Soil characteristics, cropping patterns, and use of organic resources in the coastal sandy area of Thua Thien Hue province, Central Vietnam

(eng) Sandy soils make up more than 205,000 ha in the coastal area of the North Central region of Vietnam. Thua Thien Hue province lying in the south of this region has predominantly sandy soils that cover almost 47,000 ha out of the 84,000 ha of cultivated land. Most of the population lives in the coastal area and people's livelihoods are largely dependent on crop farming in these inherently poor soils. The objectives of this study were to provide extensive information on the soil and organic resources (including agricultural and aquatic sources) in the coastal area of Thua Thien Hue province, to assess the fertilization capacity of various organic materials when applied at different rates to soils for different crops and to make a diagnosis of the balance of main plant nutrients within the farms to assess possible risks of depletion in soils or of release to the environment. Soil characteristics were measured from a survey of 300 cultivated plots in seven villages scattered in the coastal area. All these light-textured soils are acidic and are very poor in organic C. Consequently, the cation exchange capacity is very low. Soil properties are linked significantly to the different cropping patterns. Farmers in the studied area use wide diversify of organic materials for various purposes. Very large variations in the content of major elements (C, N, P, K, Ca, and Mg) are observed in the organic samples. Crop yield and soil fertility increased following the rate of farmyard manure and organic amendment application. Organic amendments brought higher profit for rice and peanut growers in coastal sandy area. The highest fertilization efficiency was observed for pig manure in case of growing rice and peanut and goat manure for sweet potato. The most frequently used species of aquatic plants by local farmers are Najas indica, Vallisneria spiralis, Potamogeton malaianus plus various algae species. Najas Indica and Hydrilla verticillata showed the highest fertilization capacity. At farm level, the N, P, K balances in 10 studied farms were positive. More detailed N-P-K balances in 9 field plots indicated that N balance was largely positive for rice and sweet potato plots. P imports exceeded P exports in all studied plots, whereas K balance was always negative in 4 cropping systems.