Inhibition of Urease Activity in Soils

Studies to evaluate more than 100 compounds as inhibitors of urease activity in soils showed that dihydric phenols and quinones were the most effective organic compounds tested and that silver and mercury salts were the most effective inorganic compounds. When the most potent inhibitors were compared using 50 1'8 of inhibitor per g of soil, their effectiveness decreased in the order: catechol > phenylmercuric acetate > hydroquinone > p-benzoquinone and 2,5-dichloro-p-benzoquinone > 2,6-dichloro-p-benzoquinone > silver sulfate > 1,2-naphthoquinone > phenol > mercuric chloride > sodium p~hloromercuribenzoate and 4-<:hlorophenol > 2,5-dirnethyl-p-ben.Zoquinone > N-ethylmaleirnide > gold chloride > acetohydroxamic acid and copper sulfate. The inhibitory effects of most of these compounds on soil urease activity decreased markedly with time, but the effect of 2,5-dirnethyl-p-benzoquinone increased, presumably because this compound is decomposed in soils to a relatively stable product more effective than the parent compound as a soil urease inhibitor. The findings concerning the rates of inactivation of urease inhibitors in soils indicate that, of the various compounds tested, 2,5-dirnethyl-p-benzoquinone, 2,5-dichloro-p-benzoquinone, and 2,6-dichlorop-benzoquinone are likely to prove the most effective for retardation of urea decomposition in soils and reduction of the problems caused by the normally rapid hydrolysis of fertilizer urea by soil urease. N-Serve and other compounds used to inhibitnitrificationin soils have very little, if any, effect on soil urease activity.