Water runoff and soil loss from wheat fields in the inland Pacific Northwest (PNW) USA is often severe during the winter when rain or snow melt occur on frozen soils. Annual precipitation in this region varies from 150 to 600 mm and characteristically 60% occurs between November and March. Water erosion in the wheat-fallow rotation is most severe during the winter of the crop year because of the winter precipitation pattern, long steep slopes, very little ground cover from crop residue or wheat seedlings, and low water infiltration rates through frozen soil. Additional management practices are needed to combat erosion events associated with frozen soil. Research was conducted at 9 on-farm sites in eastern Washington state from 1993 to 1999 to determine the effects of subsoiling fall-sown wheat on 15 to 40% slopes prior to soil freezing on soil loss, water infiltration into the soil, and grain yield. The experimental design at each site was a randomized complete block with 6 replications of 2 treatments: subsoiled and control. Two types of subsoilers were used over the 6-yr period: (i) a 5-cm-wide shank operated 40 cm deep on the contour with shanks spaced 4 or 6 m apart to cut a continuous groove in the soil, and (ii) a rotary 'sharks tooth' implement which creates a 40 cm deep, 4-litre-capacity hole every 0.7 m 2. The sharks tooth subsoiler causes less soil disturbance and less damage to wheat plants than continuous shank channels. Results show that, when water runoff on frozen soils occurs, tillage channels or holes (i) reduce soil loss by retarding rill erosion, (ii) increase water infiltration, and (iii) do not reduce or increase grain yield. Many wheat growers have started to adopt these, or similar, soil conservation practices on their farms.
