Abstract: The present invention relates to a process for inhibiting, at least in part, nitrification activity of ammonium-containing fertilizers (a1) and/or of ammonium-producing fertilizers (a2), which process comprises the steps of: a) Applying to soil and/or to foliage a nitrification inhibitory amount of one or more compounds (b) that are selected from the group consisting of (b1) polysulfides and/or (b2) thiosulfates and/or (b3) hydrosulfides; b) Wherein the one or more compounds (b) are applied simultaneously with, before or after the fertilizers (a) are applied to the soil and/or to the foliage such that the compounds (b) are in direct contact with the ammonium-containing fertilizers (a1) and/or the ammonium-producing fertilizers (a2); c) Wherein the one or more nitrification inhibiting compounds (b) are applied in an amount: i. such that the ratio (w/w) of “total protectable nitrogen” over “sulfur from compounds (b)” is at most about 8:1, preferably at most about 7.