Experimental field and site: The experiment was conducted at the Soil Science Field Laboratory of Bangladesh Agricultural University, Mymensingh during the winter (Rabi) season (January, 2015 to May, 2015). The experimental site is located about 24°75¢ North latitude and 90°50¢ East longitude. The land selected for the experiment was medium high land.
Soil: The initial soil samples were collected from 0-15 cm depth of the Old Brahmaputra Floodplain (AEZ-9) before opening the field.
Crop: The crop under the study was soybean (Glycine max. L. Merr.). The variety of the crop was PB-I (Shohag) was collected from Department of Genetics and Plant Breeding of Bangladesh Agricultural University, Mymensingh.
Climate: Details of the climatic data during the study period have been recorded.
Treatments: The experiment consists of 7 treatments with 3 replications. The treatments under study were as follows: To: Control, T1: Recommended dose of N, T2: Rhizobiun biofertilizer inoculation, T3: Rhizobium biofertilizer inoculation+75% P, T4: Rhizobium biofertilizer inoculation+100% P, T5: Rhizobium biofertilizer inoculation+125% P, and T6: Rhizobium biofertilizer inoculation +N+P (100%)
Experimental design: The total land was divided into three blocks and each block was sub-divided into 7 unit plots. The total number of plots were 7 ´ 3 = 21 and the size of each unit plot was 4 m ´ 2.5m = 10 m2.
Crop cultivation
Land preparation: Land preparation was started on November, 2014.
Fertilizer application: At the final land preparation a basal recommended dose of K and S were applied. Potassium @ 40 kg K ha-1 from muriate of potash (MoP) and sulphur @ 10 kg S ha-1 from gypsum were applied as basal dose. Triple Super Phosphate (TSP) was applied as per treatments. Urea was applied in 3 splits. The fertilizers were mixed well with the soil by spading.
Seed germination test: Germination test was conducted in petri-dish using water soaked blotting paper. It was found that more than 95% seeds were viable and germinated successfully after 4-5 days.
Preparation of rhizobial inoculants (broth): The broth culture of rhizobial strain was prepared following the method of Vincent (1970). Yeast mannitol broth medium was prepared in 250 ml Erlenmeyer flask.
Inoculation of seeds: At first uninoculated seeds were sown according to the experimental design for T0. Then inoculated seeds were taken in small polythene bag equal in weight for each plot and mixed with 40% gum acacia. Then the selected inoculum was mixed with the seeds @ 50 g inoculum/ kg seeds for T2, T3, T4, T5 and T6 treatments and mixed well with the seeds by shaking the bag thoroughly.
Sowing of seeds: Seeds were sown on the furrow on 01 January, 2015 and the furrows were covered by soils soon after seeding.
Germination of seeds: On the 8th to 9th day, the percentage of germination was more than 80% and on the 11th day nearly all plants came out of the soil.
Collection of plant samples: Plant samples were collected from each plot at 40 and 60 days after sowing (DAS) and at harvest. The shoot, root and nodule materials were first air dried and then oven dried at 65°C for 72 hours.
Harvesting: The crop was harvested on 20 may (140 days after sowing), 2015 at maturity of whole plants
Grain yield and Stover yield: From each plot randomly selected 10 square meter plants were harvested. After drying, threshing and processing were done plot wise carefully. The processed seed and stover were again dried in the sun for 3 days. Seed and stover yields were recorded plot wise, which were then converted into yield in kg ha-1.
Determination of N, P, K, and S from chemical analysis of grain and straw samples
Total nitrogen: Total N content in soil was determined by micro-Kjeldahl method
Available phosphorus: Phosphorus was extracted from plant sample with 0.5 M NaHCO3 solution at pH8.5 following the method of Olsen et al. (1954).
Exchangeable potassium: The K was determined from the extract by using flame photometer. This method was proposed by Brown and Lilleland (1946).
Available sulfur: The intensity of turbidity was measured by spectrophotometer at 420 nm wave length. The extraction method was described by Page et al. (1989).
Statistical analysis: Data were analyzed statistically using analysis of variance (ANOVA) to examine the treatment effects, and the mean differences were adjudged by Duncan’s Multiple Range Test (DMRT) (Gomez and Gomez, 1984) and ranking was indicated by letters.