Abstract:
The surface horizons of 40 profiles of the Mobilong clay hare been analysed for total nitrogen and pH, and the analyses compared with similar ones made in 1929. Soils which were originally high in nitrogen have declined, those with medium levels hare remained substantially the same, arid those which were originally low have increased in nitrogen content. The Mobilong clay thus appears to be approaching an equilibrium nitrogen content which under the present land use pattern of irrigation, drainage, and management approximates to 0.50 per cent. nitrogen. It seems probable that further declines will occur in the nitrogen content of those soils which are greatly in excess of this value, though at a slower rate than formerly. On the other hand increases in the nitrogen content of those soils with values less than this should occur. The implications of these changes, both in relation to future land use practices and in relation to general concepts regarding the effect of grass-legume pastures on soil nitrogen levels, are discussed. Changes in pH were found to ha essentially in inverse relation to the changes in nitrogen. Thus soils which have decreased in organic nitrogen have increased in pH, and vice versa. The probable effect of flooding and subsequent deposition on long term changes in the Mobilong clay is discussed.