Packing Summer Fallow Before Planting Winter Wheat: Agronomic Benefits and Environmental Concerns

Chapter 2 – Conservation Tillage Systems and Equipment, No. 17, May 1997

Authors: William Schillinger, Dryland Research Agronomist, Washington State University, Ritzville, WA; Grant Miller, Grower, Lind, WA; Ron Jirava, Grower, Ritzville, 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; Robert Papendick, Soil Scientist, USDA-ARS (retired), Pullman, WA.

Abstract

Winter wheat (Triticum aestivum L.) is sown as deep as 8 inches below the summer fallow soil surface in dry years in the semiarid Pacific Northwest (PNW). Many growers pack the summer fallow mulch in late August before planting winter wheat to improve stand establishment. But packing may increase blowing dust, which is a major soil loss and air quality concern. A 2-year study was conducted on two silt loam soil types in 11 inch and 9 inch average annual precipitation zones in eastern Washington to determine the agronomic benefits and potential wind erosion hazards associated with packing. Packing a loose, thick surface mulch increased soil bulk density (BD) between the 2-to 5-inch depth. This significantly benefited wheat seedling emergence and stand establishment, which subsequently increased grain yield 9% over non-packed plots. But packing a soil with a thin mulch layer overlying a high BD tillage pan had no effect on soil BD, wheat seedling emergence, or grain yield. Packing rendered the soil more vulnerable to wind erosion at both locations by reducing soil clod mass 55% and surface residue 38% compared to not packing. There are agronomic benefits from packing summer fallow mulch before planting, but packing should be practiced judiciously, and not considered when surface residue or soil clods are lacking. This is especially true for poorly aggregated coarse-textured soils where soil structure is difficult to maintain.

Introduction

Stand establishment is the most important single factor affecting wheat yields under dryland conditions in the PNW (Bolton, 1983). Early establishment of winter wheat on summer-fallowed soils in low (< 12 inch annual) precipitation areas of the PNW is frequently limited by insufficient seedzone water. In most years, soil water content is adequate for germination and establishment just below the firm layer created by rodweeders (Pikul et al., 1985). Rodweeders are typically operated < 4.5 inches deep during late spring and summer to control weeds and maintain a dry soil mulch. The soil mulch reduces evaporation of stored water during dry summer months by disrupting capillary flow which reduces both liquid and vapor flow to the atmosphere (McCall, 1925; Papendick, et al., 1973). In drier fallow cycles, however, soil drying extends below depth of rodweeding. Lindstrom et al. (1976) reported that -7 bar water potential was the lower limit at which to expect winter wheat emergence from deep planting on silt loam soils in eastern Washington.

The time from planting to emergence of wheat seedlings increases, and final stand count decreases, as soil water potential drops below field capacity (Hanks and Thorp, 1956; Lindstrom et al., 1976). The time to emergence and variability in emergence also increases as planting depth increases. Emergence is impeded by soil crusting when rain showers occurring between planting and emergence are followed by rapid drying. Soil crust is harder and less penetrable if surface soil has been pulverized by excessive tillage or is deficient in residue (Hadas and Stibbe, 1977). Residue within the seed row benefits wheat emergence following a rain by reducing rain drop impact on the soil surface (Awadhwal and Thierstein, 1985).

The need to improve the practice of summer fallowing has become an important environmental concern in the PNW. Traditional tillage practices are often intensive and can leave the soil vulnerable to wind and water erosion. Growers frequently have difficulty maintaining the minimum 350 lb/acre surface residue required at the end of the fallow cycle to participate in government farm programs. Blowing dust from excessively worked soils makes driving hazardous and closes roads. Very fine particulates, with diameters 10 micron and smaller (PM-10), are easily transported into urban regions and may be a health hazard, particularly to people with respiratory problems (Piper, 1989). The wind erosion problem in very dry (< 10 inch annual precipitation) wheat-fallow areas is three-fold: (i) not enough crop residue is produced; (ii) soils are generally coarse textured and low in organic matter, making it difficult to retain soil clods, and; (iii) traditional tillage techniques reduce soil roughness and bury excessive amounts of crop residue (Papendick and McCool, 1994). By the end of the fallow period the surface soil mulch is often powdery and deficient in residue.

Growers in low-precipitation dryland areas of the PNW have mixed attitudes towards packing summer fallow before planting winter wheat. Advocates report packing enhances wheat seedling emergence and allows them to obtain stands in dry years where it would otherwise not be possible. This is achieved by: (i) reducing the thickness of the dry surface mulch, allowing deeper penetration of grain drill openers into wetter soil; (ii) providing improved seed-soil contact through increased soil BD and; (iii) rendering a thinner layer of soil covering the seed. Opponents feel packing often creates an unacceptable wind erosion hazard through excessive pulverization of soil clods and residue burial.

Seedzone water loss from summer-fallowed soils tends to increase in August and September in the PNW after the annual shift in the direction of coupled heat and water flows. Packing the summer fallow surface increased soil BD in Oregon (Schillinger and Bolton, 1995), resulting in increased soil volumetric water content to a depth of 4 inches. But at time of planting 1 month after packing, there were no differences in seedzone water content or in wheat seedling emergence between packed and non-packed plots. This suggests that, if wheat seedling emergence is to benefit, planting should be performed soon after packing.

Our objective was to determine the effects of packing the surface mulch of summer fallow immediately prior to planting on: (i) soil BD; (ii) seedzone water content; (iii) reduction in soil cloddiness; (iv) surface residue burial, and; (v) emergence, stand establishment, crop growth characteristics, and grain yield components of winter wheat.

Materials and Methods

A 2-year on-farm experiment was conducted during August and September of 1993 and 1994 at field sites in Adams county, Washington. Annual precipitation at the 1993 site averages 11 inches. Soil was a Ritzville silt loam, with 1.5% OM in the surface 0-to 4 inches. The 1994 experimental site receives an average of 9 inches annual precipitation and soil was a Shano silt loam < 1.0% OM in the surface 0-to 4 inches. Soils at both test sites are > 6 ft deep to underlying bedrock and representative of the loess soils found extensively in the semiarid climatic zone of eastern Washington and north-central Oregon (Lenfesty, 1967).

The experimental design in 1993 was a split block (Little and Hills, 1978) with six replications. Main plots consisted of tillage method (packed vs. non-packed), and subplots were planting depth (deep vs. shallow). Each subplot was 23 x 13 yards. In 1994, a randomized complete block experimental design with six replications included packed and non-packed treatments but not planting depth. Each plot was 23 x 100 yards.

At both locations, soils were subsoiled at the beginning of the fallow cycle to a depth of 17 inches with shanks spaced 5 ft apart (Table 1). In late March, primary spring tillage was conducted to a depth of about 6 inches with a conventional tandem disc with 11 inch radius blades. Plots were fertilized in April, at a rate of 50 lb/acre in 1993 and 45 lb/acre in 1994, with aqua ammonia nitrogren injected through 1-inch wide shanks spaced 18 inches apart (Table 1). During late spring and summer the plots were rodweeded, three times in 1993 and four times in 1994, to control weeds and maintain a dry soil mulch. Packing was conducted immediately prior to planting winter wheat by making one pass through each designated plot with a coil packer pulled by a crawler tractor on 24 Aug. 1993 and 29 Aug. 1994. The coil packer had 1.5-inch thick solid steel coils with a radius of 10 inches. Individual coils were spaced 3 inches apart and exerted a pressure of 100 lb/ft². Total width of the implement was 70 ft.

Table 1. Field operations conducted during the 1992-1993 and 1993-1994 fallow cycles.

Soil BD and volumetric water content were determined gravimetrically, as described by Gardner (1986), within 1 day after packing from both packed and non-packed treatments. These measurements were obtained in 0.8-inch increments to a depth of 6 inches and to 9 inches in 1994 using an incremental sampler specially designed for sampling in loose dry surface soil conditions (Pikul et al., 1979). Wheel tracks were avoided. Three sample cores were obtained from each plot.

Plots were seeded 1 day after packing. In 1993, soft white common winter wheat (cv. Lewjain) was seeded in 16-inch rows with John Deere HZ deep furrow split-packer drills at a rate of 42 lb/acre. As planting conditions in 1993 were the wettest many growers had ever experienced, seeds were placed at two depths: (i) shallow, 4 inches below the soil surface, and; (ii) deep, 6 inches below the soil surface. In 1994, soft white winter club wheat (c.v. Moro) was seeded in 18-inch rows with an International model 150 deep furrow split-packer drill at a rate of 45 lb/acre. Planting conditions were very dry in 1994 (Fig. 4) and the drill was adjusted to place seed as deep as possible.

Soil cloddiness was determined within 3 days after planting by individually measuring the diameter and mass of soil clods within a 1-yard diameter sampling hoop randomly positioned within each plot. All clods with diameters > 2 inches were sorted into 0.4-inch size increments and the mass of each size group determined. Measurements were obtained from three sample hoops in each plot. Surface residue after planting was determined by clipping all above-ground dry matter within three 1-yard diameter sample hoops randomly placed in each plot.

Wheat seedling emergence was measured by counting individual plants in 1-yard row segments at 24-hour intervals beginning 8 days after planting (DAP). Three row segments were selected and marked within each plot prior to emergence of wheat seedlings. The same rows were consistently used in each plot to avoid any difference due to openers. No precipitation occurred during the packing, planting, and emergence portion of the study (August, September) either year.

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

An analysis of variance was conducted for soil BD and volumetric water content at each sampling depth, wheat seedling emergence on each sampling date, soil cloddiness, surface residue (year 2 only), and yield components and crop characteristics. Treatments were considered significantly different if the P-value was <0.05. Treatment means were separated by Fisher’s protected least significant difference.

Results and Discussion

Soil Bulk Density and Seedzone Water Content

The BD of the soil mulch at the end of the 1992-93 fallow cycle was low near the surface and increased gradually with depth (Fig. 1). Loose, relatively thick, low BD dry layers such as this are formed above depth of rodweeding and are typical of summer fallow mulches in the PNW (Pikul, et al., 1985; Schillinger and Bolton, 1995). Packing significantly increased soil BD in the seedzone (Fig. 1), resulting in increased volumetric water content to a depth of 4.3 inches (Fig. 2). Seedzone water conditions in late August 1993 were the most favorable many wheat growers in low-rainfall dryland regions of eastern Washington had ever experienced. Soil BD in 1994 increased abruptly 1 inch below the surface then remained quite stable to a depth of 9 inches (Fig. 3). The high BD layer near the surface was likely formed by the first rodweeding operation conducted when the soil was too wet; 4 days after 1.5 inches of rain in mid-May. Packing had no significant effect on either BD (Fig. 3) or volumetric water content (Fig. 4) at any depth to 9 inches. Soil drying extended below the rodweeder depth in 1994 and BD bore little relationship to volumetric water content (Fig. 3 and Fig. 4).

Wheat Seedling Emergence and Grain Yield

Packing significantly increased wheat seedling emergence in 1993. With shallow planting, plants in packed plots emerged faster until 10 DAP, after which there were no differences (Fig. 5a). With deep planting, differences in emergence between packed and non-packed treatments were significant on all sampling dates (Fig. 5b). We did not expect to find these emergence differences because of the wet 1993 planting conditions (Fig. 2). Plants survived the mild 1993-1994 winter with little injury. Grain yield components and crop characteristics at harvest are shown in Table 2. Packing combined with deep planting resulted in the highest grain yield. More grain-bearing spikes were produced and significantly higher grain yield was achieved with packing in both deep and shallow seeded plots. Deep planting produced heavier kernels and higher grain yield compared to shallow planting.

In 1994, high BD beginning 1 inch below the surface and dry soil impeded uniform penetration of grain drill openers, as evidenced by recoil of springs controlling opener placement. An average of 3.2 inch and 3.5 inch of soil covered the seed in packed and non-packed plots, respectively. Seedling emergence was variable in both packed and non-packed treatments. There were no differences between treatments in wheat seedling emergence on any date in 1994 (Fig. 6), or in any yield component and crop characteristic at harvest (Table 3).

Soil Clod Mass and Residue Reduction

Packing reduced the mass and size distribution of soil clods both years (Fig. 7, Fig. 8, Table 4). Although method and timing of tillage operations were similar throughout both fallow cycles (Table 1), differences in clod retention between the Ritzville and Shano soil types were readily apparent. Clod mass was 3 to 5 times greater in the Ritzville soil compared to the Shano soil for packed and non-packed treatments, respectively (Table 4). The coil packer eliminated clods > 4 inch diameter (Fig. 7) which may have benefited wheat seedling emergence by reducing large clod rollback into the furrow. Clods were more difficult to retain during the 1993-1994 fallow cycle (Table 4) because Shano soil is coarser-textured, lacks structure, and is low (<1%) in organic matter: Thus, maintenance of surface residue on this soil type is more critical to prevent soil loss from wind erosion. Surface residue was significantly reduced by packing in 1994 (Table 4) but, due to above average grain production in the preceding crop cycle (Table 1), surface residue levels still exceeded the minimum quantity required on highly erodible land for compliance with government farm programs.

Table 2. Yield components and crop characteristics of 'Lewjain' winter wheat in the 1993-1994 season.