The mineralization coefficient (M.C.) (the amount of nitrate nitrogen formed in one season in sift, by bare fallowing the soil, as a percentage of the amount of organic nitrogen in the topsoil at the onset of the season) was measured on two soils at Katherine, N.T. The determination of M.C. is regarded as useful in evaluating the 'true' decomposition constant of a soil under annual cropping conditions. Its relation to the 'apparent' decomposition constant is discussed. Tippera clay loam had a mean M.C. of 4.80 per cent, and the fluctuations around the mean were positively related to amount of rainfall. The M.C. for Blain sand dropped from 12.5 per cent in the first year of cultivation to 5.0 per cent in the third and fourth years. The data indicate that the initial high nitrogen yields on the sandy soil will in time decline to a low level unless a legume crop is incorporated in the cropping system.
