SPRING WHEAT YIELD TRENDS AS INFLUENCED BY FERTILIZER AND LEGUMES

The benefits and environmental impacts of using fertilizers rather than legumes to promote more sustainable agricultural production systems are in question. The objective of this study was to demonstrate the long-term consequences of withholding fertilizer N and P on wheat (Triticum aestivum L.) yields, and to determine if inclusion of legume-green manures (GM), or hay crops in rotations can maintain wheat yields. A 34-yr crop rotation experiment being conducted at Indian Head, Saskatchewan, on a Udic Haploboroll with heavy clay texture, was used to assess the influence of fertilizer (N and P), sweet-clover (Melilotus officinalis L.) GM, and alfalfa-bromegrass (Medicago sativa L. - Bromus inermis Leyss.) hay on yield trends of hard red spring wheat grown on wheat stubble in rotations of varying cropping intensity. When no fertilizer was applied, yields of wheat grown on wheat stubble trended steadily downward for fallow-wheat-wheat (F-W-W),(decline of 0.41 bu/acre/yr) due to increased soil erosion and declining fertility; yield trends declined less sharply (0.16 bu/acre/yr) for unfertilized continuous wheat, because of less soil erosion. Inclusion of GM in an unfertilized GM-W-W rotation increased the yields of wheat grown on stubble, but the yield trends were still negative (decline of 0.39 bu/acre/yr), probably because legumes do not provide any P to the system. Even though the unfertilized F-W-W-hay (H)-H-H system had low available P, it maintained stubble wheat yields at a generally constant level (decline of 0.08 bu/acre/yr) due to low soil erosion and because the hay-containing system markedly increased N-supplying power of the soil. The addition of N and P fertilizer at rates based on soil tests resulted in a positive divergence in yield trends of wheat grown on stubble in F-W-W and continuous wheat systems as both available N and P were maintained at a high level. Legumes probably will require added P if they are to sustain crop production in a manner similar to that provided by responsible use of fertilizers.