An evaluation of indices of available soil nitrogen

Abstract

Various chemical and biological measures of available soil nitrogen were evaluated as potential soil tests by estimating their relationship with the dry matter production and nitrogen uptake of ryegrass (Lolium perenne) from twelve soils treated with several rates of nitrate fertilizer. The response of the ryegrass to fertilizer was similar for all soils, and did not show any relationship with the soil tests. However, the mean dry matter yield and mean nitrogen uptake for all treatments were significantly related to some of these tests. Generally, soil nitrogen indices that measured total mineral nitrogen at planting or included this value with an estimate of the potentially available organic nitrogen gave the higher correlations. Incubation nitrogen values gave lower correlations. The best index of available soil nitrogen was based on a multiple regression involving total mineral nitrogen and alkali-permanganate soluble nitrogen. It accounted for 92 and 96 per cent of the variation in dry matter yield and plant nitrogen uptake respectively.