Recovery of field-applied fertilizer nitrogen by rice

Field estimation of the recovery of fertilizer N applied to rice (Oryza sativa L.) was carried out as an appraisal of the prevailing fertilizer management practice. The study was carried out on a farmers field for two growing seasons. Soil samples at three depths, 0-15 cm, 15-30 cm and 30-45 cm and water samples were collected every two weeks during the growing season. The plant recovery of fertilizer N was estimated by the difference between the N treated plots and the controls. Although this method could overestimate the fertilizer N recovery due to priming effect, under the conditions in the study the effect was assumed to be minimal. An increase in ammonium N content in the soil was not observed even immediately after N application. The removal of fertilizer N from the soil through either plant uptake or loss processes was very rapid. Higher ammonium N was only detected in the water samples early in the growing season but not at the mid-season after the second N application. The recovery of fertilizer N in the plant were 36% and 30% during the main and off-seasons respectively. Most of the N was in the grain. About 69 kg of the 95 kg of the total-N removed by the crops per hectare was derived from the soil. More than 64% of the fertilizer N applied was either lost or immobilized in the soil