Inorganic and organic fertilizer for lowland rice: effect on soil available nitrogen and grain yield

Long term use of chemical fertilizers may bring about a decline in the chemical and physical properties of the soil. Hence, soil properties could be improved with the use of organic fertilizers possibly due to increased soil humus content, cation exchange capacity, essential nutrients and water holding capacity. In flooded soil, however, organic N is mineralized to ammonium N (AN) or is immobilized into microbial tissue, and the rate of mineralization is determined by the carbon-nitrogen (C/N) ratio. The higher the C/N ratio, the longer it takes for organic N to be mineralized to AN. Field experiments were conducted to determine the influence of single and combined applications of organic (chicken manure with C/N ratio = 4 and rice straw compost C/N ratio = 10) and organic urea (C/N ratio = 0.4) fertilizers on soil available nitrogen or AN and grain yield. Results of the 1995 wet season (WS) field study showed that ammonium N released was highest at 80 mg N/kg soil one week after application of 50 percent of urea (U) combined with 50 percent of chicken manure (CM). However, application of uncomposted rice straw with C/N ratio of 38 contributed to the immobilization of N and hence, the lowest ammonium N of 20 mg N/kg soil was obtained. In 1996 dry season (DS), composted rice straw (RSC) was used. Results showed that ammonium N of 6=112 mg N/kg soil was released from the soil, with the highest AN coming from 50 U/50 CM and 50 U/50 RSC. In 1996 WS, 50 U/50 CM or RSC released lower AN than U alone and 75 U/25 CM or RSC. In 1996 DS, yields of PSB Rc 14 ranged from 6.7 to 7.1 t/ha for 25 U/75 CM, 50 U/50 CM, 25 U/75 CM and comparable to yields obtained in the U treatment. For RSC alone, 75 U/25 RSC and 50 U/50 RSC, yields ranged from 6.3 to 6.8 t/ha and were comparable to yields in the U treatment. Increasing the amount of RSC to 75 percent lowered yield to 5.3 t/ha. In 1995 and 1996 wet seasons, the various combinations of U and CM gave yields of 5.0 to 5.2 t/ha while U alone and CM alone gave yields of 4.6-4.7 t/ha. Low yield in the U treatment in 1996 WS was partly due to tungro infestation. Results indicate that in DS, urea and chicken manure combinations of 25 U/75 CN, 50 U/50 CM and 75 U/25 CM were as effective as urea alone. For urea and RSC, combinations with 25-30 percent RSC seem optional