Importance of soil type on the release of nonexchangeable NH 4 + and availability of fertilizer NH 4 + and fertilizer NO 3 -

Nitrogen uptake from non-exchangeable NH₄⁺ byLolium multiflorum and availability of fertilizer NH₄⁺ and fertilizer NO₃^- were studied in pot experiments with three different soil types. The luvisol derived from loess released considerable amounts of non-exchangeable NH₄⁺ when cropped. In this soil fertilizer NH₄⁺ was only weakly fixed and was as available to the crop as fertilizer NO₃^-. The recovery of fertilizer NH₄⁺ was even higher than the recovery of fertilizer NO₃^-. In the fluvisol (alluvial soil) and in the cambisol (brown earth from basalt) N recovery was higher from NO₃^- fertilizer than from NH₄⁺ fertilizer. In these soils NH₄⁺ fertilizer was strongly fixed by 2:1 clay minerals and thus less available to the grass. Particularly in the basaltic soil the content of non-exchangeable NH₄⁺ was low and so was the release of nonexchangeable NH₄⁺. At the same time this soil showed the strongest fixation of fertilizer NH₄⁺. Release and refixation of fertilizer NH₄⁺ in the loess soil appears to be an important feature of this soil type with a beneficial effect on soil nitrogen turnover and availability.