ESAM SALEH2, MATHIAS BECKER1, D. V. NI2, K.V. TINH3
1University of Bonn, Plant Nutrition in the Tropics and Subtropics, Germany
2University of Cantho, Viet Nam
In the rice-based systems of the Mekong Delta in Viet Nam, there is a trend towards replacing the traditional rice double cropping with a rotation of wet season rice and dry season upland crops (vegetables and tuber crops). However, in the prevailing acid sulphate soils, the build-up of excessive concentrations of exchangeable aluminium (Al3+) during the aerobic soil phase is strongly limiting upland cropping to few relatively Al-tolerant tuber crops that farmers grow on raised beds to enhance Al3+ leaching process. The use of organic amendments can reportedly increase soil pH and plant available P, while decreasing the concentration of active Al3+ throughout the formation of non-toxic Al-DOM complexes or the formation of humic complexes with Al and Fe. This research studied the performance of major tuber crops (cassava, sweet potato and yam) in relation to soil exchangeable Al3+ concentration and as affected by the application of locally produced biogas sludge. Experiments were conducted on three farmers? fields at the Hoa An research station of the University of Cantho, Viet Nam on a typical acid sulphate soil. Observation plots were laid out on raised beds and were categorised based on the initial exchangeable Al3+ content of the topsoil in classes of <10, 10-15, and >15 meq Al3+ 100-1 g. Biogas sludge was applied at 3 Mg ha-1 (dry matter) to tuber crops and compared with an unamended control. Soil total acidity and exchangeable Al3+ were determined from KCl extracts after subsequent NaOH and NaF titration, respectively. Dry biomass accumulation, tuber yield, and nutrient uptake by 12-week-old crops were compared. Biogas sludge tended to reduce soil exchangeable Al3+ concentrations but significantly increased the tolerance to given Al concentrations with higher tuber yield and P uptake in all tuber crops. However, Al-tolerant cassava showed stronger responses to amendment than Al"=sensitive yam. We conclude that in the absence of soil liming, the application of organic wastes can improve the performance of Al"=tolerant wile permitting the cultivation of more Al"=sensitive crop on acid sulphate soils. Further research aims at identifying most appropriate substrate types and application rates for specific acid soil conditions and crop tolerance levels.
Keywords: Active aluminium, Dioscorea sp., Ipomoea patatas, Manihot esculenta, Oryza sativa, Viet Nam