NITROGEN DYNAMICS IN SOILS AMENDED WITH DIFFERENT ORGANIC FERTILIZERS

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1 Philippine Journal of Crop Science 23, 28(3): 49-6 Copyright 25, Crop Science Society of the Philippines Released February 25 NITROGEN DYNAMICS IN SOILS AMENDED WITH DIFFERENT ORGANIC FERTILIZERS EF JAVIER & RE TABIEN Philippine Rice Research Institute, Maligaya, Science City of Muñoz, Nueva Ecija 3119, Philippines Seven organic materials, namely azolla, chicken manure (CM),, rice straw and wild sunflower, were evaluated in the laboratory, greenhouse and field to determine the nitrogen dynamics after the materials incorporation into the soil. Inorganic fertilizer (IF), commercial organic fertilizer () with ½ of the recommended IF rate, and no fertilizer treatments were also included as checks. The results showed that in the laboratory, there was higher mineralized N observed at a higher temperature (32 o C) than at a lower temperature (15 o C). Mineralized N increased as days of incorporation of OF became longer. The steady level of soil organic matter (SOM) at 32 o C and the variable trend of SOM at 15 o C showed that some of the OF tested were slightly affected by the temperature. In the greenhouse, high soil NH4-N was observed 21 days after incorporation (DAI) of OF in the DS. In the WS, soils NH4-N in pots without plants increased from 14 DAI to 84 DAI while those with plants showed high soils NH4-N only at 7 DAI then decreased thereafter. The decrease of N at 7 DAI showed temporary N immobilization. In the field, the highest N was at 2 DAI of CM and and 15 DAI of inorganic fertilizer. It was also observed that SOM did not increase by OF application. The chlorophyll readings and grain yields were not significantly different in all the fertilizer treatments. Keywords azolla, chicken manure, indigo, inorganic fertilizers, mineralized N, nitrogen dynamics, organic fertilizers, organic matter, rice straw, sunflower INTRODUCTION Most of the high-yielding rice varieties require heavy fertilizer inputs to reach their potential yields. In response to that fact, it is well known that farmers practice intensive inorganic fertilizer application to increase farm yield. For the past years, however, there has been a reversing trend of rice production despite the use of high-yielding rice varieties. To help resolve the situation, improving the nutrient-use efficiency of the variety being used while maintaining soil fertility, sustainability and productivity, has become the focus of current rice research. Continuous use of chemicals without the use of organic matter can cause deterioration in soil properties and reduce the productivity of the soil (Lian 1993, FAO 1994). If this happens, there is a need to increase fertilizer application, not only the major elements but also the minor elements. In fact there has been an observed decline in rice yield and this can be attributed to the declining soil nitrogen supplying capacity (Cassman & others 1996) and changing soil organic matter quality (Olk & others 1996). Organic fertilizer is claimed by several researchers to contain almost all the nutrients needed in plant growth. Its heavy application has been in ancient agriculture. With readily available inorganic fertilizers, the use of organic fertilizers by farmers has been declining for years. However, with the increasing cost of inorganic fertilizers and the problem of

2 sustainability of soil fertility and productivity, the use of organic fertilizers has been reconsidered. Nutrient recycling is now being considered as part of the concept of integrated nutrient management. This is done by the incorporation of crop residues, green manure and animal manure into the soils. If these are available locally, there is no need to buy them. Previous studies on the use of organic materials as fertilizer gave some promising results, like adding azolla with urea reduced the loss of urea N (Becker & others 1994). can give as high as 196 kg N/ha, and it has been observed to increase yield by 52% (Dacayo & Tabije 1992). Addition of wild sunflower to the soil has been found to increase rice yield in the rice terraces by 1% (Siota & others 199), while rice straw incorporated at the rate of 4 tons/ha has resulted in a 2.3% increase of grain yield of the succeeding rice crop (Rebuelta 1994). Similar responses have been observed when raw rice straw is added to N fertilizer (Corpuz & others 1995). Chicken dung applied at the rate of 2 to 4 tons/ha added to 127 inorganic N/ha has given a yield of 7.7 to 8.3 tons/ha (Garcia & others 1995). The initial effect of organic fertilizer is not immediately evident as with inorganic fertilizer but the long-term effect of its use may be more advantageous in the end, as far as soil fertility, sustainability and productivity is concerned. A research project aimed at determining the sustainability of the application of different organic fertilizers in paddy soils in terms of nutrient dynamics and grain yields across seasons and years was established at the Philippine Rice Research Institute (PhilRice) experimental station in From the results of that study, this paper presents the nitrogen dynamics in soils amended with different organic materials in the lab, greenhouse and field. 5 MATERIALS & METHODS Nitrogen Mineralization Expt 1 was on nitrogen mineralization in soils amended with organic materials under different temperatures. Samples of wet paddy soils at PhilRice CES were collected and passed through a 2 mm-sieve. Each sample (16 grams dry soil equivalent) was placed into the incubation test tube. The organic materials (uncomposted rice straw, chicken manure, wild sunflower, azolla, Calangtin, indigo and commercial organic fertilizer) were dried at 65-7 o C for 48 hours and were finely ground with mill to achieve thorough homogenization with the soil. An equivalent 6 kg N/ha of each of the organic materials was mixed with the soil in the test tube, after which distilled water was added to fill the gap between the soil solution and the stopper. A soil without any organic amendment served as the control. The test tubes were placed in the incubator chamber set at 15 o C and 32 o C. Determinations of the total ammonium N (NH4 + -N) by the Kjeldahl method and total organic carbon by the Walkley-Black method were done at, 7, 14, 28, 56 and 84 days after incubation (DAI). The soils from these determinations were dried for the analysis of organic carbon. Nitrogen Dynamics Expt 2 was on nitrogen dynamics in soils amended with organic materials under greenhouse conditions. An equivalent of 6 kg N/ha from the organic materials (OM) (uncomposted rice straw, chicken manure, wild sunflower, azolla, Calangtin, indigo, and a commercial organic fertilizer) were mixed with the paddy soils in the pots. No rice plants were planted in the pots. The soils were kept flooded. Wet soil samples were collected at, 7, 14, 28, 56 and 84 days after incubation (DAI) and extracted as fresh samples for the determination of nitrogen. The remaining samples were dried and pulverized for the determination of organic carbon. In the wet season, pots with the same quantities of soil and organic materials as described above were planted with rice. This was to replicate the conditions in the field. Plants Nitrogen Dynamics In Soils

3 were sampled at the same time the soils samples were taken. Nitrogen Dynamics, Field Conditions Expt 3 was on nitrogen dynamics in soils amended with organic materials under field conditions. The experiment was laid out in a randomized complete block design with 4 replications. The plot size was m 2. The constructed dikes per plot were kept intact for the next cropping season and will be maintained for the succeeding 5 cropping seasons. As test plants, 25-day old PSB Rc18 seedlings were used for the dry season (DS); 21-day old seedlings were used for the wet season (WS). In the DS, 5 tons of fresh organic materials, ie, rice straw, wild sunflower, indigo, azolla, chicken manure (partly dried) and the readily available commercial organic and inorganic fertilizers, were incorporated into the soils. The rice straw and chicken manure were applied a month before transplanting while the green manure was applied 2 weeks before transplanting. The commercial organic fertilizer (with ½ of the inorganic fertilizer recommended rate applied at early panicle initiation (EPI)) was incorporated into the soil 7 days before transplanting. With the pure inorganic fertilizer at the rate of kg NPK/ha, half of it was applied a day before transplanting and the remaining half at EPI. The regenerating azolla was re-incorporated by trampling at 29 days after it was first applied. An unfertilized plot served as the control. In the wet season, an equivalent of 6 kg N/ha from each of the organic materials was applied instead of the blanket application of 5 tons/ha. The rice straw was applied 24 days before transplanting (DBT), the chicken manure and commercial organic fertilizer at 8 DBT, and the azolla, indigo and wild sunflower at 2 DBT and inorganic fertilizers at 1 DBT. The inorganic fertilizer rate used was NPK kg/ha. A leaf color chart-based N application was added as treatments. Soil samples were taken a day before transplanting and at 7, 14, 28, 56, and 84 days after transplanting (DAT). The samples taken were extracted as fresh for the determination of available ammonium-nitrogen using the indophenol method. The remaining samples were dried and pulverized for the determination of organic carbon using the colorimetric method. Plant samples were also taken at the same time as that of soil sampling for the analysis of total nitrogen. Chlorophyll reading using the SPAD meter was taken from 1 sample plants at 14 days after transplanting and every 2 weeks thereafter until the flowering stage or at 84 DAT. Yield per plot was also taken and computed to an adjusted yield of tons/hectare at 14% moisture content. RESULTS Nitrogen Mineralization Certain temperatures enhance microbial activities and decomposition. The higher or faster the decomposition is, the faster the availability of released nutrients from the soils for the plant to absorb. This particular activity is expected to give some basic information on how the N is being mineralized under the irrigated lowland rice area and at the cool elevated area. Generally, there was a higher N mineralization of all the materials tested at a higher (32 o C) temperature than at a lower temperature (15 o C) (Figure 1). This shows that a nitrogen release is slower in a cooler environment than in a warmer environment like that in the lowland rice areas. Almost all the organic materials showed increasing N mineralization up to the 28 th day of incubation. N mineralization decreased at 56 DAI but exhibited an abrupt increase at 84 DAI. With the above trend, we can adjust the transplanting dates of rice according to the peak of N mineralization of each of the organic materials used. The first peak of N mineralization will coincide with the time that the transplanted rice will be at the start of its tillering stage, which usually requires high N. The second peak will be in time for panicle 1

4 initiation up to heading stage. Based on laboratory results, transplanting of rice in the lowland should take place 1-2 weeks after the application of the indigo, azolla, sunflower, chicken manure and Calangtin. The rice straw should be applied 2-3 weeks before transplanting. The commercial organic fertilizer may be applied 1 week before transplanting. Since the N mineralization in the cooler area is slower than in the warmer area, the rice straw will take almost 5 weeks to release N. It is advisable then to incorporate the rice straw at least 4 weeks before transplanting (Figure 1). It was observed that wild sunflower undergoes faster mineralization than the other indigenous green manure being used in the highlands. Therefore, sunflower can be incorporated 1 week before transplanting or 3 weeks after rice straw incorporation. These two organic materials will give complementary nutrients available to rice. can be incorporated a week before panicle initiation. The results showed (Figure 1) that azolla started giving a relatively high mineralized N a week after it was incorporated, and continued its high N releases in increasing amounts for another 4 weeks. The available N will support the plant during the panicle initiation and heading stage. This can be a good replacement of the topdressed chemical N. As far as the soil organic matter (SOM) is concerned, the results (Figure 2) showed that whether the soils with organic fertilizers were subjected to high or low temperature, the addition of SOM did not show an additional effect. Generally, the treatments did not show any significant differences among the organic materials or fertilizers. Even the control treatments gave almost the same average SOM as the fertilized treatments. However, rice straw showed a continuous and steady increase in OM until 84 DAT regardless of the temperature. Chicken manure showed the same trend up to 84 DAI while Calangtin, a commercial organic fertilizer, decreased at this time. Generally, it was observed that while the mineralized N increased, the soil organic matter 52 remained relatively stable (Figures 1 & 2). There was no building up of SOM despite the addition of organic fertilizers in the soil. The organic matter in the soils incubated at higher temperature was stable and the same in all the treatments. At lower temperature, however, there was some distinct trend of organic matter among the treatments tested. This could be attributed to the slower mineralization and turn-over of N from the OM pool while at higher temperature, the OM has a very fast turn-over of N from the OM as seen in the higher and increasing N released. Nitrogen Dynamics In Greenhouse There was a general lag period of about 3 weeks before the N was released in the dry season (Figure 3). All the organic materials started releasing ammonium N from 56 days after pot incubation (DAPI) until 96 DAPI. The highest NH4-N was released by the sunflower. Both the chicken manure and the control had declined in NH4-N release at 84 DAPI and abruptly increased again at 96 DAPI, giving almost the same amount of NH4-N as the other organic materials. The percent SOM did not change all throughout the experimentation. Apparently, the addition of different organic fertilizers or materials did not increase the organic matter but somehow sustained the amount in the paddy soils. In the wet season, N immobilization was observed to have taken place for 7 days (Figure 4). The release of N was observed 14 days after incorporation of the organic fertilizers. Again, the inorganic fertilizer, commercial organic fertilizer, indigo and chicken manure showed higher potential of supplying N into the soils. Among the green manures, the azolla gave the lowest NH4-N. In this case, the green manures may still need a supplemental inorganic N to complete the N requirement of the rice plants. Nutrient Dynamics In Field In the DS, the soils applied with chicken manure and inorganic fertilizer had the highest Nitrogen Dynamics In Soils

5 NH4-N contents at 14 days after transplanting or 4 weeks after the chicken manure was incorporated into the soil and 15 days after inorganic fertilizer was applied (Figure 5). Both amendments gave higher NH4-N at 28 DAT than the other treatments. But like the rest of the soils amended with different OM, the available NH4-N started to decline at 28 days after transplanting up to the flowering stage of the rice plants (approximatly 84 DAT). This was in contrast with the NH4-N releases from the potincubated soils. The differences could be attributed to the presence of the rice plant in the field to absorb the nutrient and the absence of plants in the pots (Figure 4). In the WS, the trend of N releases was more distinct than in the DS. Ammonium-N was observed to increase at 7 DAT except for the commercial organic fertilizer and the chicken manure treatments that started at transplanting time (Figure 6). Based on the data, the chicken manure should be incorporated before than 14 DBT while the commercial organic fertilizer should be applied before than 7 DBT. There were no significant differences among the organic fertilizers tested. In general, however, the N in the soil decreased and was observed to be lower than 1 ppm during the tillering and panicle stages of the plant. Regardless of the kind of fertilizer applied in both the WS and DS, accumulation of soil organic matter was not observed (Figures 5 & 6). This can be due to faster and higher N mineralization in the warm environment. Furthermore, as is shown in Figure 4, there was an increasing amount of N in the soils amended with organic fertilizers on pots without plants under greenhouse conditions. In the field, the weekly greenness of the leaf up to flowering stage was monitored using the SPAD meter. The chlorophyll readings did not differ significantly among organic fertilizers used. Higher readings, however, were observed in the WS than in the DS. If the critical reading set for transplanted rice is 35 (Quilang et al 1996), the results showed that in the DS, the plants had experienced N starvation throughout the growing period except those applied with commercial organic fertilizers and inorganic fertilizer (Figure 7). The readings in the WS likewise were high. This manifested that no N starvation by the rice plants was observed except at 56 DAT, approximately the panicle initiation stage where the SPAD readings were equal to or less than 35. In the DS, the highest yield was observed in those plots applied with inorganic fertilizers and those applied with commercial organic fertilizers with half of the inorganic fertilizer rates (Figure 8). The chicken manure and rice straw followed. In the WS, the application of commercial organic fertilizer with half of the inorganic fertilizer rate, rice straw incorporation and the LCC-based N application treatments gave similar yields. Since the yield differences were found statistically not significant, further study is to be made to determine the sustainability of the continuous application of these materials in terms of soil fertility and productivity as well as the grain yield of transplanted rice. DISCUSSION The dynamics of the availability of nitrogen and organic matter in the soils amended with different organic fertilizers were determined. The soil used in the experiments belongs to the Maligaya clay soil series. In the dry season, there were no significant differences observed among the treatment means. The trend of the nutrient content in the soils was therefore given more emphasis rather than the individual effect of each of the organic fertilizers tested. There were no statistical differences observed between the treatments. In the laboratory set-up, there was a higher N mineralization of all the materials tested at higher (32 o C) temperature than at lower temperature (15 o C). This shows that a nitrogen release is slower at a cooler environment than at a warmer environment such as in the lowland areas. Based on the peak of N mineralization, azolla, indigo, wild sunflower, chicken manure and commercial organic fertilizer can be 3

6 incorporated 1 to 2 weeks before transplanting while the rice straw is to be applied not earlier than 3 weeks before transplanting in the lowland area. In cooler areas, rice straw may be incorporated 4 to 5 weeks before transplanting. The peak of NH4-N content will coincide with the tillering stage where N is needed., because of its capacity to reproduce after its first incorporation, can be used as a fertilizer to support the panicle initiation of the plants. This can be incorporated by trampling. It was observed that while the mineralized N increased, the organic matter did not. There has been no building up of OM despite the addition of organic fertilizers in the soil. The organic matter in the soils incubated at higher temperature was stable and of similar amounts in all the treatments. At lower temperature, however, there was some distinct trend of organic matter differing from each of the treatments tested. This could be attributed to the slower mineralization and turnover of N from the OM pool while at higher temperature, the OM had a very fast turnover of N from the OM as seen in the higher and increasing N released. In the greenhouse, pots of soils amended with organic fertilizers were used. There were no significant differences between the organic fertilizers tested, although inorganic fertilizer and wild sunflower treatments seemed to give consistently higher N both in the soil and in the plant throughout the growing season. But the general trend was that N in the soil decreased and was observed to be lower than 1 ppm during the tillering and panicle stages. The concentration in the plant showed an inverse linear relationship with time. N content was observed to have decreased by 3.5% at 14 DAT and by 5% at 84 DAT. This was true during the dry season. In wet season, there was a distinct trend on the availability of N in the soils. Initially, NH4-N started to be high regardless of the organic materials incorporated. At 7 days after incorporation, the NH4-N abruptly decreased. This showed that there had been 54 temporary immobilization of N in the soil. After 7 days, N increased. At this moment, the N will be absorbed immediately by the plant before it will be leached or volatilized. It is therefore, important to know these peaks to enable the farmer to re-adjust the time of transplanting and the time of incorporation of organic materials. The green manures like azolla and sunflower, and the commercial organic fertilizer Calangtin gave lower NH4-N than chicken manure, indigo, commercial organic fertilizer and the inorganic fertilizer, which consistently had higher soil N. The results from the field seemed to be supported by the data from the pot incubation in the greenhouse. The inorganic fertilizer showed the highest peak of NH4-N 7 days after transplanting. This means that at 7 days after the application of inorganic fertilizer, the N is already available for plant use. But the decrease of N from the inorganic fertilizer started also 7 days after the peak of N release into the soils. As for the other treatments, chicken manure (incorporated 14 days before transplanting) and commercial organic fertilizer (applied 7 DBT) showed their highest NH4-N available right at the day of transplanting. To maximize therefore the availability of N, the commercial organic fertilizer should be applied in 2-3 days before transplanting while the chicken manure should be applied at least 7 days before transplanting. In the wet season, N immobilization was observed to have taken place at 7 days. The release of N was observed 14 days after incorporation of the organic fertilizers. Again, the inorganic fertilizer, commercial organic fertilizer, indigo and chicken manure showed higher potential of supplying N into the soils. Among the green manures, the azolla gave the lowest NH4-N. In this case, the green manures may still need a supplemental inorganic N to complete the need of the rice plants. The lower chlorophyll readings in the DS than in the WS implies that the application of pure organic fertilizer is not enough to supply the N needs of the rice plants. Higher N level is needed in the dry season due to higher solar Nitrogen Dynamics In Soils

7 radiation, hence in principle, higher rates of N uptake and utilization are expected. Since statistically the yields in all the treatments were similar, further study over time needs to be made to determine the sustainability of the continuous application of these materials in terms of the soils fertility and productivity as well as the grain yield of transplanted rice. Acknowledgement The authors acknowledge the assistance of Ms Luzviminda Quitos and Mr Noel de Gracia in the analyses of samples at the ASPPD-PhilRice laboratory, Ms Filomena Grospe in her assistance in the N mineralization set-up, and Mr Marcelo Villanueva in the maintenance of the field and greenhouse experiments. LITERATURE CITED Becker M, JK Ladha & JCG Ottow Nitrogen losses and lowland rice yields as affected by residue nitrogen release. Soil Society Scientific American Journal 58: Cassman KG, A Dobermann, PC Sta. Cruz, GC Gines, MI Samson, JP Descalsota, JM Alcantara, MA Dizon & DC Olk Soils organic matter and in the indigenous nitrogen supply of intensive irrigated rice systems in the tropics. Plant & Soil Corpuz AA, PI Rebuelta, RJ Lara, RT Cruz & SR Obien Management practices to maximize yield of irrigated transplanted rice. Philippine Rice R&D Highlights for PhilRice, Muñoz, Nueva Ecija Dacayo JB & FD Tabije Utilization of green manure in rainfed lowland rice. Philippine Rice R&D Highlights for PhilRice. Muñoz, Nueva Ecija FAO Fertilizer and Plant Nutrition Guide. FAO Fertilizer and Plant Nutrition Bulletin 9. Rome, Italy. 176 pp Garcia FD, WN Obcemea & RT Cruz Influence of organic and inorganic fertilizers on yield of irrigated transplanted rice. Philippine Rice R&D Highlights for PhilRice, Muñoz, Nueva Ecija Lian S Use of chemical fertilizers combined with organic manure in rice production. Extension Bulletin 371. Food and Fertilizer Technology Center for the ASPAC Region, Taipei, ROC, pp 1-13 Olk DC, KG Cassman, EW Randall, O Kinchesh, LJ Sanger & JM Anderson Changes in chemical properties of soil organic matter with intensified rice cropping in tropical lowland soils. European Journal of Soil Science 47: Quilang EJP, AA Corpuz & RT Cruz Assessing crop leaf nitrogen status with SPAD and leaf color chart. Philippine Rice R&D highlights for PhilRice, Maligaya, Muñoz, Nueva Ecija Rebuelta PI, RT Cruz & SR Obien Verification of management practices for increasing rice yield. I. Nitrogen level and rice straw incorporation. Philippine Rice R&D Highlights for PhilRice, Muñoz, Nueva Ecija Siota CM, JA Lapitan, A Sotomil & FC Tagalog Adaptation trial of promising fertilizer management practices for acid upland areas. Philippine Rice R&D Highlights for 199. PhilRice, Muñoz, Nueva Ecija 5

8 NH 4 -N (mg kg -1 ) at 15 o C C ontrol (--) Wild s unflow er Calantin Com. Org In d ig o Days after incubation NH 4 -N (mg kg -1 ) at 32 o C C ontrol (--) Wild s unflower Calantin Com. Org Days after incubation Figure 1. Ammonium nitrogen (mg kg -1 ) in soils amended with different organic fertilizers and incubated at different temperatures. SOM(%) at 15 o C C ontrol (--) Wild s unflower Calantin Com. Org D ays after incubation SOM (%) at 32 o C C ontrol (--) Wild s unflower Calantin Com. Org D ays after incubation Figure 2. Organic matter (%) in soils (SOM) amended with different organic materials at 2 different temperatures. DS Nitrogen Dynamics In Soils

9 NH 4 -N (mg kg -1 ), greenhouse condition, DS Control Sunflower Days after incorporation Soil OM(%), DS Control Sunflower D ays after incorporation Figure 3. Ammonium nitrogen and organic matter in soils amended with different organic materials under greenhouse condition. DS 99. NH 4 -N (m g kg -1 ), without plant,greenhouse, WS azolla calantin chicken m anure control indigo inorganic rice s traw sunflower D ays after inc orporation NH 4 -N (m g kg -1 ), with rice p la nt, g re e nho use, W S azolla calantin chicken m anure control indigo inorganic rice s traw sunflower Days after incorporation Figure 4. Ammonium nitrogen (mg kg -1 ) in soils amended with different organic materials, with and without rice plants under greenhouse condition. WS99. EF Javier & RE Tabien 57

10 NH 4 -N (mg kg -1 ), field expt. DS Chicken Manure Inorganic Fertilizer No Fert. Sunflower Days after transplanting Soil OM (%), field expt. DS Chicken Manure Inorganic Fertilizer No Fert. Sunflower Days after transplanting Figure 5. Ammonium nitrogen (mg kg -1 ) and organic matter (%) in soils amended with different organic materials under field conditions, DS 99. PhilRice Maligaya NH 4 -N (mg kg -1 ), field expt. WS azolla chicken manure control indigo inorganic LCC-based N rice straw sunflower Days after transplanting Soil OM (%), field expt. WS Inorganic LCC-based N No fertilizer Rice straw Sunflower Days after transplanting Figure 6. Ammonium nitrogen (mg kg -1 ) and organic mater (%) in soils amended with different organic fertilizers under field conditions, WS 99, PhilRice Maligaya 58 Nitrogen Dynamics In Soils

11 Chlorophyll readings, DS Inorganic ComOrganic Chicken manure Wild sunflower Rice straw Control Days after transplanting Chlorophyll readings, WS Days after transplanting Inorganic chicken manure Wild sunflower Rice straw Control LCC-based Figure 7. Chlorophyll readings by the SPAD meter of rice plants applied with different organic fertilizers in the WS and DS 1999, PhilRice Maligaya 9

12 Adj. yield/ha, DS and WS Inorganic chicken manure Wild sunflower Rice straw Control LCC-based DS 99 WS 99 Figure 8. Adjusted yield (t ha-1) in 14% moisture content of the PSB Rc 18 applied with different organic fertilizers in the WS and DS 1999, PhilRice Maligaya 6 Nitrogen Dynamics In Soils