Fertilizer mixture containing nitrification inhibitor

A process for preparing 2-(N-3,4-dimethylpyrazole)succinic acid or a salt thereof by:

• (i) reaction in water of 3,4-dimethylpyrazole with maleic acid and/or maleic anhydride in the absence of organic solvents or diluents at a temperature from 70° C. to 105° C. to produce a reaction mixture, and
• (ii) following step (i), in the absence of organic solvents or diluents cooling the reaction mixture to induce crystallization of 2-(N-3,4-dimethylpyrazole)succinic acid or salt thereof having a purity of at least 99.7%.