N CREDIT OF SOYBEAN TO A FOLLOWING CORN CROP IN CENTRAL ONTARIO

Field studies were conducted to determine the nitrogen (N) fertilizer replacement value (NFRV) when soybean (Glycine max [L.] Merrill) prece ded corn (Zea mays L.) in the rotation (S-C), compared to corn precedi ng corn (C-C). Large, replicated blocks of soybean and corn were estab lished in 1993 and 1994 near Elora, Ontario. In the following year, ea ch large block was subdivided into smaller plots. Fertilizer N was app lied at six rates from 0 to 200 kg N ha(-1) to the second-year corn cr op. Corn grain yield responses to fertilizer N were fitted by quadrati c regression. Maximum economic rate of N was calculated for each crop sequence and NFRV's were determined Corn yields were consistently high er when grown after soybean (S-(C) under bar) than after corn (C-(C) u nder bar). Maximum corn yields were 10.4 and 12.3 Mg ha(-1) in 1994 an d 1995, respectively. NFRVs for S-(C) under bar, compared to C-(C) und er bar, were 41 and 59 kg N ha(-1) in the two years. As a result of th ese studies and numerous other experiments, recommended fertilizer N r ates have been changed to 30 kg N ha(-1) less for S-(C) under bar than for C-(C) under bar in central Ontario.