A simple, precise and rapid alkaline hydrolysis method for determining nitrogen (N) availability index of soils is described. It involves direct steam distillation of 1 g field-moist soil and 1 M KOH, NaOH, LiOH or phosphate-borate buffer (pH 11.8) and the amount of NH4+-N released trapped in boric acid and its concentration determined successively every 5 min for a total of 40 min. The cumulative N hydrolyzed was fitted to a hyperbolic equation to determine the maximum hydrolyzable N (Nmax) and the time required to hydrolyze one-half of Nmax (Kt) by linear regression of the transformed data. First-order equation was also used to estimate the potentially hydrolyzable N (No), hydrolysis rate constant (k) and the time required to hydrolyze one-half of No (t1/2). Results showed that for each soil and reagent, Nmax and No values were similar, but differed significantly among soils, suggesting differences in the chemical nature or reactivity of organic N in the soils. In general, Nmax and No values ranged from 401 to 1667 mg kg-1 soil and accounted for 12-56% of total organic N in the soils. The Kt values ranged between 15 and 30 min. Among the reagents tested, KOH and NaOH showed the best promise for estimating the total hydrolyzable organic N pool in the soils. The Nmax and No values were significantly correlated with the amounts of N mineralized in two weeks under aerobic and anaerobic conditions at 30 oC, N released by 2 M KCl extraction at 80oC for 20 h, and the initial NH4+-N present in the soils. We concluded that direct steam distillation of soils with 1M KOH or NaOH offer a quick and precise mean for estimating the potentially mineralizable organic N pool and availability index in soils
