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Relation of Ammonia and Nitrous Acid to Suppression of Phytophthora in Soils Amended with Nitrogenous Organic Substances. Peter H. Tsao, Professor, Department of Plant Pathology, University of California, Riverside 92521; J. J. Oster, staff research associate, Department of Plant Pathology, University of California, Riverside 92521. Phytopathology 71:53-59. Accepted for publication 10 March 1980. Copyright 1981 The American Phytopathological Society. DOI: 10.1094/Phyto-71-53.

Amendment of natural soils with chicken manure and urea at 2 and 0.1%, respectively, caused soil pH to rise from near neutral to above 8.6 and then fall to the acid range. Soil nitrogen analyses confirmed correspondence of this pH pattern to the processes of ammonification and nitrification. Results of in vitro tests with NH4OH and KNO2 solutions demonstrated that Phytophthora propagule germination was inhibited by low concentrations of NH3 and HNO2. A minimum of 4 ppm NH3 or 0.3 ppm HNO2 reduced Phytophthora cinnamomi sporangium germination to ≤ 1%; 6 ppm NH3 or 0.5 ppm HNO2 reduced P. parasitica sporangium germination to ≤1%; 17 ppm NH3 or 0.9 ppm HNO2 reduced P. parasitica chlamydospore germination to less than 10 and 5%, respectively. Additional in vitro tests with ammonium and nitrite salts showed that toxicity resided in the NH4+ or NO2 portion of a compound. Furthermore, the pH dependency of germination inhibition in the ammonium/ammonia and nitrite/nitrous acid solutions demonstrated that NH3 and HNO2, respectively, are primarily responsible for the inhibition. NH4OH at a given concentration was more toxic at pH 8 than at pH 6, but KNO2 was more toxic at pH 6 than at pH 8. Sufficiently high concentrations of NH4+ and NO2 were detected in amended soils at soil pH values favoring their presence in the nonionized forms, NH3 and HNO2, respectively, to account for significant germination inhibition. Therefore, these toxicants are responsible, at least in part, for Phytophthora suppression in amended soils.

Additional keywords: organic amendment, suppressive soil.