Pot experiment was conducted to observe the effects of green manures on the chemical properties of soil. The seived soil was well mixed with a shovel in 18 pots. Pots were filled with five kilogram soil up to 12 cm of the top and soil moisture was maintained at approximately field capacity (0.33 g water g soil -1 ) by weighing individual pots and replacing lost water when necessary and room temperature was maintained uniform during the experiment.
Pot experiment and location of sampling: A long-term pot experiment was initiated at Soil Science Discipline, Khulna University, Khulna, Bangladesh during 2009. It was laid out with five treatments including control have three replication. Each pot was 12 cm by length and 8 cm by wide. The experimental soil from Bajoa soil series (Typic Endoaquepts ) had pH 8.67, electrical conductivity 2.76 dS m-1 , total nitrogen 0.094%, phosphorus 6.5µg/g soil, potassium 0.27 meq/100g soil, sulfur 26.88µg/g soil, boron 0.36µg/g soil, calcium 10 me/100g, magnesium 5.03me/100g, zinc 0.47µg/g soil, cation exchange capacity or CEC 23.73 meq/100g .
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 of seedlings establishment 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 3 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. Soil samples were then collected from each pot including control for laboratory 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. The larger aggregates of the samples were broken gently by crushing it with a wooden hammer. A portion of the crushed soils was sieved with a 2.0 mm sieve. The chemical analyses of these collected soil samples were carried out in the laboratories of the Soil Science Discipline, Khulna University. Khulna and SRDI, Dhaka.
Soil chemical Analysis: Soil pH was determined electrochemically with the help of a glass electrode pH meter as suggested by Jackson (1962). The electrical conductivity of the soil was measured at a soil: water ratio of 1: 5 by the help of EC meter (USDA, 2004). The CEC of the soils were determined by extracting the soil with 1N KCL (pH, 7.0) followed by the replacing the potassium in the exchange complex by 1 N NH4OAc. The displaced potassium was determined by a flame analyzer at 589 nm wavelength (Jackson, 1962). Organic carbon of soil sample was determined by Walkley and Black’s wet oxidation method as outlined by Jackson (1962). Organic matter was calculated by multiplying the percent value of organic carbon with the conventional van-Bemmelene’s factor of 1.724 (Piper, 1950). Total nitrogen of the soil was determined by Micro-Kjeldahl’s method following H2SO4 acid digestion as suggested by Jackson (1962).
Total content of Zn of the sample was determined by Atomic Absorption Spectrophotometer (Jackson, 1962), after digestion with HNO3: HClO4 (2: 1) acid mixture. The available K was determined from NH4OAc. (pH, 7.0) extract as described by Jackson (1962). The extract was analyzed for available K by a flame analyzer at 589 nm wavelength (Jackson, 1962). Available Phosphorus was extracted from the soil with 0.5 M NaHCO3 (Olsen’s Method) at pH 8.5 and Molybdophosphoric blue colour of analysis was employed for determination of phosphorus (Jackson, 1962). Available sulfur content was determined by turbidimetric method as described by (Jackson, 1962). It was measured by spectrophotometer at 420 nm wavelength. The available Ca and Mg were determined from NH4OAc. (pH 7.0) extract as described by Jackson (1962). The contents of Ca and Mg were measured by atomic absorption spectrophotometer. Available Boron was determined by hot water method using buffer solution and Azomethine Solution on Perkin-Elmer Lambada 11 (2.2) UV/VIS spectrometer at 420nm wavelength (Jackson, 1962).
Statistical analysis: This experiment was designed as completely randomized design with three replications. 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% and 1% level of significance with the statistical package of MINITAB. Statistical mean values of different treatment were tested using independent Student t-tests at 5% and 1% level of significance and not to correlate the different soil properties (Table 2 and Table 3) (Ryan et al., 1985).