Composting of Straw Residues and Its Effects on Rice Yield and Soil Nutrients

This study was carried out to optimize and process of conversion of straw residues into enrich compost by adding organic materials rich in nitrogen and examine its effects on rice yield and soil nutrients. Two experiments were conducted viz; 1) composting of straw residues and 2) pot and field plot experiments at RDA farm, Bogra. In the composting experiment straw residues along with blackgram plants and cow dung were used and that had been collected from RDA farm. Straw residues and blackgram plants were cut into small pieces (4-5 cm) and mixed thoroughly along with least amount of cow dung as inoculants and composted in the compost chamber of RDA farm. Cow dung and blackgram plant residues (after harvesting of pods) were used as inoculants and N-materials, respectively to achieve efficient and quicker composting of rice straw residues to avoid wasting of useful natural resources as well as keep long-term crop productivity and soil fertility. Cow dung and blackgram plants were added 10-50% by dry weight of straw residues in six different mixtures and composted for two months. At the end of composting process nutrient contents were tested. In the pot and field plot experiments the composts obtained from the compost chamber was tested with rice (BR14) as a test crop in a randomized block design with three replications. Total yield and yield contributing characters were recorded. Soil samples were collected and tested their nutrient contents before planting and after harvesting of rice in 2006 to know the effect on residual soil nutrients as compared to the initial soils.

The mixture of S₆D₁B₃, 60% straw residues to 40% N-materials (10% cow dung + 30% black gram plants) was found efficient for composting of relatively larger amount of straw residues (60%) with C/N ratio of 12.10. The mixture of S₆D₁B₃ increased the decomposition rate and nutrient contents especially N (0.53% in straw residues to 2.66% in the compost), P (158 mg 100^-1g in straw residues to 394 mg 100^-1 g in compost), K (1473 mg 100^-1g in straw residues to 3436 mg 100^-1 g in compost), Ca (42 mg 100^-1g in straw residues to 840 mg 100^-1 g in compost and Mg (182 mg 100^-1g in straw residues to 365 mg 100^-1 g in compost), with no significant difference as compared to composts obtained from the mixtures of 50% straw residues to 50% N-materials. Thereafter, no significant decrease in C/N ratio was noticed with the increase of 10% N-materials over S₆D₁B₃ mixture. Therefore, this mixture was the highest stability and the best fertilizer-value of compost. The C/N ratio serves here as a measure of stability of composts. It has been stated that when C/N ratio is < 20 the compost is mature and can be used without any restrictions.

The efficacy of composts was compared on rice, S₆D₁B₃ compost was found to be superior. Using rice as a test crop, the straw residue composts were applied at the rate of 10 t/ha with and without using any chemical fertilizers. The grain yields in S₆D₁B₃ applied plot was found 207% and 197% higher than that of control (no compost) significantly, respectively in pot and field plot experiments, and sustains soil nutrients for successive crops with highest potential for sustainable soil management. Except for total N, the residual soil nutrients, OM (1.20% initial soils to 1.08% and 1.46% in control and S₆D₁B₃ soils respectively), P₂O₅ (27.85 mg 100^-1g initial soils to 25.34 mg 100^-1g and 48.71 mg 100^-1g in control and S₆D₁B₃ soils respectively), K₂O (20.56 mg 100^-1g initial soils to 13.14 mg 100^-1g and 68.00 mg 100^-1g in control and S₆D₁B₃ soils respectively), CaO (210.31 mg 100^-1g initial soils to 170.43 mg 100^-1g and 285.87 mg 100^-1g in control and S₆D₁B₃ soils respectively), MgO (33.25 mg 100^-1g initial soils to 18.76 mg 100^-1g and 51.92 mg 100^-1g in control and S₆D₁B₃ soils respectively), EC (0.028 mS/cm initial to 0.026 mS/cm and 0.050 mS/cm in control and S₆D₁B₃ soils respectively), C/N ratio (11.19 initial to 10.58 and 11.29 in control and S₆D₁B₃ soils respectively) and TN (.064% initial to .060% and .074% in control and S₆D₁B₃ soils respectively) were found significantly high in S₆D₁B₃ compost applied soils even after growing of rice. The results of the study also confirmed that a combination of S₆D₁B₃ compost with moderate doses of chemical fertilizers may be used as an alternative to high doses of chemical fertilizers for increased rice production and environmental conservation. This finding is to be a solution to the soil depletion problem in places where intensive rice is grown and cow dung is scarce.