Tillage Depth and Timing Effects on Soil Water Profiles in Two Semiarid Soils

A 2‐yr winter wheat (T riticum aestivum L.)–fallow rotation continues to be the most common cropping system in much of the U.S. Pacific Northwest. The sustainability of soils in the region depends on our ability to halt or greatly reduce wind and water erosion. An incomplete understanding of how tilled summer fallow preserves seed‐zone moisture for timely fall crop establishment has slowed efforts to optimize tillage techniques for creating profitable and erosion‐resistant systems. This 2‐yr study created a series of soil mulches at two sites representing major soils in the region. It was found that timing and depth of mulch creation had consistent effects at all four site‐years. Tillage performed in mid‐June to depths of 10 and 15 cm preserved up to 0.01 kg kg −1 greater water content than no or 5‐cm tillage, an amount of water that can make substantial differences in the germination of winter wheat. The later or shallower tillage treatments produced water contents similar to zero tillage below the 15‐cm depth. Temperature profiles at 1‐cm resolution demonstrated different shapes under different mulch treatments, which may prove useful in making quick mulch performance comparisons in the field. To optimize the timing and depth of summer fallow tillage, it will be necessary to characterize spring water storage plus the potential for end‐of‐summer water storage for each soil type.