Process for preparing fertilizers releasing nitrogen slowly

A process for preparing a fertilizer, releasing nitrogen slowly, containing ureaform and at least one other fertilizing material, wherein urea and formaldehyde are reacted in aqueous phase at temperatures ranging from 30 DEG to 60 DEG C., and at the following molar ratios: urea/formaldehyde from 1.2 to 2; water/urea from 0.7 to 10, the reaction being carried out at a pH ranging from 2 to 4, by addition of an acid substance; the so-obtained ureaform aqueous suspension is mixed with at least one other fertilizing material and with a portion of the (recycled) final product in an amount such as to give to the mixture a pH ranging from 4.0 to 5.5; the mixture is granulated at temperatures ranging from 50 DEG to 85 DEG C.; before the granulation ends, the pH of the product is increased to values ranging from 5.8 to 7 by the addition of an alkaline substance; the product is dried, and a portion thereof recycled as indicated.