Field studies on the fate of radioactive sulphur fertilizer applied to pastures

Abstract

Nine field trials involving the application of³⁵S-labelled gypsum fertilizer to various soil—pasture systems were conducted on five different soil types belonging to the New Zealand recent (Entisol) and yellow-brown earth (Inceptisol) soil groups. After application to the soil, the fertilizer sulphur (S) was converted rapidly into organic forms by microbial immobilization. Within 34 to 75 days from the time of S fertilizer application, 17 to 40% of the applied S was in organic forms in the topsoil (0–15 cm depth). A higher rate of incorporation occurred in improved pasture sites compared to unimproved sites. A lower rate of fertilizer S application enhanced the extent of organic incorporation while a difference in the time of fertilizer application reduced the fertilizer conversion. At most sites, leaching of fertilizer S beyond the topsoil was most important at two months after fertilizer application. The rate of plant uptake of fertilizer S decreased with time and was similar to the immobilization rate. The significance of these findings is discussed in relation to possible manipulations of the pasture—soil system to improve the efficiency of fertilizer S utilization.