Influence of different sources of nitrogen fertilizer on the value of soil nitrogen tests

Abstract

This paper examines the influence of nitrogen fertilizer form on the predictive ability of 12 soil nitrogen indices in a glasshouse study using ryegrass (Lolium perenne) as the test crop with six representative soils from southern New South Wales.The nitrogen fertilizers were added in the form of urea, ammonium sulphate and sodium nitrate. Total soil nitrogen and organic carbon were not satisfactory tests for predicting responses in dry matter and nitrogen uptake for the three forms of nitrogen. Total nitrogen was only significantly related to yield with urea (coefficient of determination of 75 per cent). Acid hydrolysis nitrogen was shown to be a useful test when relating plant yield response and nitrogen uptake with the urea form (coefficients of determination of 80 and 77 per cent, respectively). The anaerobic incubation test showed a significant relationship with plant yield with coefficients of determination of 80, 57 and 66 per cent for the urea, ammonium sulphate and sodium nitrate fertilizer forms respectively. Only with urea was this test significantly related (coefficient of determination of 68 per cent) to nitrogen uptake. The rapidly distillable nitrogen and the magnesium oxide distillable tests proved to be satisfactory for predicting the response of dry matter yield to the three forms of nitrogen. Coefficients of determination of 72 to 88 per cent were common. The magnesium oxide distillable test was also significantly related to nitrogen uptake with coefficients of determination of 78 to 85 per cent for the three forms of nitrogen. The data presented indicate that although rapid and single tests can give good predictions of yield and/or nitrogen uptake, care is needed in interpreting results where only one form of nitrogen is used.