The effect of two levels of K and P applications on an Italian vertisol was investigated adding known quantites of KC1 and Na2HPO4solutions, respectively. The extractions were carried out using electro‐ultrafiltration (EUF) at a constant electric field and different temperatures. The EUF data were compared with the NH4OAC extracted K and Olsen‐P values. We found that both the soluble K fraction, extracted by moderate EUF conditions, and the reserve K fraction, extracted by stronger EUF conditions, did not increase significantly with fertilization treatments. On the contrary, in the case of P the first moderate extraction assessed the different “fertilization”; levels better than the successive, more powerful procedure.