Conservation of the natural land resources of the Mekong delta

Physical fertility of typical Mekong delta soils (Vietnam) and land suitability assessment for alternative crops with rice cultivation

Most of the soils in the Mekong delta, Vietnam are formed and developed during the Holocene period. The first Viet people came to reclaim and exploit this plain at the beginning of the 17th century. As a result, in the middle of the 19th century, the Mekong delta had become the largest region of agricultural production, essentially rice produce for marketing purposes. Around 1980 three rice crops in a year were applied. That system continued to increase with time within the recent past 10 years. This land use system has mainly enlarged on the areas along Hau and Tien river.

Nowadays, in the Mekong delta, rice yield and production in the developing and well developed alluvial soil groups tend to be stable. However, there is a tendency to gradual decrease year to year, although chemical fertilizers and other amendments were intensively applied. This study is carried out in order to conserve the natural land resources of the Mekong delta. The study focuses and evaluates mainly physical soil characteristics which are related to physical soil degradation of the selected major soil groups in the area of mono and intensive rice cultivation as Chau Thanh (Tra Vinh province), Vung Liem (Vinh Long province), Cai Lay (Tien Giang province), Cao Lanh (Dong Thap province), Tan An (Long An province), Phung Hiep (Can Tho province), Vinh Chau (Soc Trang province), O Mon (Can Tho province), Tinh Bien (An Giang province), and Moc Hoa (Long An province).

A quantitative land evaluation is carried out for some alternative crops (soybean, mungbean, maize and cotton) in order to change and develop a new land utilization type for sustainable agricultural production. Results show that soil compaction, the so-called “plow pan” has already existed between 20-40 cm soil depth and its thickness varies from 35-50 cm in the intensive rice cult!ivation area. Soil resistance penetration tends to increase from 40 cm to 100 cm in the soil profile and its value varies in the range of 0.8-1.2 MPa. Infiltration rate is low in the topsoil (average 0.8 cm/hr) and very low in the subsoils. Surface hardsetting and subsoil plinthization exist in the Degraded Grey Soils and sodification is found in the area of saline intrusion. Soil aggregate stability over 10 study locations is low and it is strongly influenced by soil organic matter and soil texture. The value of pF2-pF2.5 is approximately representing the matric head at field capacity. The volumetric water content at wilting point is 0.05 cm_/cm_ and 0.25 cm_/cm_ for loamy sand and heavy clay textured soils respectively. About 62 % of the cases, in rice intensive area, show a cumulative available soil water storage between 100-120 mm water at 0-50 cm soil depth, and it will be mainly lost by evaporation. Due to the intensive rice cultivation, local farmers usually irrigate the fields year around. In combination with a shallow low water table level (50-80 cm from soil surface) in the rice area, the soil water storage in the subsoils is not depleted to the wilting point even in the dry season and that happens on most of the study locations.

Muscovite is a dominant clay mineral in recent alluvial soils and kaolinite is a main soil mineral in old alluvial soils and soils derived from hard rock of the mountainous area. Soil compaction, soil hardsetting, plinthization, nutrient depletion, sodification and acidification (from oxidation of pyrite) are the main soil degradation types found in the different study locations. Soil compaction tends to increase in the existing mono-rice cultivation and it is a main soil constraint influencing to the crop yield and soil productivity in future.Soybean represents the best perspective crop to be alternated in the area of intensive rice cultivation and also suitable for the other areas of the study locations. However, few soil constraints need to be reclaimed and improved, the crop yield can reach more than 2 tons/ha. Cotton and maize can be planted in saline and alkaline soils. Organic matter, balanced mineral fertilization, soil preparation under the proper moisture condition and alternative land use application are the necessary activities to be done to conserve soil resources, to improve the rice yield and to develop a sustainable agricultural production in the Mekong delta.

The obtained results will help and support to the local farmers and even local government, specially the department of agriculture and rural development, in the Mekong delta, Vietnam to select and to develop the adapted land utilization types and the proper soil management packages for sustainable agricultural production at the present time and in future.