EFFECTS OF p H AND NITRIFIER POPULATION ON NITRIFICATION OF BAND-APPLIED AND HOMOGENEOUSLY MIXED UREA NITROGEN IN SOILS

Abstract

Two soils, Keld silty clay loam (pH 5.4) and Wellwood clay loam (pH 6.6), were used to investigate the effects of modifying soil pH upon the nitrification of band-applied urea at 20 C. The pH of the Keld soil was adjusted to 6.5 and 7.3 with Ca (OH) 2 , whereas the Wellwood soil was adjusted to 5.6 with H 2 SO 4 . In addition, nitrification of nitrogen in a homogeneously mixed soil–urea suspension at 20 C was studied using the two soils mentioned and also Morton very fine sandy loam (pH 8.2). There were marked differences in the rate of oxidation of NH 4 between the natural Keld and acidified Wellwood soils at comparable soil pH values. The oxidation was found to be very slow in the Keld soil at all pH values and no appreciable nitrite was formed. In the Wellwood soil, nitrification was more rapid and was accompanied by nitrite formation. However, the rate of oxidation was temporarily retarded by decreasing the pH of the Wellwood soil, but the oxidation of band-applied urea in the Keld soil remained unchanged with modified pH of 6.5 and 7.3. Increase in nitrification and nitrate accumulation occurred when the limed Keld soils (pH 7.3) was inoculated with Nitrosomonas europaea. Soil suspension studies confirmed that the difference in nitrifying capacity among the soils was related to the initial nitrifier numbers whose activities were affected by the initial soil pH.