SOIL NITRATE PROFILE AND RESPONSE OF POTATOES TO FERTILIZER-N IN RELATION TO TIME OF INCORPORATION OF LUPIN (LUPINUS-ALBUS)

Lupin (Lupinus albus L. 'Ultra') was evaluated as a green manure crop the year preceding potato production on Prince Edward Island for 3 yr. One pass of tandem disc was used to incorporate lupin green manure on 1 September (early) or 1 October (late). Barley residue following com bine harvest was also incorporated in the same way in early September. One treatment of lupin was left unharvested with no fall incorporatio n (none), and in another treatment of lupin the seed was fall harveste d with no fall incorporation. All treatments were moldboard plowed the following spring. Potato (Solanum tuberosum L. 'Russet Burbank') was planted with three rates of nitrogen (0, 50, and 200 kg ha-1) band app lied. Tuber yield and tuber N content increased with increasing rates of N applications in all incorporation treatments. Following late inco rporated lupin, potato had higher tuber yield at 0 N than following th e early incorporated lupin, while both treatments were higher than bar ley. Potato petiole nitrate-N levels and the mean spring nitrate-N con centrations in the soil were lower following barley than following lup in, and generally lower following early compared to late incorporation of lupin green manure. These data indicate the time of fall incorpora tion of a legume can have a significant effect on the N response of a succeeding crop. Early fall incorporation can result in significant fa ll N mineralization and winter leaching. Research on N response after legumes in rotation should consider the impact of time and method of r esidue incorporation.