The positive effects of soil organic matter (OM) on soil properties that influence crop performance are well documented. But definitive and quantitative information of differential effects of soil OM contents is lacking for the northern Great Plains. The objective of this study was to quantify the contribution of a unit quantity of soil OM to productivity. Experiments were conducted on Williams loam (fine-loamy, mixed, Typic Argiboroll) for 4 yr in the same field. The variables were soil OM content of the upper 30.5 cm together with all combinations of three postplanting soil available N levels (55, 90, and 125 kg N ha-1 as NO3-N to 1.2 m) and three water levels. Water levels were uniformly maintained with a trickle system that independently metered water to each plot for each soil available N level. Pretillering spring wheat (Triticum aestivum L.) plant population decreased as soil OM content decreased in 3 of 4 yr. On an annual basis, highest total aerial dry matter and grain yields were associated with highest OM contents. The contribution of 1 Mg OM ha-1 to soil productivity, across the range of 64 to 142 Mg OM ha-1, was calculated as equivalent to 35.2 kg ha-1 for spring wheat total aerial dry matter and 15.6 kg ha-1 for grain yield. Loss of productivity associated with a depletion of soil OM in the northern Great Plains is primarily a consequence of a concomitant loss of fertility.
