Evaluation of Nutrisphere-N As a Urease Inhibitor.

A maleic-itaconic polymer (MIP), Nutrisphere-N, is widely sold as a soil urease inhibitor in the USA.  Six laboratory experiments were conducted to evaluate the effectiveness of MIP as a urease inhibitor.  In the first study, concentrated solutions of urea or urea + MIP were applied to two soils as small droplets, to simulate the reaction of individual urea granules with soil.  There was no effect of MIP on the rate of urea hydrolysis.  In a second study, commercial urea granules or treated urea granules were placed on the surface of three soils.  There was no effect of MIP on urea hydrolysis but a commercial NBPT product significantly slowed urea hydrolysis.  In a third study, application of 1, 5, or 50 mg kg^-1 of MIP to three soils had no effect on urea hydrolysis.  Application of 500 mg kg^-1 of MIP provided a slight inhibition (16-17%) of urea hydrolysis with a sandy soil, but not with the other two soils.  By contrast, soil urease activity was inhibited by an average of 64 and 71% by addition of 1 and 5 mg kg^-1 of NBPT, respectively.  The mode of action of MIP, nickel sequestration, was evaluated in the fourth study.  Thirteen carboxylic acids with a wide range of stability constants for nickel, MIP, and NBPT were tested as urease inhibitors with three soils.  The carboxylic acids and MIP were, in general, ineffective as urease inhibitors, but NBPT inhibited urease as in the previous trial.  In two studies with purified urease at pH 7, hydrolysis was completely inhibited by 1 mg L^-1 of NBPT in the reaction mixture.  Addition of MIP had no effect at concentrations of 1 mg L^-1 or less, and MIP stimulated urea hydrolysis at higher concentrations.  In general, MIP was ineffective as a urease inhibitor.