Description of the Study Villages: The research was conducted in Bilpabla and Lebubunia villages, which are located about 7 and 5 kilometers (km) west from the headquarters of the Khulna district, respectively. Bilpabla village was selected because it is one of the typical villages in RPG farming. The prawn-producing environment/factors (such as water quality, weather, rainfall, temperature, and soil quality) of Bilpabla are very similar to other villages where RPG farming is being practiced. Lebubunia village was selected because its cropping pattern is similar to Bilpabla prior to RPG farming. Moreover, Lebubunia and Bilpabla are neighboring villages. The altitude of rice fields in Lebubunia is slightly higher than those used for RPG farming in Bilpabla. As a result, farmers in Lebubunia village could not convert their paddy field into gher farms. The farmers in Lebubunia village mainly grow MV paddy twice a year. The climate of the study area is tropical monsoon-type with wide seasonal variations in rainfall, moderately warm temperatures, and high humidity. The rainy season formally starts in June and continues until October, when the monsoon air stream sweeps in from the Bay of Bengal. During this time, Bangladesh receives heavy rainfall and most places receive more than 2,538 millimeters (mm). In 2001, rainfall was between 1,937 to 2,949 mm (BBS 2005). The Khulna district annually receives, on average, 1,696 mm with a wide range of 1,159 to 1,994 mm. In 2005, the average monthly humidity was 77 percent, which ranged between 61 percent (March) and 84 percent (August). In 2006, the average monthly humidity was 79 percent, ranging between 66 percent (March) and 87 percent (July). The mean temperature was 270C in 2005 (BBS 2005). Soil Sampling To assess the impact of the RPG farming system on soil quality of MV paddy fields, soil samples were taken from RPG and YRMV paddy farms. A total of 40 farmers (20 farmers from RPG and 20 from YRMV) were randomly selected from two study villages. Each of the sampled 20 RPG and YRMV paddy farmers owned 30 farm plots. Soil sample collection procedures were conducted in two phases. The first phase was conducted at the beginning of paddy transplanting (26-29 December 2005 in Bilpabla and 30-31 December 2005 in Lebubunia) and at harvest time (22 April 2006 in Bilpabla and 27 April 2006 in Lebubunia).
Each soil sample was a mixture of nine sub samples that were collected from nine different places in a particular farm plot. The soils were taken from a depth of 0-15 centimeters (cm), which represented the cultivated topsoil. After collecting soils, the sample soils were placed in polythene bags and sun-dried. After drying, the soil samples were again placed in polythene, labeled numerically and sealed for transportation to the laboratory for testing. As the study aimed to explore the impact of RPG farming on soil quality (fertility), the samples as well as plot numbers were identified using the same numerical value at the beginning of paddy transplanting and during harvesting. Soil and Analytical Methods: Soils were air-dried, ground, and sieved with 0.5 mm mesh. Some soil chemical properties were analyzed by routine methods: H2O pH (H2O, 1:2.5), pH (KCl, 1:2.5), EC (1:5), total carbon and nitrogen by the combustion method (C-N analyzer, Sumigraph NC-1000), exchangeable cations extracted with ammonium acetate, phosphorus absorption coefficient, available P by the Troug method, hot-water extractable NH4 -N and B, available zinc and copper extracted with 0.1 mol/L HCl, and easily reducible Mn.