When isotopic P-labeled soil is used, estimation of the sources of P in plant tissues is easily estimated based on the specific activity in the plants. By amending a neutral-calcareous soil with 32P-45Ca-tricalcium phosphate, Azcon-Aguilar et al. (1986) were able to estimate the effect of AMF (Glomus mosseae and Glomus spp.) and two phosphate solubilizing bacteria on the growth and nutrition of soybean. Barea et al. (2002) were able to evaluate the interactive effects of P-solubilizing rhizobacteria, AMF, and Rhizobium in legumes using the isotopes 32P and 15N.