Recovery of available soil nitrogen by annual fodder crops at Katherine, Northern Territory

In 1958-59, a field trial was carried out at Katherine, N.T., to compare the recovery. of available soil nitrogen by fodder sorghum, Sudan grass, and bulrush millet when grown after an all-grass ley, a legume ley, and a long period of fallow. After 6 years of grass, available soil nitrogen remained low throughout the cropping season at all soil levels to a depth of 5 ft, and nitrogen yield from the fodder crops averaged only 17 lb/acre. After 7 years of Townsville lucerne, available soil nitrogen was low at the start of the season, but soil nitrogen was quickly mineralized and, in the absence of a crop, was leached to give a peak concentration of nitrate nitrogen at 2–3 ft. All three crops intercepted this mineral nitrogen efficiently to give an average aboveground nitrogen yield of 75 lb/acre, approximately half the nitrate nitrogen available in the 5 ft soil layer on uncropped land at the end of the season. After 5 years of clean fallow, soil nitrogen had been mineralized and leached to a considerable depth, 260 lb of nitrate nitrogen per acre being accumulated in the 0–5 ft layer. Fodder sorghum and Sudan grass depleted soil nitrogen appreciably in the 0–2 ft layer, and gave an average above-ground nitrogen yield of 62 lb/acre. Bulrush millet depleted soil nitrogen throughout the 0–5 ft profile to give a nitrogen yield of 154 lb/acre. Bulrush millet is therefore regarded at Katherine as an outstanding crop for the recovery of deep accumulations of soil nitrate nitrogen and their conversion to harvestable fodder protein.