Uptake of Sulphur-35 by pasture plants from different depths in two Canterbury soils under field conditions : A dissertation submitted in partial fulfilment of the requirements for the Degree of Bachelor of Agricultural Science (Honours) in the University of Canterbury

Abstract

In recent years investigations using radioisotope tracers have suggested that the subsoil is an important source of plant nutrients. However, soil tests in New Zealand are usually made on samples from the top 15cm or so and no information is obtained concerning the subsoil fertility and its possible contribution to plant nutrition. The main aim of the present study is to look at the contribution of all the compartments in the soil profile especially the subsoil in determining the sulphur status of pasture soils. Two soils in Canterbury, namely Kowai (recent) and Gorge (lowland yellow brown earth), were chosen for this purpose. The uptake of s³⁵ isotope placed at four different depths in the profile (i.e. 22, 52, 75 and 100 cm) by perennial ryegrass and white clover plunts was examined. Prior to the work presented in this dissertation, a major study of the dynamics of sulphur movement in these soils had been carried out by P.E.H. Gregg as part of his Ph.D. studies. In these studies, (pers.comm.) also looked at s³⁵ uptake from four different depths with 52 cm being the deepest depth of placement. The present study was undertaken to confirm and extend this work of Gregg and in so doing it also assessed the availability to plants of adsorbed sulphate in the subsoil. To do this the following determinations were made. (1) Uptake of s³⁵ isotope by plants under field conditions was studied from applications of the isotope placed at four different depths, the deepest being 100cm, (2) Plant species differences (loliumm perenne and Trifolium repens in pasture) in s³⁵ uptake were examined, and , (3) The lateral spread of uptake of sulphur by plants from a source of application was assessed.