SINGLE SUPERPHOSPHATE - REACTIVE PHOSPHATE ROCK MIXTURES .3. THE USE OF CONCENTRATION RATIOS OF ELEMENTS TO IDENTIFY THE NATURE AND AMOUNTSOF UNACIDULATED ROCK RESIDUES IN THE MIXTURES

Water insoluble residues (WIR) of unreactive phosphate rocks in single superphosphate-reactive phosphate rock (SSP-RPR) mixtures are conside red to reduce the agronomic value of these mixtures. A technique using concentration ratios of elements to identify the quantities of WIR of ground North Carolina (NC), ground Nauru and as received NC phosphate rocks in a SSP-RPR was developed. Of 22 elements tested P/Sr ratios w ere found to be the only element ratios that could be used to distingu ish between WIR's derived from Nauru and NC. P/Sr ratios in Nauru and NC were markedly different and provided a useful index for differentia ting between the two rocks. During acidulation the P/Sr concentration ratio remained essentially constant in the WIR's from both rocks. Usin g the element ratio technique the percentage of total P in the SSP-RPR sample was determined to be 60-61% water soluble, 5% water insoluble Nauru, 2% water insoluble ground NC and 32-33% water insoluble as rece ived NC. The element ratio technique developed in this study can be ap plied to partially acidulated P fertilizers made with rocks other than NC and Nauru provided elements which satisfy the same conditions as S r in this study can be found.