Inorganic nitrogen movement in tropical soils

A field experiment was conducted to follow the transformation of ammonium fertilizer applied in rainfed-upland fields. The study was done during the rainy season on a heavy-textured, poorly rained soil. Ammonium fertilizer was applied in rows, and soil samples from different depths were analyzed to check the vertical distribution of inorganic nitrogen. In order to learn the relation between nitrogen loss and nitrification, the effect of nitrification inhibitor was also checked. Ice variety IR5 was used to check nitrogen uptake.

The ammonium content decreased by about 40 per cent up to I month after fertilizing and about 90% up to 2 months after fertilizing in the unplanted plots. Up to 5 months it decreased to a level equal to or lower than that before fertilizer application. The accumulation of nitrate was observed in the lower soil layers-after 3 months, but the accumulated amount was too low to account for the loss of ammonia from the surface. The overall loss of mineral nitrogen within 60 cm of the surface was observed after 5 months in unplanted plots. The addition of nitrification inhibitor retarded the decrease of ammonium nitrogen up to 2 months. After 3 months, the nitrifying activity began to recover.

Only 23 to 27 per cent of the fertilizer nitrogen was taken up by the rice plants, indicating low efficiency.