YIELD AND NITROGEN-UTILIZATION BY RICE AS AFFECTED BY METHOD AND TIME OF APPLICATION OF LABELED NITROGEN

Field experiments using labeled fertilizer N were carried out during 1974 and 1975 utilizing different times and methods of application of (NH4)2SO4 on small field plots of lowland rice (Oryza sativa L.) (2.32 m2). Fertilizer N was applied either by deep placement at the beginning of the season or at 1 or more times during the growing season as surface application. Relative contributions of native soil N and fertilizer N sources to plant uptake were followed during the growing season, and total uptake of native soil N and fertilizer N by rice was determined at harvest. Highest grain yields were obtained where all of the N was applied early in the growing season or where part of the N was applied in early season and part applied no later than mid season. Late season application of part of the N was not as effective. In 1974, recovery of fertilizer N (in grain and straw) was highest in the plots receiving all N by deep placement, while in 1975 split application (half at early season and half at mid season) gave greatest N utilization. For the 2 yr, the recovery of fertilizer in grain and straw ranged from 33-61% of applied N. Rice plants derived more fertilizer N during the early part of the growing season with the rate of uptake decreasing as the season progressed. Recovery of fertilizer N calculated from labeled N data showed lower values, compared to the recovery of fertilizer N estimated by a method based on total N uptake difference between the plots receiving N and the untreated plots. More native soil N was taken up by the crop from the plots receiving fertilizer N than from the check plots in 1974, but there was little difference in uptake in 1975.