QUANTIFICATION OF THE EFFECT OF SOIL ORGANIC-MATTER CONTENT ON SOIL PRODUCTIVITY

The positive effects of soil organic matter (OM) on soil properties th at influence crop performance are well documented. But definitive and quantitative information of differential effects of soil OM contents i s lacking for the northern Great Plains. The objective of this study w as to quantify the contribution of a unit quantity of soil OM to produ ctivity. Experiments were conducted on Williams loam (fine-loamy, mixe d, Typic Argiboroll) for 4 yr in the same field. The variables were so il OM content of the upper 30.5 cm together with all combinations of t hree postplanting soil available N levels (55, 90, and 125 kg N ha-1 a s NO3-N to 1.2 m) and three water levels. Water levels were uniformly maintained with a trickle system that independently metered water to e ach plot for each soil available N level. Pretillering spring wheat (T riticum aestivum L.) plant population decreased as soil OM content dec reased in 3 of 4 yr. On an annual basis, highest total aerial dry matt er and grain yields were associated with highest OM contents. The cont ribution 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 sp ring wheat total aerial dry matter and 15.6 kg ha-1 for grain yield. L oss of productivity associated with a depletion of soil OM in the nort hern Great Plains is primarily a consequence of a concomitant loss of fertility.