YIELD RESPONSE OF SUBTERRANEAN CLOVER TO APPLICATIONS OF SUPERPHOSPHATE AND ROCK PHOSPHATE MEASURED IN THE YEAR OF APPLICATION AT THE SAME SITE IN DIFFERENT YEARS

The relationship between yield of dried clover herbage (Trifolium subterraneum) and the level of phosphorus (P) applied (yield response to P) was studied in the year of application for two successive years at two sites and five successive years at one site. Single superphosphate and two rock phosphates were compared at each site. Data for the relationship were adequately described by a Mitscherlich equation. At each site, for each fertilizer, the yield response to P was different in different years, with both the maximum yield plateau and the shape of the relationship differing between years. Expressing yield as a percentage of the maximum (relative) yield reduced the variations so that only the shape of the relationship, described by the exponent (curvature) coefficient of the Mitscheriich equation, differed between years. This is attributed to the effect of different seasonal conditions on the yield responses of clover herbage to freshly-applied P. The value of the curvature coefficient was related to rainfall for five successive years at one site. The following trends were observed: (i) for superphosphate, the value of the coefficient tended to decrease with increasing rainfall so that more P was required to produce the same relative yield; and (ii) for rock phosphate, the value of the coefficient tended to increase with increasing rainfall so that less P was required to produce the same relative yield.