Abstract. Pot experiments, and field trials with temperate pastures, were used to compare pelleted Citraphos (ground calcined Christmas Island C rock phosphate) with soluble phosphates. The relative effectiveness of Citraphos increased markedly with time during a growing season. This increase occurred with residual Citraphos, topdressed twelve months previously and more, as well as with freshly applied Citraphos. It was not observed when Citraphos was compared with residual superphosphate.The apparent improvement of Citraphos thus derived very largely from the seasonal pattern of phosphorus uptake associated with soluble phosphate sources. As the availability of the latter decreased progressively after the early part of the season, the effectiveness of the more constant Citraphos improved by comparison. There is considerable arbitrariness in values of relative effectiveness assigned to slow acting phosphate fertilizers on the basis of single measurements. Direct assessments of the residual values of phosphatic fertilizers, by comparison with freshly applied soluble phosphates, appear to suffer similar limitations.The sharply decreasing availability of soluble phosphates has implications for the production of improved, long lasting phosphatic fertilizers.Comparisons between fertilizers in the field have usually been made over a whole season whereas comparisons in pot experiments have been limited in time by water supply in a small volume of soil or other artificial circumstances [8].