Degree Type


Date of Award


Degree Name

Doctor of Philosophy



First Advisor

John M. Bremner


The growing importance of urea fertilizer in world agriculture has stimulated extensive research to find compounds that will retard hydrolysis of urea by soil urease and thereby reduce gaseous loss of NH[subscript]3 and other problems resulting from the normally rapid hydrolysis of urea fertilizer. Numerous compounds have been proposed for inhibition of soil urease, but the only compounds that have shown promise are phosphoroamides such as N-(n-butyl) thiophosphoric triamide (NBPT) and thiophosphoryl triamide (T·PTA). Studies to evaluate these two compounds as soil urease inhibitors showed that they are poor inhibitors of plant or microbial urease and that their ability to inhibit urea hydrolysis by soil urease is due to their decomposition in soil with formation of compounds that are potent urease inhibitors. Studies using [superscript]31P-NMR and FT-IR to identify these decomposition products showed that NBPT decomposes with formation of N-(n-butyl) phosphoryl triamide and that TPTA decomposes with formation of phosphoryl triamide. Enzyme kinetic studies of inhibition of plant and microbial ureases by phosphoroamides showed that these compounds are slow-binding or slow, tight-binding inhibitors of urease;Most of the fertilizer N applied to soils is in the form of ammonium or ammonium-producing compounds such as urea. The nitrate formed through nitrification of this fertilizer N by soil microorganisms is susceptible to loss by leaching and denitrification. The adverse effects of oxidation of ammonium to nitrate by the nitrifying microorganisms in soil have stimulated research to find compounds that can be used as fertilizer amendments to retard nitrification of fertilizer N. Studies to evaluate compounds showing the most promise as soil nitrification inhibitors demonstrated that their effectiveness decreased in the order: 2-ethynylpyridine > etridiazole (Dwell) > nitrapyrin (N-Serve) or 3-methylpyrazole-1-carboxamide (MPC);Studies of the relationships between the structures of heterocyclic N compounds and their effectiveness of inhibition of nitrification in soil showed that unsubstituted heterocyclic N compounds containing two adjacent ring N atoms inhibit nitrification in soil and that two of these compounds, pyrazole and 1,2,4-triazole, are potent inhibitors. They also showed that several substituted pyrazoles and thiadiazoles are effective soil nitrification inhibitors. (Abstract shortened with permission of author.)



Digital Repository @ Iowa State University,

Copyright Owner

Gregory William McCarty



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144 pages