Lead uptake from soils by perennial ryegrass and its relation to the supply of an essential element (sulphur)

The lead status of 16 soils of England and Wales was studied by pot-culture and soil chemical procedures. Perennial ryegrass was grown on the soils, with and without added sulphur, in a controlled environment cabinet with carbonfiltered air. Plant-available lead comprised uptake in 4 successive harvests of tops plus that in roots at the final harvest. The concentration of lead in the tops of healthy plants, i.e. those with adequate sulphur, was lower than in the roots, e.g. at harvest 4 the means were 5.0 and 12.9 ppm, respectively. However, with sulphur-deficient plants the concentration of lead in the tops was often higher than in the roots, the means at harvest 4 being 16.3 and 13.0 ppm, respectively. The marked increases in the concentration of lead in the tops of sulphur-deficient plants coincided with decreases in dry-matter yield, but for any one soil the tops of such plants contained similar amounts of lead to those of healthy plants. The lead content of the tops was poorly correlated with soil lead whereas that of the roots, in terms of both concentration and total amount, was highly correlated. The amount of lead extracted by 0.5 M BaCl₂ or 0.05 M EDTA provided a slightly better assessment of availability than 'total' content or the amount extracted by 2.5 per cent acetic acid. The solutions of acetic acid, BaCl₂ and EDTA extracted, on average, 1.0, 16.3 and 32.7 per cent respectively, of the total lead in the soils. The greater replacement of lead by the Ba ion than by the H ion (acetic acid) is ascribed to valence and the similar radii of Pb²⁺ and Ba²⁺. It is concluded that in soil-grown ryegrass the roots restrict the movement of lead into the tops of high-yielding plants, but when growth is limited by sulphur deficiency the concentration in the tops increases markedly.