The Mekong delta is divided into seven divisions in terms of physiographic character relating to rice culture. They are Trans-Bassac Horst, Floodplain, Modern Delta, Coastal Complex, Broad Depression, Plain of Reeds and Western Coastal Zone, and some of them are further subdivided. Trans-Bassac Horst is a slightly uplifted geologic block with flat ground surface. Floodplain is a graven filled up by the Bassac and the Mekong's fluviation. Modern Delta is the downward extension of Floodplain and has deltaic network of rivers of great tidal range. Broad Depression is a blocked-in swamp behind Coastal Complex. These five divisions are important rice growing area of the delta. The last two, Plain of Reeds and Western Coastal Zone, are swampy areas on the margin of the delta and still waiting reclamation. The distributions of the divisions and their subdivisions are given in Fig. 3.

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 Vietnamese part of the Mekong delta is a wet terrain, being signified by the plentiful monsoonal rainfall with even distribution over the rainy months, and by low lying topography. The delta is, however, subdivided into two parts with respect to the hydrographic conditions, in which the alternation of dry and wet cycles are more pronounced in one region, and less significant in the other. These two classes are solely determined by physiography of the area. Trans-Bassac Plate, Flood Plain and Coastal Flat comprise the former region, and Modern Delta, Broad Depression and Lagoonal Swale belong to the latter category.
 A perfect adaptation of the rice growing methods to the given hydrographic conditions is the character that defines the rice culture of the delta. Rice growing does not rely on [table] the river water, if not at all, but it depends more on rainfall and its localized run-off which is controlled by micro-relief. At the present stage, abundant flow of rivers, creeks and canals which are often perennial being induced by the significant tidal fluctuation of water level in the sea and in the major rivers, are not playing the major role in supplying sufficient water for rice plants at their critical periods, although they are readily usable. However, the river water will have to be tapped to meet the requirement of water for TN rice which is rapidly extending its acreage as the early rainy season's crop as well as the dry season's crop. The possible types of water control and utilization are illustrated in Table 1,in view of giving the favorable conditions to the extended TN rice cultivation in the delta. The characteristics of the present hydrography in the delta are also summarized in the same Table in corresponding to the physiography and the possible types of water control.
 The most parts of the modification of the hydrographic conditions in the area can be achieved by the efforts of individual farmers and of communal level. The government will be required to participate in building major water works that are a prerequisite of those efforts made by communal human powers.

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 The material characteristics of 410 sample soils from tropical Asian countries have been described in terms of texture, clay mineralogy, and total chemical composition.
 Paddy soils of Indonesia, Malaysia, and the Philippines are generally heavy-textured, while those of Sri Lanka are sandy and poor in silt. Bangladesh soils are characterized by a high content of silt. Soils from Burma, Cambodia, India, and Thailand are more heterogeneous with respect to textural composition.
 There seems to be a general parallelism between clay mineral composition and total chemical composition and in terms of these two material characteristics the following three groups may be set up :
 7Å mineral dominant, high silica and low bases-Cambodia, Malaysia, Sri Lanka and Thailand,
 14Å mineral dominant, low silica and high bases-Indonesia and the Philippines,
 7-10-14Å minerals evenly contained, medium chemical composition-Bangladesh, Burma, and India.
 As the material characteristics are correlated with each other, separate discussions on individual items seem to be insufficient. In one of the papers to follow we will deal with these material characteristics collectively in order to set up a system for classification of paddy soil materials in tropical Asia.

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