This field trial was conducted in High Ganges River Floodplain Soils (AEZ-11) of Ishurdi, Pabna during the period of 2000-2020 to estimate the contribution and efficiency of individual nutrients and to find out Mustard-Mungbean-T.aman cropping pattern based fertilizer recommendation. A description of nutrient status of initial soil prior to fertilization is presented. The experiment was laid out in RCB design with eight treatments and replicated four times. The unit plot size was 6m x 5m. There were three levels each of N (80, 120 and 160 kg ha-1), P (18, 36 and 54 kg ha-1) and K (35, 70 and 105 kg ha-1) in the treatment combinations. BARI Sharisha-16 variety was used as first crop for the study and 40 kg S, 3 kg Zn and 1 kg B ha-1 was applied as a blanket dose in all the treatment except the control treatment. All PKSZnB and half of N were applied at the time of final land preparation and the remaining N was applied before flowering stage. The seeds were sown in the 2nd week of November 2019 and harvested on last week of February 2020. Only 20 kg N ha-1 was applied in mungbean except control plot. For the second crop mungbean (var. BARI Mung-6), the seeds were sown in the 1st week of March, 2019 and first picking was started in the 1st week of May and continued up to the 3rd week of May. After the harvest, the total biomass was ploughed down in the soil. The fertilizer was applied in T. aman (BINA dhan 7) according to the schedule. The seedling were transplanted on 1st week of July. Records on some yield parameters were taken at certain growth stages of all the crops of the pattern. Duncan Multiple Range Test (DMRT) was used to determine the significant difference in response of different treatment combinations.
Mustard:
Yield increased in mustard seed and straw due to different treatment combinations over native nutrient treatment in different years. With the increasing rate of nitrogen application up to 120 kg N ha-1, there was significant effect in increasing seed yield. Beyond this, higher application rate had some beneficial effect. The seed yield also increased progressively and significantly with the increased rate of P application. The calcareous soil being deficient in available phosphorus showed positive effect in increasing seed yield due to higher rate of P application. Application of 35 kg K ha-1 was enough to produce significant seed yield. Higher K addition had beneficial effect. The available K in the soil was below the critical level. So, a maintenance dose of potassium was sufficient for higher yield. The effect of N120P36K70S40Zn3B1 kg ha-1 (T2) produced the highest (average of twenty years) seed (1.49 t ha-1) and straw (4.32 t ha-1) yields and the lowest 0.80 t ha-1 and 1.49 t ha-1 respectively, in the native nutrient treatment (T8).
Mungbean:
No significant differences were observed in seed and biomass yield of mungbean due to fertilizer treatments in the year of 2000-01. But in the rest of the year showed significant differences of both grain and biomass yield over native nutrient treatment. The residual effect of phosphorus increased the yield. The highest yield (average of twenty years) of 1.41 and 7.67 t ha-1 of grain and biomass were obtained in T2 (residual effect of N120P36K70S40Zn3B1 kg/ha) treatment and the lowest 0.98 and 4.98 t ha-1 in native nutrient treatment.
T. aman:
Incorporation of green biomass of mungbean and residual effect of previous applied fertilizers influenced the grain and straw yield of T. aman significantly. The highest grain yield (average of twenty years) 5.58 t ha-1 and straw yield 6.21 t ha-1 were obtained in T2 treatment and the lowest 3.02 t ha-1 and 3.74 t ha-1 in native nutrient treatment (T8).
Total rice (system) yield:
Total rice (system) yield as influenced by different treatments has been presented. Highest total rice (system) yield of 14.95 t ha-1 year-1 was obtained from T2 treatment. Lowest total rice (system) yield of 8.93 t ha-1 year-1 was obtained from control i.e. native fertility treatment (T8) treatment.
Cost and return analysis:
The benefit cost analysis of Mustard-Mungbean-T. aman cropping pattern has descrived. Highest gross return of 299028 Tk ha-1 yr-1 was obtained from T2 treatment with a total variable cost 156370 Tk ha-1 yr-1. Highest gross margin of 142658 Tk ha-1 yr-1 was also obtained from T2 treatment. The highest benefit cost ratio of 1.91 was found in T2 treatment.
Soil fertility status:
The status of soil pH, organic matter, total N, available P and S in initial soil as well as in soil after completion of twenty cycles of Mustard-Mungbean-T.aman cropping pattern have descrived. Initially, the pH of the soil was 7.3. But after twenty cropping cycles, the pH reached near neutrality ranging from 7.1 to 7.2. The highest amount of organic matter was found in T2 treatment where high amount of biomass was added. The soil fertility was improved i.e. OM, total N, P, S & Zn content of soil was increased. But more K is to be added to soil to improve the K content of the soil.