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PNW Conservation Tillage Handbook Series
Chapter 2 - Conservation Tillage Systems and Equipment, No. 18, May 1997

Tillage Mulch Depth Effects During Fallow on Wheat Production and Wind Erosion Control Factors

Authors: William Schillinger, Dryland Research Agronomist, Washington State University, Ritzville, WA; Curtis Hennings, Grower, Ritzville, WA; Robert Papendick, Soil Scientist (retired), USDA-ARS, Pullman, WA; Harry Schafer, Agricultural Research Technician, Washington State University, Ritzville, WA; Roger Veseth, Extension Conservation Tillage Specialist, Washington State University and the University of Idaho, Moscow, ID.


Blowing dust from summer fallow is a major soil loss and air quality concern in winter wheat production areas of the inland Pacific Northwest (PNW). The objective of our 3-year on-farm study in a 11.3 inch precipitation zone in eastern Washington was to determine the effects of tillage mulch depth during fallow on surface soil roughness, residue retention, seedzone water storage, wheat establishment, and grain yield. Soil was a Ritzville silt loam. Mulch depth combinations were created by primary spring tillage with non-inversion implements at 4-or 6-inch depths, and with subsequent rodweedings at 2-or 4-inch depths. Tillage mulch depth during fallow did not affect seedling emergence after two wet fallow cycles, but wheat spike density was consistently greatest in deep-tilled plots. In a dry fallow cycle, when dry soil extended beneath the rodweeder or secondary tillage layer, deep tillage increased stand establishment from 3-to-6 seedlings/ft2, grain yield from 65-to 79 bu/acre, and residue production from 2.5-to 3.7 ton/acre compared to shallow tillage. Surface soil clods > 2 inches diameter, desirable for wind erosion control, increased with tillage mulch depth from 6.2-to 9.4 tons/acre in 1994, and from 9.8-to 16.5 tons/acre in 1995. A drawback to deep tillage mulches was the need to reduce tractor speed during planting. Surface residue retention was not affected by tillage mulch depth. Results show that surface clod structure and roughness during fallow can be maintained to protect the soil from erosion, mostly benefiting wheat production potential.



A winter wheat (Triticum aestivum L.)-summer fallow rotation is practiced on about 3 million acres in the low-precipitation (< 12 inch annual) dryland areas in the inland Pacific Northwest (PNW) (Ramig et al., 1983). The climate of this region is characterized by winter precipitation with warm, dry summers. Successful establishment of winter wheat on fallow from late summer planting depends on carryover moisture from the previous winter and is essential for high yields and protection of soil from both wind and water erosion (Leggett et al., 1974). Additionally, growers plant cultivars with the ability to emerge under conditions of poor seedzone water, high temperature, and deep planting (Donaldson, 1996). In dry years, winter wheat is planted as deep as 8 inches below the summer fallow soil surface to reach adequate water for germination, and seedlings emerge through as much as 6 inches of soil cover (Schillinger, 1996). In the driest years, deep planting is not attempted. Rather, seed is shallowly 'dusted in' to dry soil, or planting is delayed until the arrival of fall rains, or postponed until spring.

The benefits of tillage during fallow on water retention and winter wheat stand establishment have been reported by several workers. Over-summer water loss from the seedzone depth occurs by evaporation across a dry soil layer generally 3-to 5-inches thick (Hammel et al., 1981). Seedzone water is best conserved by a loose soil mulch of maximum resistance to vapor and liquid water flow, and maximum thermal insulation, overlying a seedzone having good capillary continuity with deeper soil layers (McCall and Hails, 1921; Papendick et al., 1973; Lindstrom et al., 1974). Finely-divided soil aggregates within the soil mulch are most effective in retarding water loss during fallow, but tillage to create such a mulch often buries excessive surface residue and may pulverize surface clods.

A soil surface deficient in roughness, clods, and residue may pose a serious wind erosion threat (Fryrear, 1984; Fryrear and Bilbro, 1994), especially with large (160-to 640-acre) fields and the frequent high winds common in the inland PNW. In the semiarid Canadian prairies, measured soil loss from summer fallow during individual wind storms has exceeded 13 tons/acre (Larney et al., 1995). In addition to loss of soil, blowing dust from excessively tilled soils is a major air quality concern. The Federal Clean Air Act of 1990 mandates control of dust particulates 10 micron and smaller (PM-10) which may lodge in lung tissue and be a health concern (Saxton, 1995). Surface residue, roughness, and clods are effective in reducing wind erosion from summer-fallowed soils.

Many growers in low-rainfall wheat-fallow areas of the PNW use V-shaped sweeps or similar non-inversion implements for primary spring tillage to maximize surface residue and clod retention. Rodweeders are used for secondary tillage during late spring and summer to control weeds and maintain the dry mulch layer. Papendick et al. (1973), operating sweeps and rodweeders at several depths during fallow, found that: (i) increasing the depth of the tillage mulch reduced seedzone water loss enough to benefit wheat seedling emergence and; (ii) seedzone water was best retained when rodweeding operations were conducted at the depth of initial tillage, i.e. conducting all tillage operations at the same depth to create an abrupt break between tilled and untilled soil.

More information is needed concerning both the agronomic feasibility and environmental friendliness of conservation tillage practices during fallow in the PNW. This paper compares the effects of four fallow tillage mulch depth combinations created by non-inversion tillage implements on:

surface residue retention, surface and subsurface soil cloddiness, seedzone water retention, winter wheat seedling emergence, and grain yield components and crop characteristics.



An on-farm experiment was conducted between August 1992 and July 1996 on the Curtis Hennings farm in Adams County, Washington. Annual precipitation at the site averages 11.3 inches and the cropping pattern is winter wheat - fallow. The soil was a Ritzville silt loam with 1.5% OM in the surface 4 inches. Soil depth was > 6 ft and slopes were < 2%. Precipitation and minimum-maximum air temperature were recorded daily at the Hennings's farmstead located 1 mile from the experiment site.

The experimental design in 1992-93 and 1993-94 was a randomized complete block of three tillage mulch depth treatments replicated four times. In 1994-95, a split-plot design included four tillage mulch depth treatments (main plots) and two tractor speeds for rodweeding (subplots). Main plots were 620 ft by 72 ft and subplots 310 ft by 72 ft. Paired adjacent parcels of land were used so that data could be collected each year from both crop and fallow phases of the experiment.

Soils were subsoiled at the beginning of each fallow cycle to a depth of 16 inches with shanks spaced 3 ft apart (Table 1). In March, plots were sprayed with glyphosate herbicide to control weeds and then lightly disked to create a thin (1 inch) soil mulch. In mid-April in 1994 and mid-May in 1993 and 1995, primary tillage was conducted with V-shaped sweeps at shallow (4 inch) or deep (6 inch) depths (Table 1). Aqua ammonia nitrogen was delivered with the V-shaped sweeps at rates ranging from 50 to 58 lb N/acre. Between June and August, plots were rodweeded two times at either a shallow (2 inch) or deep (4 inch) depth. Three tillage mulch depth combinations were compared in 1993 and 1994. These were: (i) shallow primary tillage at 4 inches and shallow rodweedings at 2 inches (shallow-shallow); (ii) shallow primary tillage at 4 inches and deep rodweedings at 4 inches (shallow-deep), and; (iii) deep primary tillage at 6 inches and deep rodweedings at 4 inches (deep-deep). Tractor speed for rodweeding operations was 6 miles/hour. In 1995, an additional tillage mulch depth treatment was included: deep primary tillage at 6 inches and shallow rodweedings at 2 inches (deep-shallow), and subplots were slow (3.5 miles/hour) and fast (6 miles/hour) tractor speeds for rodweeding. To reduce variability, all measurements described hereafter were obtained from a 9-ft-wide swath made by a specific section of the rodweeder implement.

Water measurements in the 6 ft soil profile were made each spring prior to primary tillage and again in late August before planting. Soil volumetric water content in the 1-to 6-ft depth was measured in 0.5-ft increments by neutron attenuation. Volumetric water in the 0-to 1-ft depth was determined from two 0.5-ft core samples using gravimetric procedures (Gardner, 1986). Additionally, in late August, volumetric water content in the seedzone was determined in 0.8-inch increments to a depth of 6 inches in 1993, and to 9 inches in 1994 and 1995, using an incremental soil sampler. Soil water measurements were always obtained from three locations in each plot.

Surface soil cloddiness was determined at the end of the fallow cycle in 1994 and 1995 by measuring the diameter of individual soil clods within a 1-yard diameter sampling hoop randomly positioned at three locations in each plot. All clods with diameters > 2 inches were sorted into 0.4-inch size increments and the mass of each size group determined. Surface residue at the end of the fallow cycle in 1994 and 1995 was measured by collecting and weighing all aboveground drymatter within three 1-yard diameter sample hoops randomly placed in each plot.

Subsurface soil cloddiness was measured before planting in 1995 by gently sieving 0.5 ft3 of soil from the 0-to 4-inch mulch layer through stacked 2 inch2, 1 inch2, and 0.5 inch2 mesh screens. Mass of clods not passing through each of the three mesh screens was then measured. Subsurface clods measurements were obtained from the same area where surface clods had been removed; thus surface clods > 2 inches diameter were excluded from subsurface samples.


Table 1. Field operations conducted during three fallow cycles at Ralston, WA.


1992-93 fallow cycle

1993-94 fallow cycle

1994-95 fallow cycle


Subsoil 16 in. deep, 3 ft spacing Subsoil 16 in. deep, 3 ft spacing Subsoil 16 in. deep, 3 ft spacing


Spray glyphosate herbicide, 12 oz/acre. Light disking. Spray glyphosate herbicide, 12 oz/acre. Light disking. Spray glyphosate herbicide, 12 oz/acre. Light disking.


  Primary tillage with 16-inch wide sweeps with attached harrow. Aqua N injection, 60 lb/acre + sulfur 12 lb/acre.  


Primary tillage with 32-inch wide sweeps with attached rotary hoe. Aqua N injection, 55 lb/acre + sulfur 11 lb/acre. First rodweeding. Primary tillage with 18-inch wide sweeps with attached harrow. Aqua N injection, 75 lb/acre + sulfur 15 lb/acre.


First rodweeding   First rodweeding


Second rodweeding Second rodweeding  


  Plant 'Moro' winter wheat, 35 lb/acre. Second rodweeding


Plant 'Eltan' winter wheat, 35 lb/acre.   Plant 'Lewjain' winter wheat, 23 lb/acre.

Plots were planted to soft white winter wheat in late August or September (Table 1) with deep furrow split-packer drills. Row spacing was 16 inches and planting rate varied from 23-to-35 lb/acre. Seed placement and soil roughness after planting were determined in 1994 and 1995 by measuring: (i) furrow ridge height, i.e. the distance to the ridge from the bottom of the furrow; (ii) depth of soil covering the seed, and; (iii) depth of seed placement below the tillage layer created by the rodweeder. Wheat seedling emergence and stand establishment were determined by counting individual plants in 1-yard row segments at 24-hr intervals beginning 8 days after planting (DAP). Three row segments were selected and marked within each plot prior to emergence of wheat seedlings.

Grain yield and spike density were measured from hand-cut samples obtained from three 1-yard row sections in each plot at harvest in July or August. Clean grain yield, kernels/spike, 1000 kernel weight, and dry matter were determined from these samples.

Statistical analysis was conducted using standard ANOVA procedures for 6 ft soil profile and seedzone soil water, surface soil cloddiness, subsurface soil cloddiness, surface residue, seed placement characteristics, wheat emergence on each sampling date, and yield components and crop characteristics. Treatments were considered significantly different if the P-value was < 0.05.



Precipitation and Soil Water Retention

The 1992-93 and 1994-95 fallow cycles were wetter than average, whereas the 1993-94 fallow cycle was one of the driest on record (Table 2). About 12 inches of stored water was present in the 6 ft soil profile in the spring of the wet fallow cycles, compared to < 8 inches of stored water in the spring of the dry year (Table 3). Despite spring and summer rainfall, over-summer soil water loss occurred every year in all treatments, but was lowest after the dry 1993-94 winter and greatest following the wet 1994-95 winter (Tables 2 and 3). These data agree with other findings showing summer rainfall in the PNW rarely adding to stored soil water (Massee and Siddoway, 1970). Total 6 ft soil profile water retention was not affected by depth of tillage mulch during the wet fallow cycles, but was significantly improved with deep tillage mulching during the dry fallow cycle (Table 3).


Table 2. Precipitation (inches) occurring during three 13-month fallow cycles compared to the 20-year average at Ralston, WA.

Time Period 1992-1993 1993-1994 1994-1995 20-yr avg.
























13-Month Total





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