Nitrogen (N) management practices of corn producers have been called into question because of nitrate contamination problems in surface and ground water. Fertilizer N has been strongly implicated as the major source of much of the nitrate that now exists in our water resources. Cultural practices used in corn production lend themselves to possible nitrate leaching because practical limitations require a majority of the fertilizer N to be applied well ahead of when it can be used by the crop. Nitrogen uptake by corn is anything but uniform throughout the growing season. Little N is used by a corn crop before mid June and by silking in mid to late July 70 to 90% of total N uptake at harvest will already be in the plant. This scenario implies that only 10 to 30% of total N uptake occurs after silking even though the crop accumulates about half of its dry matter after silking. This simple analogy assumes that once N enters the plant it is not lost until the crop matures.