Consequences of banding nitrogen fertilizers in soil

Summary An account is given of the growth of wheat roots in the environment produced by banded urea and ammonium sulphate in both sterile (γ-irradiated) and non-sterile soil. In the non-sterile soil, (i) roots were absent from the fertilizer-affected zone (diameter about 10 cm) for both fertilizers at 2 weeks, presumably owing to unfavourable osmotic suctions there, (ii) roots proliferated enormously in the zone at both 4 and 8 weeks with ammonium sulphate, and (iii) roots were absent from the zone at both 4 and 8 weeks with urea (presumably owing to the large amounts of nitrite there), but they proliferated at the edge of the zone and completely encased it by eight weeks. In the sterile soil there was no marked proliferation of roots with ammonium sulphate, and with urea there was a region of intense proliferation surrounding a small zone (diameter 2 cm) which was devoid of roots. There were no universal correlations between the proliferations and the distributions of nitrate, nitrite, ammonium, pH, or (presumed) activity of nitrifiers. Nevertheless, ammonium seemed to be the main stimulus for the roots; it only failed to correlate with the root distributions if the concentration of nitrite was high or if the pH was below 4.