SPRING WHEAT YIELD TRENDS AS INFLUENCED BY FERTILIZER AND LEGUMES

The benefits and environmental impacts of using fertilizers rather tha n legumes to promote more sustainable agricultural production systems are in question. The objective of this study was to demonstrate the lo ng-term consequences of withholding fertilizer N and P on wheat (Triti cum 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, Saskatc hewan, on a Udic Haploboroll with heavy clay texture, was used to asse ss the influence of fertilizer (N and P), sweet-clover (Melilotus offi cinalis L.) GM, and alfalfa-bromegrass (Medicago sativa L. - Bromus in ermis Leyss.) hay on yield trends of hard red spring wheat grown on wh eat stubble in rotations of varying cropping intensity. When no fertil izer was applied, yields of wheat grown on wheat stubble trended stead ily downward for fallow-wheat-wheat (F-W-W),(decline of 0.41 bu/acre/y r) due to increased soil erosion and declining fertility; yield trends declined less sharply (0.16 bu/acre/yr) for unfertilized continuous w heat, because of less soil erosion. Inclusion of GM in an unfertilized GM-W-W rotation increased the yields of wheat grown on stubble, but t he yield trends were still negative (decline of 0.39 bu/acre/yr), prob ably because legumes do not provide any P to the system. Even though t he unfertilized F-W-W-hay (H)-H-H system had low available P, it maint ained 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 posi tive 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 su stain crop production in a manner similar to that provided by responsi ble use of fertilizers.