LIME EFFECT ON SOIL-N AVAILABILITY INDEXES AS MEASURED BY PLANT UPTAKE

Lime application to acid soils usually increases N mineralization but little is known about how it affects the N determined by methods that assess organic N availability. One incubation and three chemical metho ds were compared in twenty samples of unlimed or limed soils in a pot experiment with maize (Zea mays, L). The N availability methods includ ed the NH4+-N released from soils by: a) anaerobic incubation for 7 da ys at 40-degrees-C; b) 2 mol/L KCl at 100-degrees-C for 4 hours and di stilled with MgO (hot KCl); c) this same procedure but distilled with 5 mol/L NaOH (hot KCl-NaOH); and d) 30% v/v H2O2 and MnO2. In addition , inorganic N, total N and organic C were also determined in the soil samples. Readily available inorganic N presented the highest correlati on coefficient with N uptake by maize but anaerobic incubation, hot KC l, hot KCl-NaOH, and total N were also good predictors of soil N avail ability. The H2O2/MnO2 procedure and organic C produced inconsistent r esults. The amounts of N extracted by the methods tested were little a ffected by lime application. Multiple regression analysis showed that, among the methods that assess mineralizable organic N, the hot KCl me thods accounted for most of the variation in N uptake by maize.