Residual nutrients and crop residue utilization in rice-based cropping system

Rice yields increased with increasing rates of N fertilizer application, and showed an increasing yield pattern during the observation period from 1991-1999. With 30 kg N/ha, rice yields increased from 1.3 t/ha in 1991 to 3.1 t/ha in 1999; with 60 kg N/ha, 1.6 t/ha to 3.7 t/ha; with 90 kg N/ha, 2.1 t/ha to 4.3 t/ha; and with 120 kg N/ha, 1.5 t/ha to 4.7 t/ha. Rice yield increase in year 2000 was not attributed to the fertilizer application, but to the change in rice variety from BPI Ri 10 to PSB Rc 14. The application of crop residues increased rice yields by 0.13 -0.60 t/ha during the period 1997-2000. Except for mungbean, the dry season crops obtained yield increases with fertilizer application. Mungbean, being a leguminous crop, did not respond to the applied fertilizer. The yield responses of the relay crops indicated that the residual nutrients in the soil after the dry season crop, i.e., the second crop after rice, could actually support another crop. Except for mungbean after corn, the third crop (mungbean after garlic, and corn after mungbean, sweet pepper and tomato) showed higher yield than those in the control, where no fertilizer was applied on the second crop. Mungbean after corn showed a decline in yield on the first three years of cropping (1994-1996), and gradually increased on the fourth to the sixth year (1997-1999), and declined again on the following year (2000). An analysis of the nutrient contribution of the crop residues in the different cropping patterns showed that rice-garlic-mungbean had the highest average amount of nutrient contribution, 50.51 kg N/ha, 6.57 kg P/ha, and 257.95 kg K/ha. The high amount of K was attributed to the mulch applied on garlic. The monetary value of the nutrient contribution of the crop residues, translated into bags of fertilizer materials, ranged from P345.00 to P5,304.00/ha. Rice-garlic-mungbean gave the highest monetary value, P7,424.00. The cropping patterns and fertilizer application resulted in an increase in the levels of nitrogen and carbon in the soil. Soil nitrogen, measured as % CHN-N, increased from 0.088 in 1993 to 0.108 in 1999. Soil carbon, measured as % carbon, increased from 1.238 in 1993, to 1.421 in 1997, and 1.388 in 1999