A study was conducted to evaluate the physical properties of soil before and after adding green manure crops to the soil by pot experiment. Silty clay loam soil from Bajoa soil series of Bangladesh was collected by proper method and kept in eighteen pots. The soil type was endoaquepts (Rahman, 2005). The soil was stored for about 2 weeks before sieving (2 mm) and mixed it extensively with a shovel in 18 pots. Pots were filled to 12 cm of the top and soil moisture was maintained at approximately field capacity (0.33 g water g soil-') by weighing individual pots and replacing lost water when necessary and room temperature was maintained in the pots.
Pot experiment and location of sampling A long-term pot experiment was initiated at the Soil Science Discipline, Khulna University, Khulna, Bangladesh during 2009. It was laid out with five treatments and one control experiment. Each experiment was carried out in three pots as replication to avoid experimental error. The pot size was 12 cm x 8 cm. The experimental soil from Bajoa soil series (Typic Endoaquepts ) was Silty clay loam, cation exchange capacity or CEC was 23.73 me/100g and Organic matter was 5%.
Cultivation and application of Green manure crops Five green manure crops were pigeon pea (Cajanus cajen), mung or green or golden gram (Vigna radiate), common bean (Phaseolus vulgaris), cowpea (Vigna unguiculata), yard long bean (Vigna sesquipedalis) were grown in five pots. One pot was used as control experiment where no green manure crop was grown. 10 gm seeds were broadcasted in each pot to a loosely tilled, fine seed bed for satisfactory germination during the last week of September 2009. Shallowly seeds were incorporated with garden rake and leaf rake to a depth of 0.25 to 0.75 inch deep. After 7 days seedlings established and plants were allowed to grow for 30 days. Before incorporation green manure crops were cut and same amount of green manure crops (300g) from each pot was measured by an electrical balance to ensure nutrient release from each experiment from same amount of green manure application at the flowering stage. The green manure crops were thoroughly mixed to the soil by a shovel; soil was watered daily for hastening decomposition and was kept for 6 weeks for proper decomposition.
Collection of soil samples Green manure crops were well mixed to the soil for three weeks. After three weeks the color of the soil was changed and there was not a portion of fresh plants that means the plants were completely decomposed. Then the soil samples were collected by proper method from each pot for laboratory analyses. Similarly samples were collected from control experimental pots for the same analyses.
Preparation of Soil Samples The collected soil samples were air-dried by spreading on separate sheet of papers after it was transported to the laboratory. After drying in air, the larger aggregates were broken gently by crushing it in a wooden hammer. A portion of the crushed soils was passed through a 2.0 mm sieve. The sieved soils were then preserved in plastic bags and labeled properly. These were later used for various physical and physico-chemical analyses. The physical properties and physico-chemical analyses of these collected soil samples were carried out in the laboratory of the Soil Science Discipline, Khulna University and SRDI Dhaka.
Soil physical Analysis Bulk density was measured by Core method (Blake and Hartge, 1986a). It was determined by pycnometer method as described by Blake and Hartge (1986b). Total porosity of soil was calculated from the data of particle density and bulk density using the formula as described by Strickling (1956). % porosity = ( I- Bulk Density/ Particle Density) X 100 Particle size analysis of the soils was done by hydrometer methods (Day, 1965). Textural classes were determined using textural Mashall triangle digram (Brady and Weil, 2002). Soil moisture was measured by oven dry method (Black, 1965). The saturated hydraulic conductivity of water in soil (or the intrinsic permeability of the soil) was measured by constant head method (Klute, 1965). The hydraulic conductivity was calculated using the equation which is also known as Darcy's equation (Hillel, 1980).
Soil Physico-chemical Analysis The CEC of the soils were determined by extracting the soil with 1N KCL (pH, 7.0) followed by replacing the potassium in the exchange complex by 1 N NH4OAc (Ruhlmann et al., 2006). The displaced potassium was determined by a flame analyzer at 589 nm (Jackson, 1967). Soil pH was determined electrochemically with the help of a glass electrode pH meter as suggested by Jackson (1973). The ratio of soil and water was 1:2.5 as suggested by Jackson (1962). The electrical conductivity of the soil was measured at a soil: water ratio of 1:2.5 by the help of EC meter (USDA, 2004).
Statistical analysis This experiment was conducted in one factor completely randomized design with three replications. A total of five treatment including control was used. The factor was green manure. So, total pot number was eighteen. Data were subjected to a one factor (different green manure treatment) analysis of variance (ANOVA) to determine if the materials mineralized differently with several green manures with F test at 5% level of significance with the statistical package of MINITAB (Ryan et al., 1985).