SOIL ACIDIFICATION UNDER SOME TROPICAL AGRICULTURAL SYSTEMS .1. RATESOF ACIDIFICATION AND CONTRIBUTING FACTORS

A paired site (developed v. undeveloped) approach was used to calculat e acidification rates in several agricultural systems of tropical and subtropical Queensland. The systems considered were summer crop-winter fallow, grass or grass-legume pastures for hay production, tobacco, s ugarcane, table grapes, and bananas. Mean acidification rates varied f rom -2.4kmol H+/ha year for tobacco to 34.2 kmol H+/ha year for banana s. Acidification rates were higher than for comparable systems in temp erate Australia. Subsurface acidification occurred under all systems, and was particularly severe under bananas despite the surface applicat ion of at least 2.5 t lime or dolomite/ha . year. As bananas can be co nsidered to be a generic perennial horticultural system, subsurface ac idification may be a widespread problem in such systems despite surfac e applications of amendments. There was a wide range in acidification rates within a particular agricultural system, suggesting that managem ent practices can be manipulated to reduce acidification. As the N cyc le terms were the major contributors to the acidification under croppi ng systems, N fertiliser management is likely to be the most critical acidification factor.