The use of inorganic or organic poly acids is claimed for treating mineral fertilisers containing a nitrification inhibitor. The use of compounds of formula (I) is claimed as nitrification inhibitors. R1 = H, halo or 1-4C alkyl; R2 = 1-4C alkyl; R3 = H or CH2OH, and when R3 = H, the compound may be converted to a phosphoric acid salt. A mineral fertiliser containing a poly acid and optionally (I) is also claimed. 3,4-dimethylpyrazole (Ia), 4-chloro-3-methyl-pyrazole (Ib), N-hydroxymethyl-3,4-dimethylpyrazole (Ic), N-hydroxymethyl-4-chloro-3-methylpyrazole (Id), acid addition salts of (Ic) and (Id), and mixtures of two or more of (Ia)-(d) are new.