The uptake by crops of fresh and residual phosphatic fertilizers by simultaneous measurements with 32P and 33P

The effects of various residual P fertilizers, with or without fresh fertilizers cannot be measured directly by field or pot experiments. A radiotracer technique using 32PO4 and 33PO4 is described. Available soil phosphorus is 32P-labelled and fresh P fertilizer is 33P-labelled. Counting 33P and 32P in crops is carried out by measurements making use of the Cerenkov effect and using liquid scintillation procedures. From the results obtained in the example given, it can be concluded that the various kinds of residual P fertilizers can modify the uptake of the fresh P fertilizers applied