INFLUENCE OF APPLIED NITROGEN ON POTATO .2. RECOVERY AND PARTITIONING OF APPLIED NITROGEN

Maximizing fertilizer nitrogen (N) uptake efficiency, while maintaining crop productivity may reduce potential nitrate contamination of groundwater. A two-year field investigation tvas conducted to evaluate the effects of applied N on fertilizer N uptake, uptake efficiency, and total fertilizer N recovery of potato (Solanum tuberosum L, var. Russet Burbank) grown on irrigated sandy soils in Michigan. Nitrogen was applied as N-15-depleted ammonium sulfate [(NH4)(2)SO4] at rates 0, 56, and 112, kg N ha(-1) in a single application at planting or 112 and 168 kg N ha(-1) in split applications during the growing season. Fertilizer N uptake efficiency was relatively unaffected by the N treatments. Fertilizer N uptake efficiency for the whole crop at onset of senescence averaged 52 percent, while values calculated for tubers at harvest were 34 percent. An average of 27 percent of the applied N was present in the soil to a depth of 120 cm after harvest. Approximately 83 percent of this N was found in the 0-30 cm depth. Over 90 percent of the recovered soil N was in the organic form. In this investigation, crop fertilizer N uptake and fertilizer N recovery in soil averaged 63 percent and was largely unaffected by the rate or timing of fertilizer N applications.