Pot experiments using barley and ryegrass on 12 soils with different textures,organic matter,total nitrogen contents and C:N ratios were conducted and mineralized and mineralizible nitrogen in soils were determined by different methods in order to investigate the relationship between nitrogen uptake by plants and nitrogen in soils determined.The results obtained showed that N uptake by plants was well correlated with NO₃-N in soils extracted either by saturated CaSO₄ or KCl,but less correlated with NH₄⁺-N while the soils N mineralized after a 2-week aerobic incubation was closely related to plant uptake nitrogen,the soils N mineralized after a l-week waterlogged incubation did not reflect the nitrogen supplying capacities of the soils in non-waterlogged condition.The soil N released by boiling KCl was found to be highly correlated with the N in plants,and also with the soil N mineraized after aerobic incubation.Both aerobic incubation and boiling KCl method have the same value in evaluation of soil nitrogen supplying capacities.