POTATO RESPONSE TO LEGUME AND FERTILIZER NITROGEN-SOURCES

Production of potato (Solanum tuberosum L.) in rotation with an N2-fixing legume requires better information on the N contribution from the legume and on management of N fertilizer following a legume. Objectives of this research were to: (i) quantify N accumulation by legumes grown as either green manure or hay; and (ii) evaluate the vine and tuber yield response of a subsequent potato crop to legume and fertilizer N. We established rotations at two Michigan locations in 1987 on McBride sandy loam (coarse-loamy, mixed, frigid Alfic Fragiothods) and Oshtemo sandy loam (coarse-loamy, mixed, mesic Typic Hapludalfs). First-year crops included: alfalfa (Medicago sativa L.), birdsfoot trefoil (Lotus corniculatus L.), and red clover (Trifolium pratense L.) grown as both green manure (0 harvests) and hay (two or three seeding-year harvests); non-dormant 'Nitro' alfalfa hay; sweetclover (Melilotus spp.) and hairy vetch (Vicia villosa Roth) green manures; corn (Zea mays L.); fallow; and potato. The second-year crop was potato, fertilized with 0, 75, 150, or 225 kg N ha-1. Plowdown N yield [PDN = fall(herbage + root)N + spring herbage N] of legumes ranged from 33 (fall-seeded hairy vetch) to 238 kg N ha-1 (sweetclover), with the PDN yield of alfalfa, red clover, sweetclover, and spring-seeded hairy vetch generally exceeding 150 kg ha-1. Seeding-year harvest of legumes tended to have little effect on PDN yield, compared to a green manure crop of the same species. Potato vine dry matter and N content late in the season were 61 to 100 and 75 to 145% higher, respectively, following legumes than following non-legumes, but total and marketable tuber yields were not affected by rotation at either location. Fertilizer N rate increased total, marketable, and cull tuber yield at MRF, with optimum N rates of about 120 and 170 kg ha-1 for marketable and total tuber yield, respectively. Nitrogen rate had no effect on tuber yield at KBS, suggesting that moisture, not available N, limited tuber yield. The difference in vegetative and tuber yield responses to rotation indicates that legume N eventually became available to the potato crop, but probably not early enough in the growing season to affect tuber initiation.