottom half contained subsoil from the Bt1 horizon (26% Al saturation) of a Creldon silty clay loam. Both soils originally contained 0–9 kg B ha−1. When root growth was measured in the bottom half of the high‐A1 subsoil, all measurements—depth of rooting, total root growth, final root lengths and root dry weight—demonstrated greater root growth in treatments where additional B was incorporated into the high‐Al subsoil. Results from this soil study extend those obtained in our hydroponic study in which supplemental B presented Al inhibition of root growth. Boron concentrations may need to he increased under acidic ‘high‐Al’ soil conditions to promote root penetration into these soil zones, and this could be especially important during periods of drought stress.