|Title||Soil knowledge for farmers, farmer knowledge for soil scientists : the case of acid sulphate soils in the Mekong delta, Viet Nam|
|Author(s)||Mensvoort, M.E.F. van|
|Source||Agricultural University. Promotor(en): J. Bouma, co-promotor(en): V.T. Xuan. - S.l. : Van Mensvoort - ISBN 9789054855552 - 135|
|Department(s)||Laboratory of Soil Science and Geology
|Publication type||Dissertation, internally prepared|
|Keyword(s)||zure gronden - kattekleigronden - landevaluatie - grondvermogen - bodemgeschiktheid - vietnam - acid soils - acid sulfate soils - land evaluation - land capability - soil suitability - vietnam|
Half the Mekong delta in Vietnam, i.e. around 2 million hectares, suffers soil related problems due to acid sulphate soils. These soils generate sulphuric acid due to the oxidation of pyrite after aeration. Pyrite is most easily formed in tidal swamps. Human interference through land drainage is the most important way in which the acidification takes place. The processes of pyrite formation, of acidification and of the solution chemistry of these soils have been well explained (e.g. van Breemen 1976, Dent 1986). The translation of this knowledge into practical recommendations for farmers is still problematic.
The Can Tho University (CU), the only institution for higher education in the delta, saw it as its duty to assist the farming community on these soils and come up with practical recommendations for improved agricultural use. From 1980 - 1992 a project of co-operation between CU and the Wageningen Agricultural University, particularly the Soil Science Department, where specialised knowledge on the processes of formation and the chemistry of these soils was present, was carried out. Objectives of the project were (1) to train CU staff in acid sulphate soils formation and survey; (2) to carry out research for improved management of these soils and (3) to equip CU with the necessary facilities for such a research and training programme.
The aims of this thesis are:
1 . to review the recent literature on acid sulphate soils in order to screen themethodological developments for survey and identification, for chemical and physical support in field and laboratory, for simulation and mathematical modelling of the soil processes, for management and land use and for acid sulphate land evaluation;
The review of the recent literature (chapter II of this thesis) shows how difficult the diagnosis of acid sulphate land is. Some indications can be expected from the vegetation or the drainage patterns. Coastal wetlands, inland marshes and swamps and mine spoils are the land forms where potential acid sulphate soils occur. Surface water usually gives a first warning of acidification having taken place by an oily skin at the surface (iron) or suspicious water clarity (acidity and aluminium). Identification of acidified soils is usually easy through the appearance of pale-yellow jarosite mottles, but is much more difficult in potential acid sulphate soils (pyrite is invisible) or in acidified soils without jarosite. Field tests such as oxidation by hydrogen peroxide for potential acidity, or the azide-soap and the red lead paint for sulphide may help in identification. Moist incubation for prolonged periods is recommended to make sure. Soil survey is difficult because of land inaccessibility, high spatial variability of the diagnostic characteristics and the need for specialised laboratory assistance to identify acid sulphate components. The dynamic modelling of acid sulphate soils received much attention in recent years and resulted in sophisticated models encompassing the processes but inevitably requiring many detailed data for model application. Regardless of all research efforts many management decisions have still to be taken after the problems have already become manifest. Local farmers have, particularly in Southeast Asia, succeeded in adapting to the situation and they have developed interesting management systems for cultivation of rice, shrimp and fish, yams, pineapples, sugar cane and fruit trees.
The proceedings of the three last conferences dedicated to acid sulphate soils show that most knowledge is communicated specialists to specialists; that some attention is paid to generating knowledge suited for the needs of the local experts such as soil survey methodologies and for the needs of the farmers and the extension workers such as fertiliser recommendations, on-farm water management strategies and crop choice. Only in a few cases, however, the indigenous knowledge of the farmers has been used to its full potential. It is particularly in the Mekong delta that the local farmers' knowledge has played a dominating role in acid sulphate soils research for practical application.
The VH 10 project of CU and WAU profited from the strong embedding of CU, in particular its Faculties of Agriculture and of Water Management and Rural Engineering, in the rural society of the Mekong delta. The network of contacts with provincial and district agricultural services, with state farms and with private farmers could be used for gathering local farmer knowledge, for experimentation in line with farmers experience, for extrapolation of the findings to other locations in the delta and for knowledge dissemination through workshops and TV programmes.
The research set-up of the project changed from a top-down technology driven approach in the early and mid-eighties to a system of a balanced knowledge exchange between farmers, local experts and soil/water specialists in the late eighties and early nineties. This approach generated much more successful recommendations for farmers than the top-down approach.
The project had a number of deviating organisational aspects which might be interesting for application elsewhere: (1) no permanent foreign staff present in Vietnam; (2) large share of the responsibility for the execution of the project left with the Vietnamese counterparts; (3) long term continuity of the project with the same staff (twelve-and-a-half years); (4) mutual interest in project objectives by both partners; (5) applying the principle of learning from each other.
The project profited from the political change of economic liberalisation in Vietnam in the second half of the eighties since recommendations for improved use of acid sulphate soils, emphasising small-scale development, could much better applied by small farmers than by large scale state farms.
Two evaluation studies of the acid sulphate land in the Mekong delta (chapter V) showed that fresh water availability is the most important constraint to farming. Moderately and slightly acid land, characterised by a sulphuric horizon deeper than 50 cm, of which large tracts are present in the Mekong delta, can become suited for rice and tolerant upland crops such as yam, cassava or sweet potato when fresh water is available. However, severely acid land will only become marginally suited and should therefore not be given priority for development. Well constructed raised beds can improve the land and makes moderately acid land highly suited for pineapple and sugar cane, provided the coastal salt intrusion in the dry season is short (less than 3 months) and the annual Mekong river flood does not exceed a depth of about 60 cm. Both evaluation studies describe in detail farmers' practices of an unexpected wide variety of land use types and thereby emphasise the main focal point of this thesis which is the major input of farmer's expertise in developing innovative management schemes for these problem soils.