The present invention relates to additive solutions of at least one urease inhibitor and/or at least one nitrification inhibitor, wherein the solution comprises: urea, an urea-aldehyde adduct and/or an urea-triazone fertilizer, water and at least one co-solvent, and further at least about 1 wt%, preferably at least about 2 wt% of urease and/or nitrification inhibitors.Up to about 30 wt% and more of the inhibitors could be dissolved as such. Additive solutions of the invention are highly suitable for the adding to liquid N and/or S fertilizers. A liquid 28-0-0-5S fertilizer can for instance be made from materials of the invention.
