In the Spanish southeast, a greenhouse with a sandy soil was cultivated with a cucumber crop (Cucumis sativus L.) in a short productive season and fertilized by fertigation with a nitrate/ammonium mixed nutrition with and without DMPP (3,4‑Dimethylpyrazole phosphate). Three soil samplings were made up to 20 cm depth to analyse microbial populations by the successive dilutions method. Over various fungi obtained by the analysis, the DMPP toxicity was tested in vitro on two diverse culture medium and various concentrations (0, 1, 2, 5, 10, 15, 20, 50, 100, 150, 200 and 300). DMPP application increased the number of colony forming units of various fungi in soils, but this behaviour was not consistent. Under in vitro conditions generally the DMPP application produced a higher mycelial growth on the tested fungi and although at higher DMPP concentrations produced inhibition in growth, the effect was only fungistatic, and did not differ markedly from the ammonium nitrate which it was compared with.