For the determination of boron soluble in acid, in fertilizers, 6,0 g of mannitol are sufficient. The influence of the ion NH4+ is eliminated by boiling the alkaly solution for 30 minutes. The sulfate ion affect the phosphate precipitation with Pb2+, beeing necessary 1 ml of 10% Pb (NO3)2 solution for each 1% of P2O5 of the sample. The method presents suitable precision and accuracy.