William F. Schillinger1. Abstract. Water runoff and soil loss from wheat fields in the inland Pacific Northwest (PNW) of the USA is often severe during the winter ...
This is not a peer-reviewed paper. Pp. 32-35 in Soil Erosion Research for the 21st Century, Proc. Int. Symp. (3-5 January 2001, Honolulu, HI, USA). Eds. J.C. Ascough II and D.C. Flanagan. St. Joseph, MI: ASAE.701P0007
Reducing Water Runoff and Erosion from Frozen Agricultural Soils William F. Schillinger1 Abstract Water runoff and soil loss from wheat fields in the inland Pacific Northwest (PNW) of the 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 nine on-farm sites 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 six replications of two treatments: subsoiled and control. Two types of subsoilers were used over the 6-year 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-liter-capacity, hole every 0.7 m2. The sharks tooth subsoiler causes less soil disturbance and less damage to wheat plants compared to 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. Keywords. Frozen soils, Water infiltration, Pacific Northwest, Subsoiling, Wheat production. Introduction Water erosion is often severe during winter of the crop year in the dryland winter wheat production areas of the inland Pacific Northwest (PNW). A winter wheat-fallow rotation , where only one crop is grown every two years, is practiced on more than 2 million hectares. Soil freezing may occur to depths of 10 cm several times during the winter with occasional freezing to 40 cm (Papendick and McCool, 1994). Partial or complete soil thawing frequently occurs between freezing events. Soil loss from water erosion can be especially high when snowmelt or rain occur on thawed soil overlying a subsurface frozen layer. Inclement weather conditions frequently limit the amount of crop residue available for erosion control. Dry seed zone soil conditions in the fall often reduce winter wheat stand establishment and consequently limit the amount of crop residue produced. Winter kill, drought, fire and disease also may result in insufficient residue for erosion control. Additional management practices are needed for the Inland PNW to combat erosion events associated with frozen soil. Where soil freezing is common, growers routinely chisel or subsoil wheat stubble after harvest at the beginning of the fallow cycle, but do not employ this practice during the crop year. Pikul et al. (1992), using a rainfall simulator on a Walla Walla silt loam soil, found that contour shanking increased water infiltration into frozen soil during the crop winter. But there were no differences in water infiltration between shanked and non-shanked plots when the depth of freezing was greater than the depth of the tillage channel (Pikul et al., 1996). Saxton et al. (1981) reduced water runoff and erosion during the crop winter by filling soil slots with crop residue to prevent freezing and surface sealing. For these systems to be successful, a connecting water channel from the soil surface to below the frozen layer must be maintained during the time frozen soil/runoff events occur. Forming tillage slots at the time of planting in late summer-early fall does not improve infiltration because the loose dry surface soil sloughs back into the tillage channel. One solution to the problem of maintaining the tillage slot integrity during the critical erosion period is to till the soil after it freezes. Wilkins and Zuzel (1994) developed a special tillage tool to create tillage channels through the frozen layer with minimum soil disturbance. This tool was used in northeast Oregon on frozen fields that were planted to winter wheat. Frozen soil tillage destroyed wheat plants in and adjacent to the tillage slot but grain yields were not reduced due to stand reduction or disease (Wilkins and Zuzel, 1994). 1
Corresponding author: William F. Schillinger, Associate Scientist and Extension Specialist, Department of Crop and Soil Sciences, Washington State University, P.O. Box B, Lind, WA 99341; tel.: (509) 677-3673; fax: (509) 6773676; e-mail: .
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Although this technique of tilling when the soil is frozen is successful, there is a narrow window of time when tillage can be performed. Wilkins and Zuzel (1994) tilled when there was 5-to 10-cm of frozen soil. If the frozen layer is thin it will not support the tractor and ruts are formed and the soil compacted. The other extreme is a thick layer of frozen soil that requires excessive power to fracture and large chunks of soil are uprooted causing difficulty at harvest. Another approach for providing tillage channels through frozen soil in wheat fields is to plant in the fall, wait for rain, and then make channels with deep tillage shanks prior to soil freezing. Fall rains moisten and firm the loose dry surface mulch created during summer fallow. Tillage channels formed with these soil conditions may tend to stay open during the winter when frozen soil runoff occurs. It is also more practical than tilling frozen soil, allowing growers a longer time period in late fall to create the tillage channels. The objectives of this research were to determine the effects of deep subsoiling fall-sown wheat on steep slopes prior to soil freezing on: water infiltration into the soil, soil erosion, and grain yield. Materials and Methods On-farm experiments were conducted at 9 sites in eastern Washington from 1993 to 1999. Annual precipitation at the sites ranges from 240 to 360 mm. The soil type at all sites is silt loam and soil depth is greater than 2 m. Slope of sites ranged from 15 to 40%. The experimental design was a randomized complete block with six replications of two treatments: subsoiling and control. Plot size varied at each location but, on average, each treatment was 100 m long and 20 m wide. Soft white winter was sown into summer fallow in September, October, or November at rates of 45 to 90 kg ha-1 along the contour of the hillside. Uniform wheat seedling emergence and plant establishment were attained at all sites during all years. After the onset of fall rains, subsoil tillage plots were established on unfrozen soil. In 1993, 1994, and 1995, subsoiling was with a shank operated at . 40 cm deep which ripped the soil and created a continuous tillage channel. These contour tillage slots were established at intervals ranging from 4-to 6-m along the slope. The shank, pulled by a crawler tractor, was lifted out of the soil when crossing control plots. From 1996 to 1999, a rotary "shark's tooth" subsoiler was used in lieu of the continuous shank. The rotary subsoiler features staggered rows of 55-cm-long points that form deep pits in the soil. This implement produces one pit every 0.7 m2 and reduces disturbance to the wheat stand compared with deep ripping with a continuous shank. Depth of soil penetration was 40 cm and was controlled by adding water to storage tanks built on the shark's tooth implement. Each pit can hold 4 liters of water. Water Infiltration, Soil Loss, and Grain Yield Soil water content was measured within 3 days of the subsoiling operation and periodically thereafter throughout the winter. Volumetric water content of the 30- to 180-cm soil depth was measured in 15-cm increments with a neutron probe. Water content of the 0-to 30-cm soil depth was measured gravimetrically, in two 15-cm core samples. In 1993, 1994, and 1995 (when using the continuous shank) neutron probe access tubes were installed in the tillage channel, and 30, 90 and 150 cm down-slope of a tillage channel. Access tubes were placed in the same general lateral locations in control plots, i.e. where the tillage channel would have been if the shank had not been lifted out of the soil when crossing control plots. With the rotary "shark's tooth" subsoiler (1996 to 1999) access tubes were placed 30 cm down slope from a pit created by the implement, with three access tubes per plot. Soil loss from rill erosion during the winter was measured using the voided rill method (Everts and Riehle, 1980). Total cross-sectional area of rills near the top, middle and base of each plot were averaged to determine soil loss on a whole-plot basis. Precipitation, minimum-maximum air temperature, and soil temperature at 5, 10, 20, and 30 cm depths were recorded hourly at all sites during all years at a weather station placed near each site. Grain yield was measured either from i) hand-cut samples obtained from 1-m row sections near each access tube in all plots at harvest in August, with clean grain yield, kernels per spike, 1000 kernel weight, and dry matter determined from these samples, or by ii) harvesting a 8-m-wide swath through each plot with a commercial combine and then auguring grain into a truck fitted with weight pads under each tire. An analysis of variance was conducted for water content in 15 cm depth increments and for the total 180-cm soil profile on each sampling date, soil loss from rill erosion on each sampling date, and grain yield. Treatments were considered significantly different if the P-value was
