SIMULATED EROSION AND FERTILIZER EFFECTS ON WINTER-WHEAT CROPPING INTERMOUNTAIN DRYLAND AREA

Topsoil loss from erosion in the intermountain dry-farming area reduces crop yields. This study tested the hypothesis that the effects of erosion on water storage and wheat (Triticum aestivum L.) yield could be partially alleviated by applying appropriate fertilizers. Two sites were used, one on Rexburg silt loam, a coarse-silty, mixed, frigid Calcic Haploxeroll, and the other on Newdale silt loam, a coarse-silty, mixed frigid Calciorthidic Haploxeroll. Topsoil-depth treatments were +15, 0, -15, or -30 cm changes relative to the original surface. After making the soil depth changes, 54 kg P ha-1 were incorporated on one half of each topsoil depth plot and the other half received no P. These P or no P plots were split for applications of 0, 34 or 68 kg N ha-1. Phosphorus had no effect on wheat yield. Without fertilizer N, yields on -15 and -30-cm plots were reduced 46 and 55%, respectively, but increased 69% from the addition of 15 cm of topsoil, compared with the 0-cm plot. Removing 15 and 30 cm of topsoil also reduced the upper limit of N-fertilized production to 80 and 65%, respectively, of production on undisturbed N-fertilized plots. Three kilograms fertilizer N ha-1 each crop year offset each centimeter of soil removed, but only to the new lower production limit. All plots had similar amounts of stored, available soil water in the spring, but a large fraction of this water remained unused at harvest on plots with 15 and 30 cm of topsoil removed because the low-yielding wheat did not use as much water. Profile water differences at harvest were no longer apparent by the next spring, following winter recharge. Unused water at harvest, which partially filled the soil profile, reduced winter infiltration and contributed to subsequent runoff from precipitation on those plots. Adding N fertilizer was only a partial solution to topsoil deficiencies.