Evaluation of Nutrisphere-N as an Inhibitor of Urease in Soils with pH Values Near Five

Nutrisphere-N is a maleic-itaconic polymer (MIP) marketed as a soil urease inhibitor. Recently, it was shown that the MIP in Nutrisphere-N was able to inhibit purified urease at a pH of 5.0. Furthermore, Ni was removed from purified urease by MIP at a pH of 5.0. The objective of this study was to determine if these findings with purified urease at pH 5.0 could be replicated with soils with pH values near 5.0. Three soils with pH values of 4.5, 5.2, and 5.7 were incubated with 5 mu g g(-1) of N-(n-butyl) thiophosphoric triamide (NBPT) as Agrotain Ultra, and 5 and 50 mu g g(-1) of MIP as Nutrisphere-N. The inhibitors were allowed to react with the soil for 1 d before application of urea (sequential application) or applied at the same time as urea (simultaneous application). For the sequential application, 5 mu g g(-1) of NBPT as Agrotain Ultra inhibited soil urease activity by 49%, versus -3 and -1% for 5 and 50 mu g g(-1) of MIP as Nutrisphere-N (average of three soils). For the simultaneous application, averaged across the three soils, 5 mu g g(-1) of NBPT inhibited soil urease activity by 59%, versus -1 and -4% for 5 and 50 mu g g(-1) of MIP. The observation that MIP inhibits purified urease at a pH of 5.0 could not be repeated with soils with pH values near 5.0. It remains to be demonstrated that Nutrisphere-N functions as a soil urease inhibitor at the concentrations likely to be applied to soil.