Developing management packages for acid sulphate soils based on farmer and expert knowledge : field study in the Mekong Delta, Viet Nam

Effective interaction of farmers' expertise and expert knowledge has been a special point of attention for this study. The objectives of the study were to describe the process of interaction between farmers and experts in improving the use of acid sulphate soils and to point out difficulties encountered. Actual conditions for four major areas were described including variabilities. Four representative areas: Tan Thanh, Tri Ton, Phung Hiep, and Hong Dan in the Mekong Delta, Viet Nam were selected for this study. Physical conditions were defined in terms of soil properties, as reflected by soil classification, and hydrological conditions were defined by climate data including flooding. Methods for land improvement in different areas were first defined by describing and analyzing measures taken by farmers, and by next developing schemes for improvement, using expert knowledge. These schemes were expressed by decision trees as a part of expert systems. But many questions were left and a series of experiments was designed and executed to answer those questions that could not be answered by experts.Four study areas were selected. Tan Thanh and Tri Ton (in the Plain of Reeds and Long Xuyen Quadrangle, respectively) have acid sulphate soils with sulphuric horizons within 50 cm from the soil surface, and deep flooding (>100cm) during the rainy season. The Phung Hiep area with moderately acid sulphate soils is not deeply flooded and is located in the fresh water area of Ca Mau peninsula. The Hong Dan area contains mainly moderately and strongly acid sulphate soils with salt water intrusion during the dry season. The main agricultural problems in these study areas, where most farmers are poor, arise from the fact that most modem practices or new cropping patterns are applied incompletely and at unequal levels. Also unfavourable soil acidity coincides with high flooding depth or salt water intrusion, poor infrastructure for irrigation and drainage and shortage of capital for farm production. Furthermore, price fluctuations of farm products are very high, which makes economic production very difficult.A study at farmers' level shows that the history of fanners' land use, including their soil-water-crop management practices clearly indicates changes of land use over time and the important role of expert knowledge in initiating these changes. Developments also illustrate the positive effects of the creative interaction between fanners and experts in developing innovative management systems. Ten, four, sixteen and eleven actual land use types, including land and water practices, were described for the Tan Thanh, Tri Ton, Phung Hiep, and Hong Dan areas, respectively. Water-soil-crop management practices such as construction of canals and infrastructure for irrigation, construction of high raised beds for cultivation of upland crops, and construction of a ditch system for the prawn/shrimp-rice system were essential parts of these land use systems.Studies by experts paid more attention to the cultivation of crops, mainly rice, and the associated water management practices. Sixteen promising land use types were defined for a land suitability classification study based on present land use systems of four study areas. Double cropping of modem rice varieties is present in all study areas. Eight land qualities were identified for a farm-level land evaluation. Land quality "(potential) soil acidity" is important for all land use types selected. Land qualities: "flood hazard", "drought hazard", "salt water intrusion", and "fresh water availability" are important for double cropping of modem rice varieties. Land qualities: "potential for daily tidal flooding and drainage" are critical for raising of prawn/shrimp. For land evaluation at farm level, decision trees were developed and used as an decision support system. Decision trees were made for each promising land use type. Using decision trees as a decision support system for land evaluation study at the farm level was shown to be helpful to identify optimal management decisions. Conditions for improvement of suitability were identified and visualized in those decision trees, in terms of improved soil and water practices based on farmers' expertise and expert knowledge.While developing these schemes, it became clear that many unanswered questions on land use requirements and water management practices were encountered. These were formulated and field experiments were carried out to answer the questions. Some conclusions were drawn from these experiments: (1) For yam cultivation in acid sulphate soil areas with a deep flood in the wet season, application of a mulch layer on top of the raised bed resulted in a 46% higher yield as compared with urimulched plots; (2) the use of fresh water in an irrigation frequency of 20 days, when available during the dry period of the cropping season, increased yields of yam by 31%. (3) Rolled-carpet raised beds gave higher yields as compared with mixed-raised beds. (4) A relatively high yield of yarn can be obtained by application of N 120 -P 60 -K 60 as is already being practised by farmers in the area or N 80 -P 160 -K 100 . Higher fertilizer applications give no better yields. Staking yam vines is of limited importance and does not increase yarn production. A planting density of 40 cm. x 50 cm is proposed for yam cultivation. (5) For pineapple cultivation in areas with dry season salinity, supplementary irrigation in the early dry season had no effect on pineapple yield. (6) Production was higher on high raised beds as compared with traditional lowraised beds because excess water in the rainy season. Old raised beds gave significantly higher yields as compared with younger raised beds because of lower acidity due to longer leaching by rain. (7) A yield increase of pineapple of about 20% can be obtained by a combination of high P and K fertilization ( 10N-7g P 2 O 5 -10K 2 O/plant). And (8) application of the zero-tillage technique was profitable as it shortened the duration of double cropping of Winter-Spring modem rice followed by Summer-Autumn rice in areas with long duration flooding. These accumulated results provided quantitative data to characterize land qualities and land use requirements relating to fresh water availability, soil acidity, and flooding hazard . Also data for agronomic practices were obtained by experiments to be used for formulating optimal management packages.Under different physical conditions, optimal management packages on acid sulphate soils were finally formulated for the four study areas in. These optimal management packages were based on the farmers' practice, expert knowledge, results of land evaluation study at farm level and accumulated results obtained by field experiments. There are many options including various cultural, soil and water management practices for the different physical conditions encountered. Double modem rice cropping, and growing yam, pineapple, and sugarcane, are suitable in areas where fresh water is available during the dry season. In areas with salt water intrusion during the dry season, land use types incorporating shrimp are more profitable. Melaleuca spp. is most suitable for remote and unfavourable areas for crop growth as a means for reforestation in order to protect natural environments. These results provides a good start to obtain description of realistic and acceptable systems. In the future, these description can also be used to identify areas of additional research by measurement or by simulation modelling.