A long-term field experiment under different fertilization treatments had been conducted to explore the effects of rice yield and soil chemical properties from 1978 to 2008 in Suwon, Korea. The paddy was applied eight fertilization treatments which were F0 (no fertilizer), PK (phosphorous and potassium), NK (nitrogen and potassium), NP (nitrogen and phosphorous), NPK (nitrogen, phosphorus and potassium), NPKC (NPK with compost), NPKS (NPK with straw) and NPKL (NPK with lime). Results of 31 years experiment showed that yield index (the ratio of yield in each treatment to NPK) was the lowest in F0 (0.52) and the highest in NPKC (1.18). Yield index was gradually increased in NPKC but decreased in F0 and NK. The yield index of PK, NP, NPKS and NPKL were not changed long-term treatment. Soil acidity of NPKL showed the highest with pH 7.9, and that of other treatments ranged from pH 6.3 to 6.8. Available phosphorous content of soil was increased in all plots by long-term fertilization, was the highest in PK and NPKC. Soil organic matter was higher in NPKC (1.8%) and NPKS (1.8%) than other treatments (1.3~1.4%) in the early experiment, but that was remarkably increased in only NPKC (2.5%) according to annual long-term application. Thus we suggest that annual compost application with optimum NPK could make stable and sustainable rice production.

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Field experiment was carried out to identify the proper water exchange interval for the rice cultivation on reclaimed saline soil with different nitrogen levels in southwestern area. The nitrogen fertilizer was applied 14, 17 and 20 kg per 10a by ingredient, and intervals of water exchange treated 3, 6 and 9-day periods from transplanting of rice(Oryza sativa var. Cheongho) to maturing stage in Munpo soil series. The salinity levels ranged 0.10∼0.24% and 0.24∼0.32% of 3-day and 6-day respectively, whileas it ranged 0.35∼0.52% for 9-day interval of water exchange during vegetative stage. Water exchange and nitrogen level showed significant effects on the plant growth, yield, and quality. The yield of milled rice on 3-day and 6-day interval of water exchange showed 497 kg/10a and 492 kg/10a and that were significantly higher than that of 9-day interval in 2008 and 2009. Milled rice yield of 9-day interval of water exchange was lower than that of 3-day and 6-day interval of all nitrogen levels. Plant growth and yield components were not significantly different between 3-day and 6-day interval of water exchange of all nitrogen levels. The combination of nitrogen fertilizer level of 17 kg/10a and 6-day interval of water exchange after transplanting might be recommended for maximum yield realization and minimize salt injury at reclaimed medium saline soil in southwestern area.

A field study was conducted to understand nitrogen use efficiency of high yielding Japonica rice varieties under three levels of nitrogen fertilizer (90, 150 and 210 kg N ha -1 ) in Iksan, Korea. Two high yielding rice varieties, Boramchan and Deuraechan, and an control variety, Dongjin2, were grown in fine silty paddy. Nitrogen use efficiencies (NUE) were 83.3, 56.3, and 41.2 in 90, 150, and 210 Nitrogen is the most critical input that limits rice productivity (Mae, 1997). The required amount of nitrogen depends on soil type, variety, climate, method of application and type of fertilizer. However, supply of proper amount of nitrogen based on the physiological requirement is the key factor. Through proper nitrogen management, one kg of nitrogen accumulation can produce 50 kg of grain under irrigated conditions (Yoshida, 1981). However, acquisition of applied nitrogen by the rice crop is typically less than 40% in farmer's fields (Cassman et al., 1993). Further increase of yield from the existing cultivars is possible by increasing nitrogen use efficiency (NUE).For crops, NUE can be defined as grain yield per unit nitrogen supply (Moll et al., 1982). NUE is understood by the combination of the nitrogen uptake efficiency (NUpE) and nitrogen utilization efficiency (NUtE). Nitrogen use efficiency (NUE) largely depends on nutrient balance, water availability, light intensity, disease pressures and cultivated variety. Nitrogen efficient genotype is considered in two different terms: the ability to convert high nitrogen input into yield comparatively better than other genotypes or the ability to realize an above average yield at suboptimal nitrogen level. Rice genotypes showed different nitrogen uptake, nitrogen translocation efficiency, and also NUE (Broadbent et al., 1987; Koh, 2007, Singh et al., 1998). In some researches, increasing NUpE is focused as the strategy to increase NUE with high grain yield (Cassman et al., 1993;Rauna and and Johnson, 1999;Feng et al., 2011).Grain yield of rice is the final product of the combination of number of panicles per unit area, spikelet density, percentage of filled spikelets and grain weight. The primary yield determinant of yield in rice is the number of spikelets per unit land (Gravoid and Helms, 1992). For achieving higher yield in rice, the sink size should be increased by increasing panicle size or number or either. The cultivars having large panicles may the best option but the adequate numbers of panicles need to be maintained properly in terms of sink-source balance (Kim et al., 1993). Therefore, this study has undertaken to evaluate the morpho-physiological response of high yielding Japonica rice varieties under variable nitrogen levels. The experiment was laid out in a split plot design with three replications, where nitrogen levels were in the main plot and rice varieties were in subplots. The subplot size was 28 m × 2.6 m. The nitrogen levels were 90, 150, and 210 kg N ha -1 and three Japonica rice varieties were Boramchan, Deuraechan and Dongjin...