Barrow, N. Fertilizer Research (1985) 8: 85. doi:10.1007/BF01048909
The effectiveness of different sources of the same nutrient should be compared using plots of yield versus fertilizer applied to find how much more of the less-effective source is needed to provide the same yield. This gives the same result as measuring the relative response to the two fertilizers — provided response is defined as the instantaneous slope of the yield-fertilizer curve rather than the yield produced by a given amount of fertilizer. In most cases, and especially for immobile nutrients, the relative effectiveness, measured as suggested, will not be affected by the level of application. It is reasonable to assume this simple case until this assumption can be disproved — rather than assuming a more complicated case in the first instance.