SummaryThe influence of different factors on the nitrification of an added ammonium salt and inherent soil‐N in soils from arid and semi‐arid areas of Israel was investigated.Nitrification of ammonium‐N proceeded rapidly at 28°C but was inhibited partially or completely in soils incubated at 37·40°C. In contrast, nitrate formation from inherent soil‐N proceeded better at 37·40 °C than at 28 °C.Bacteriological examination showed that a temperature of 37·40 °C had an injurious effect on the population of nitrifiers, especially the nitrate‐forming bacteria. Nitrification by the Nitrosomonas‐Nitrobacter group in culture media was also markedly inhibited at 37 °C as compared with that at 28 °C.Chloromycetin at a concentration of 25 mg per 100 g soil, and potassium chlorate at a concentration of 10‐3m suppressed the formation of nitrate from ammonium, but did not exert any appreciable effect upon nitrate formation from inherent soil nitrogen. Sodium sulphacetamide inhibited the production of nitrate from ammonium‐N more strongly than that from inherent soil‐N.Marked differences in the two nitrification processes in the soils investigated provided good evidence that the greater part of nitrate originating from soil‐N is produced by some process other than that which is responsible for nitrification of ammonium‐N.