Nitrogen uptake of cereals grown on sandy soils as related to nitrogen fertilizer application and soil nitrogen fractions obtained by electro-ultrafiltration (EUF) and CaCl2 extraction

Abstract

Field trials were carried out to examine to what extent nitrogen fertilizer and various soil nitrogen fractions contribute to the nitrogen uptake of cereals. The study focused on soil N tests which are easy to handle, in order to improve their acceptance by farmers. The soil nitrogen fractions estimated were NH4+, NO3−, and organic nitrogen extracted by electro-ultrafiltration (EUF) or CaCl2 solution, respectively, sampled at various times in early winter and spring. The investigation comprised 36 different sites; all sandy soils derived from mottled sandstone or alluvium. Nitrogen fertilizer rates were 0, 40, 80, and 120 kg N ha-1 applied in the form of calcium ammonium nitrate. The relationship between nitrogen fertilizer rates and nitrogen uptake was weak (r2 = 0.36). The coefficient of determination (r2) for the nitrogen uptake by the crop was improved to 73% if, in addition to the N fertilizer rate, the inorganic N (NH3− + NH4+) present in the soil profile (0–90 cm) in spring was included in a multiple regression. Soil sampling to a depth of 90 cm in early spring is laborious work and therefore empirical models of « available N » were evaluated based on soil samples taken from the top, soil layer (0–30 cm) in early winter. Satisfactory coefficients of determination for the relationship between nitrogen uptake of crops v.s. soil test were found for soil tests in which the samples were taken from the top layer (0–30 cm) in December and in which, besides inorganic nitrogen, organic nitrogen was included in the regression. The coefficients of determination thus obtained were 0.65, 0.68 and 0.73 for the EUF-Norg, the CaCl2-Norg, and total soil nitrogen, respectively. We believe that such a soil test method based on soil sampfing from the upper soil layer in early winter will be more readily accepted by farmers than the Nmin method.