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 N analyzes confirmed correspondence of this pH pattern to the processes of ammonification and nitrification. Results in vitro testes 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 P. cinnamomi sporangium germination to .ltoreq. 1%; 6 ppm NH3 or 0.5 ppm HNO2 reduced P. parasitica sporangium germination to .ltoreq. 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. 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. These toxicants are responsible, at least in part, for Phytophthora suppression in amended soils.