How do soil P tests, plant yield and P acquisition by Lotus tenuis plants reflect the availability of added P from different phosphate sources

The relative effectiveness (RE) of each one of three different sources of P-P in solution (Psol), triple superphosphate (TSP) and phosphate rock (PR)-for reflecting the availability of P in a P-deficient soil were assessed by measuring in Lotus tenuis variables associated with growth, organ morphology, and plant tissue P-content together with the amounts of P extracts from soil by two of the currently used soil-P tests-Bray I and Olsen. A hyperbolic equation was used to fit the response curves of each one of those plant variables to added-P. The ratio between the shapes of paired response curves of any P-sources was used to compute the RE and substitution rate (K) of one source relative to the other. More P was needed from TSP and PR compared to Psol-100% soluble P-source. On the average P applications as TSP relative to Psol and PR relative to TSP were only 68 and 63% effective respectively for plant growth. Plant roots were more sensitive than soil-P tests to detect shifts in P-availability from different P-sources. Because soil tests are commonly used to estimate the current P status in soil in order to calculate the optimum application levels of fertilizer P for a crop or pasture, these results would have practical agronomical consequences if reproduced in other cultivated species because they show that the response curve of a plant species as a function of added P and soil test might differ among fertilizer types, measured plant variables, and the test used to measure P availability in the soil.