2008 年 52 巻 2 号 p. 107-117
Laboratory experiments on ammonium (NH_4^+-N) releasing from lowland (Soil 1) and upland (Soil 0) soils classified as Eutropepts in Leyte of the Philippines were studied to evaluate its response to urea type fertilizers with different NH_4^+-N releasing time. Although the X-ray diffraction (XRD) indicated a dominant presence of kaolin group minerals, halloysite or kaolinite was not clearly identified under the scanning electron microscope. Both Soil 1 and Soil 0 samples possessed variable charge. Without fertilizer and totally anaerobic condition, the highest NH_4^+-N amount in soils was observed at two- week incubation. Applications of common urea and slow-release urea: Meister 40 and Meister 70 (M40 and M70 are expected to release 80 % of their nitrogen at 400 days and 700 days at 20℃, respectively) to lowland soil samples significantly released a larger amount of NH_4^+-N than the upland soil. In addition, at 84 days, the rate of release of NH_4^+-N from the slow-release fertilizers evaluated was enhanced by increasing the temperature condition to 30℃. The low selective adsorption capacity of NH_4^+-N in kaolin group minerals found in the lowland and upland soils, as well as the variable charge of the soil containing a slightly negative charges should be considered when planning N fertilization to avoid losses of large amounts of unutilized N especially to perennial crops whose N requirement might be lower at early growth.
