Effectiveness of 3,4-Dimethylpyrazole Phosphate as Nitriflcation Inhibitor in Soil as Influenced by Inhibitor Concentration, Application Form, and Soil Matric Potential¹ 1 Project supported by the Federal Ministry of Education and Research, Bonn, Germany (No. 423-40003-0339812) and BASF, Ludwigshafen, Germany.

The efficacy of nitriflcation inhibitors depends on soil properties and environmental conditions. The nitriflcation inhibitor 3,4-dimethylpyrazole phosphate (DMPP) was investigated in a sandy loam and a loamy soil to study its efiectiveness as influenced by inhibitor concentration, application form, and soil matric potential. DMPP was applied with concentrations up to 34.6 mg DMPP kg^-1 soil as solution or as ammonium-sulfate/ammonium-nitrate granules formulated with DMPP. DMPP inhibited the oxidation of ammonium in both soils, but this efiect was more pronounced in the sandy loam than in the loamy soil. When applied as solution, increasing DMPP concentrations up to 7 mg DMPP kg^-1 soil had no influence on the inhibition. The efiectiveness of DMPP formulated as fertilizer granules was superior to the liquid application of DMPP and NH₄⁺, particularly in the loamy soil. Without DMPP, a decline in soil matric potential down to -600 kPa decreased nitriflcation in both soils, but this efiect was more pronounced in the sandy loam than in the loamy soil. DMPP was most efiective in the sandy loam particularly under conditions of higher soil moisture, i.e., under conditions favorable for nitrate leaching.