AVAILABILITY OF SUBSOIL SULPHATES TO BARLEY AND RAPESEED

Increased use of sulphur (S) fertilizers in southern Alberta led to a series of field and phytotron experiments to investigate the importance of S fertilizers and the role of subsoil reserves of sulphate sulphur (SO4-S) for barley (Hordeum vulgare) and rapeseed (Brassica napus). Two types of experiments were conducted. In the field-plot studies, barley was grown on dryland soils low in surface SO4-S but underlain by subsoil high in SO4-S. Neither elemental nor SO4 forms of S significantly increased barley yields in a series of 10 experiments on those soils. The comparisons were made at three levels of N fertilizers. In a series of lysimeter studies, three successive crops were grown in soil low in inorganic S (2.0 μg SO4-S∙g−1), or soil supplemented with 25 μg 35SO4-S∙g−1 soil at specified depths in the lysimeters. Barley was adequately supplied with S from SO4-S at a depth of 54–72 cm. It obtained 55% of its S from a high SO4-S (25 μg∙g−1) layer of soil at that depth, although 40 days growth were required before the S was effectively utilized. Five times as much S was taken up by the barley when the entire soil received an additional 25 μg SO4-S∙g−1 as when only the 54- to 72-cm depth was supplemented; however, the yields were unaffected. Most of the excess S was retained in the straw. Rapeseed took up an increasing amount of SO4-S as the proportion of the lysimeters that initially contained SO4-S was increased. Rapeseed was also able to utilize SO4-S from a depth of 54–72 cm. Rapeseed showed deficiency symptoms when most of the added and soil reserves of S had been depleted by previous crops; its growth habit became indeterminate and seeds did not develop. Although total dry matter yield was not greatly affected, seed yield was markedly reduced in S-deficient rapeseed. Rapeseed took up 10 times as much S as did barley when the S supply was just adequate for seed production. Although rapeseed had a much higher S requirement than barley, both crops were adequately supplied by subsoil reserves of SO4-S under the field and controlled environment conditions studied. The studies suggest that fertilizer recommendations should be based on soil analysis to a depth of at least 60 cm. Key words: Sulphur fertilizer, sulphates, 35S, barley, rapeseed, nutrient uptake