Figure 1. June 15th water content, by depth, for chemical fallow (light-residue) and tillage fallow plots. Data are from three Morrow County, Oregon sites where soil depth isequal to five ft.
Farmers in north-central Oregon use tillage fallow (TF) to store soil moisture for winter wheat produced the following year. Chemical fallow (CF) is an alternative to the conventional, tillage-based system. Utilization of CF has been limited due, in part, to concerns about soil water storage. This project was designed to compare CF and TF water storage in a production zone characterized by deep soils, limited annual precipitation (8-12 in), and dry, hot summers. Soil water storage in side-by-side CF and TF Ritzville silt loam profiles was compared at six sites in 2001 and at four additional sites in 2002. Water storage comparisons were made by evaluating the gravimetric water content of samples collected from each foot of the soil’s profile. Water storage in CF, with 1.5 tons residue/acre, was statistically similar to that observed in TF. Results from statistical analysis of a smaller data set indicate water storage in CF might be increased if the amount of residue from the previous year’s crop is approximately 4.5 tons/acre. Additional work is needed to verify effects of the quantity of post-harvest residue on water storage in CF.
Chemical fallow, conventional fallow, soil water storage, tillage fallow, winter wheat
Winter wheat production in north-central Oregon occurs mostly on deep soils and in areas where average annual precipitation ranges from 8 to 12 in. Summers are dry and hot. Wheat is normally grown after a year of tillage fallow (TF). An alternative to conventional, tillage-based fallow is chemical fallow (CF). A renewed interest in CF can be attributed to (1) improvements in direct-seed equipment, (2) limited success with annual cropping, (3) repeated failures to grow alternative crops economically, and (4) the recent availability of a soil-active herbicide that may improve control of troublesome weeds such as kochia (Kochia scoparia) and Russian thistle (Salsola kali). Optimism about this alternative fallow method, though, is tempered by speculation of inadequate water storage. The objective of this project was to compare CF and TF water storage in representative fields of the region.
Soil water storage in side-by-side CF and TF plots was compared at six sites in 2001 and at four additional sites in 2002. Most sites were located within a 6-mile radius of 45? 34? 9? N, 19° 49? 8? W (Morrow County, Oregon). CF and TF plot areas were 3,600 ft2 (60 ft x 60 ft). Each plot was a subset of a much larger field and was managed with standard practices for the area. CF and TF fields at three sites were located immediately adjacent to one another. The distance between CF and TF sampling areas ranged from 40 to 80 ft. CF and TF fields at the seven other sites were separated by either a county road or farm road. The distance between sampling areas at these locations ranged from 210 to 265 ft. This larger separation distance was used in an attempt to avoid “edge effects” that might have been encountered next to the road or in areas where “outside rounds” were made.
The soil at each of the 10 sites is a Ritzville silt loam (course-silty, mixed, mesic Calciorthidic Haploxeroll) with a slope of less than 2 percent, an effective rooting depth of 4 or 5 ft, and an available water holding capacity of about 8 in. Total precipitation in 2000-2001 and 2001-2002 was 9.89 and 7.94 in, respectively. Rainfall during each of the April 1st to June 30th growing seasons ranged from 25 to 30 percent of the year’s total.
TF was maintained with a single “sweep” operation in April or May, followed by rod-weedings as necessary. Stubble in CF was left “as is” following harvest. Weed control in CF was accomplished with applications of glyphosate, 2,4-D, and/or gramoxone. CF and TF plots were weed-free during the experiment. Soil disturbance in CF occurred only at planting time and after sampling was completed.
The cropping history at three sites included 2 years of TF and 1 year of soft white winter wheat production, followed by either CF or TF (2001 project year). The estimated amount of residue in CF plots was 4.5 tons/acre. Stubble height was 16-21 in. The cropping history at seven sites included 1 year of TF and 1 year of soft white winter wheat production, followed by either CF or TF (2001 and 2002 project years). The estimated amount of residue in CF plots, at these locations, was 1.5 tons/acre. Stubble height ranged from 4 to 7 in.
Soil sampling was conducted on June 15th (? 7 days) and again on September 1st (? 3 days). Sampling took place at three randomly selected locations within each CF and TF plot. Samples, which were collected with a bucket auger, were removed from each foot of the profile and placed in a 5-gal bucket where they were mixed, by hand, for approximately 30 sec. A small portion of the mixed sample was placed into a 4-oz soil moisture can. Wet weights were determined in the field using a leveled and calibrated electronic balance. Dry weights were determined 24 hours after samples were placed in a 230?F oven. Calculation of the total water content of soil profiles (Tables 1 and 2) was accomplished by summing the product of volumetric water contents and soil depth (12 in) for each 1-ft layer. Volumetric water contents were calculated from gravimetric numbers and an assumed bulk density of 1.25 gm/cm3. The statistical significance of data was determined by an analysis of variance procedure for a randomized block design that used sites as replications. Treatment means were separated using Fisher’s protected least significant different test.
Data from this research is preliminary evidence of a relationship between potential water storage in CF and the quantity of post-harvest residue. Subsequent comparisons of water storage for the two fallow methods are discussed accordingly. Plots with small amounts of residue (1.5 tons/acre) will be referred to as “light-residue” CF. Plots with substantially more residue (4.5 tons/acre) will be referred to as “heavy-residue” CF.
Table 1. Average water content of seven side-by-side chemical fallow (light-residue) and tillage fallow plots in Morrow County, Oregon.
| Date | Chemical fallow |
Tillage fallow |
LSD(0.05) |
| Water content (inches) | |||
| June 15 | 5.48 | 6.03 | NS+ |
| Sept. 1 | 5.18 | 5.85 | NS+ |
+ Not significantly different
Water storage in the two fallow systems was statistically identical (Table 1). There was, however, a consistent trend of decreased storage in CF. Water storage in CF was 0.55 in less than that in TF on June 15th. Increased water storage in TF, on the earlier sampling date, is probably a consequence of primary tillage operations conducted in April or May. A single “sweep” operation reduces the bulk density of the top 6 in of the soil profile. The reduction in bulk density inhibits upward migration of water to the soil surface. Water on or near the soil surface is usually “lost” to the atmosphere. Rod-weeding operations during July and August maintained the “moisture cap” and reduced evaporative losses in TF.
The distribution of water in soil profiles was similar for light-residue CF
and TF (Figures 1 and 2). Maximum water contents were measured in the second
foot. Soil moisture values above and below this depth were very low and a sharp
decrease in moisture, over time, was noted in samples removed from the top
foot of the profile.
Figure 1. June 15th water content, by depth, for chemical fallow (light-residue)
and tillage fallow plots. Data are from three Morrow County, Oregon sites
where soil depth isequal to five ft.
Figure 2. September 1st water content, by depth, for chemical fallow (light-residue)
and tillage fallow. Data are from three Morrow County, Oregon sites where
soil depth is equal to five ft.
The average water content of heavy-residue CF was significantly greater than that observed in TF (Table 2). The decreased amount of stored water over time was similar (? 0.65 in) for both fallow methods. Increased CF water storage is, in all likelihood, the result of improved infiltration rates and a reduction in evaporative water loss, compared to light-residue CF, during July and August. The capillary continuity of an undisturbed soil surface layer remains intact. The soil is more effective at converting light rainfall into stored soil moisture.
Table 2. Average water content of three side-by-side chemical fallow (heavy-residue) and tillage fallow plots in Morrow County, Oregon.
| Date | Chemical fallow |
Tillage fallow |
LSD(0.05) |
| Water content (inches) | |||
| June 15 | 7.05 | 6.13 | 0.77 |
| Sept. 1 | 6.41 | 5.46 | 0.86 |
Stem, leaves, and fractured pieces of stubble on the soil surface formed a
fairly dense mat in some locations of the field. The overall effect of this
mat on evaporation, however, was probably minimal, since it was observed
infrequently within plot boundaries. Thick, standing stubble probably played
a more significant role in reducing evaporation. A portion of the sun’s
incoming radiation, which is absorbed by the stubble, never reaches the soil.
Heat input to the soil, relative to that of soils with little residue, is
reduced during the summer. Standing stubble also reduces wind turbulence
above the soil surface. Lower soil temperatures and reduced air movement
decrease the vapor pressure gradient between the soil and the atmosphere.
The effect is less evaporation.
The distribution of water in CF and TF profiles was similar on June 15th (Figure 3). Maximum water contents ranged from about 9.5 percent to almost 11 percent in the second foot. Soil moisture values above this depth decreased by approximately 1 percent. Minimum water contents were observed in the 4- to 5-ft soil layers.
The distribution of September 1st water in CF and TF profiles was not alike (Figure 4). Maximum CF water contents occurred in the second and third foot. Soil moisture values below 3 ft decreased by an average of 0.88 in/ft. The maximum water content in TF occurred in the second foot of the profile and decreased by an average of 0.58 in/ft in the 3-, 4-, and 5-ft layers. Soil moisture was minimal (? 7 percent) in the first and fifth foot. The dissimilar distribution of water may be related to differences in heat transfer (temperature gradients) between soil layers.
Figure 3. June 15th water content, by depth, for chemical fallow (heavy-residue)
and tillage fallow plots in Morrow County, Oregon.
Figure 4. September 1st water content, by depth, for chemical fallow (heavy-residue)
and tillage fallow plots in Morrow County, Oregon.
Readers are reminded that increased water storage with heavy-residue CF was observed at a very limited number of sites. Verification of results presented in this report will require additional research. Results from this project apply to CF and TF on Ritzville silt loams only. The storage of water and the distribution of water in heavy-residue CF will almost certainly be different for “heavier” or “lighter” soils. Year-to-year variations in temperature and precipitation will also have an influence on soil water storage. [an error occurred while processing this directive]