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Movement and Distribution of Ammonium-N Following Deep Placement of Urea in a Wetland Rice Soil¹

ABSTRACT

The vertical distribution of NH₄⁺-N following subsurface placement of different forms of urea was studied in incubated, undisturbed wetland soil cores. For prilled urea, supergranule urea, and prilled urea in mudballs placed at the 10-cm depth, peak concentration of NH₄⁺-N was near the placement site and decreased with time, whereas after placement of sulfurcoated urea (SCU-21) at the same depth, peak concentration of NH₄⁺-N increased over a period of 4 weeks. With time, the NH₄⁺-N tended to move downward more than upward from the placement sites, probably because of the mass flow of percolating water.

In another experiment, the movement and spatial distribution of NH₄⁺-N were studied following application of 2-g supergranules of urea (SGU) and sulfur-coated supergranules of urea (SC-SGU) at a depth of 10 cm in transplanted and cultivated wetland fallow plots. After 2 weeks, NH₄⁺-N concentration gradient for SGU was 1,850 to 32 µg N/cm³ wet soil, over a distance of 10–12 cm from the placement site. The corresponding gradient for SC-SGU was 287 to 32 µg N/cm³ wet soil, over a distance of 5–7 cm from its placement site. For SGU in transplanted plots, the concentration gradient decreased steadily through 8 weeks, whereas for SC-SGU it increased during the first 4 weeks and then decreased. The disappearance of NH₄⁺-N with time and distance from the site of application is attributed to diffusive transport or convective transport, or both, and root-sink effect. The general movement of NH₄⁺-N was downward > lateral > upward. The apparent benefits of deep placement of urea in a wetland rice soil are discussed.

¹ Contribution from the Dep. of Agronomy, The Int. Rice Res. Inst., Los Baños, Laguna, Philippines.

² Postdoctoral Research Fellow and Head, respectively, Dep. of Agronomy, The Int. Rice Res. Inst., Los Baños, Laguna (mail address: P. O. Box 933, Manila), Philippines.

Received for publication July 16, 1979. Accepted for publication January 2, 1980.