ESAM ADAMS SALEH1, FOLKARD ASCH2, MATHIAS BECKER2
1University of Bonn, IPE, Faculty of Agriculture, Germany
2University of Bonn, Plant Nutrition in the Tropics and Subtropics, Germany
Agricultural land use in the Mekong Delta of Vietnam is dominated by intensive irrigated rice double and triple cropping systems on both alluvial and acid sulphate soils. A generally observed decline in productivity is linked on the alluvial soils to low N use efficiency and a low soil organic matter content. On the acid sulphate soils, rice production is constraint by nutrient imbalances such as iron toxicity and P deficiencies. Faced with the productivity declines, farmers increasingly diversify their cropping system. The most prominent diversification strategy is the replacement of the dry season rice by high-value horticultural crops grown under upland conditions. While short-term economic benefits may arise from these new systems, upland cropping on the alluvial soils is likely to further accelerate the mineralisation of soil organic matter and hence to exacerbate the problems of declining soil N supply. On the acid sulphate soils, an aerobic soil phase can result in Al toxicity and exacerbate the problem of P deficiency.
Organic substrates from decentralised waste/water management are widely available but are unlikely to be applied to lowland rice. However, high value crops that favourably respond to substrate addition will justify farmers' investment in organic fertiliser strategies. Organic substrates may help alleviate the problems related to a low soil organic matter content and may help buffer toxic Al. During the dry season of 2003/2004, the effect of the application of various types and rates of locally available waste products on the growth and nutrient uptake of cereal, tuber and vegetable crops was evaluated on both an alluvial and an acid sulphate soil site. The results from this research are seen to guide future substrate use in the rice-based systems in the rural areas of the Mekong Delta, through the development of a decision tool, providing site- and system-specific indications as to the expected effects that a given substrate/application rate is likely to achieve on a range of possible target crops.
Keywords: Nutrient cycling, organic waste, Oryza sativa, Vietnam