Leaching of Fertilizer Ions in a Kaolinitic Ultisol in the High Rainfall Tropics: Leaching of Nitrate in Field Plots under Cropping and Bare Fallow
Leaching and crop recovery of nitrogen (N) applied as calcium ammonium nitrate (CAN) to bare fallow and cropped plots were studied in a coarse-textured, kaolinitic Ultisol under high rainfall conditions (mono-modal). Maize (Zea mays) was planted in the first cropping season (March–July) and upland rice (Oryza sativa) in the second season (August–December). Rates of N applied to maize and rice were 150 and 90 kg/ha, respectively. Soil samples up to 120 cm in depth were taken at 4-week intervals throughout the two growing seasons. Nitrate leached readily in the fallow and cropped plots. Liming (2 t/ha) shortened the initial time lag of nitrification, increased the nitrification rate, and, consequently, increased leaching of nitrate in both fallowed and cropped plots. During the first season, substantial amounts of NH₄-N persisted in the surface soil for over 4 weeks in plots that received CAN application. Splitting of N from one to three applications reduced the leaching loss from 53 to 28% of applied N. The limed plots with single application of N lost the largest amount of N through leaching. Splitting of N into two applications significantly increased grain yield of maize and rice. Further splitting into three splits increased the yield of rice only. Percentages recovery of applied N by maize were 22, 35, and 41% and 31, 39, and 61% by rice for one application, two and three splits, respectively. With one application of N during each cropping season, bare fallow retained 37% of the total applied N (240 kg N/ha) within the 0- 120-cm depth at the end of the second cropping season.
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