Fig. 11. The substitution value index (SVI) is the relative amount of P as phosphate rock (PR) compared to water soluble phosphorus (WSP) fertilizer needed to give the same relative yield. An example of this would be if a phosphate rock (PR) with 80% relative agronomic effectiveness (RAE) is used. To get a 75% of the maximum yield (75% relative yield), 52 kg P ha^–1 of PR compared with 23.5 kg P ha^–1 of WSP is needed. The SVI is thus 52/23.5 = 2.2.