**2. Data**

The primary data of this study was collected in a field survey in two agro-ecological areas of the Mekong Delta of Vietnam in 2004. In the Mekong Delta, rice is the main crop. Farmers often apply mixed farming systems such as one-rice and one-fish crop, or two-rice and onevegetable crop to improve income and soil conditions. Consequently, farmers grow soybean once a year. The soybean crop is usually cultivated in January and February after the Win‐ ter-Spring rice crop and harvested in March and April. In this study, farmers who grow two-rice and one-soybean crop were selected for interviews.

The interviews were conducted in Can Tho Province, representing the lower reaches of the Mekong Delta, and An Giang Province, representing the upper one. The total sample of 113 farmers, of which 58 farmers were in Can Tho and 55 farmers in An Giang, was interviewed following a stratified random sampling procedure. The survey was conducted by the staffs of Can Tho University, Vietnam.
