Few studies conducted in western Canada have assessed how crop rotations and fertilization influence soil organic matter content on land that has been cropped for many years. We monitored soil organic matter in the 0- to 0.15- and 0.15- to 0.3-m depths of a 24-yr crop rotation experiment conducted on a medium-textured Aridic Haploboroll in southwestern Saskatchewan. Prior to the study, the land had been in a hard red spring wheat (Triticum aestivum L.)-fallow rotation for {approx}50 yr. Only the 0- to 0.15-m segment showed significant treatment effects. Due to good weather and crop yields in the first 15 yr, soil organic matter had increased under well-fertilized annually cropped rotations, and it remained constant under fallow-containing rotations and under continuous wheat receiving inadequate N fertilizer. Because of several dry years in the final 9 yr of the study, all rotations except a well-fertilized, fallow-winter cereal-wheat system lost organic matter. Changes in organic matter were directly related to the amount of crop residues produced by these systems and their ease of eroding. Soil organic matter was inversely related to apparent N deficit (i.e., N exported in grain minus N applied as fertilizer). The fallow-flax (Linum usitatissimum L.)-wheat rotation receiving N and P fertilizer had the lowest soil organic matter, partly due to low production of crop residues by flax, partly to greater leaching of NO3, and partly due to some loss of flax residues blown from the plots. Soil organic matter in the well-fertilized fallow-winter cereal-wheat rotation remained constant because its shorter fallow period reduced soil erosion, and due to its more efficient use of N, as evidenced by minimal leached NO3-N.
