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PNW CONSERVATION TILLAGE HANDBOOK SERIES
Chapter 3 - Residue Management, No. 5, October-November 1985


Fall Stubble Management for Storage of Winter Precipitation

Roger Veseth

The Pacific Northwest receives about 2/3 to 3/4 of its annual precipitation during the winter months. In dryland cropping areas, maximum efficiency in storing this overwinter precipitation is critical for top yields. . . making the most of one of the most important crop yield limiting factors, Fall residue management and tillage practices used after harvest are the main options growers have to influence the amount of winter precipitation that is stored for the next crop.

Impacts of a Residue-free Surface

Leaving crop residue on the soil surface overwinter effectively reduces water loss from runoff and evaporation. Removal of all or most of the residue from the surface, through fall burning or complete burial by moldboard plowing or other tillage operations, can significantly reduce water storage potential.

Soil Water Storage Comparisons

In studies of fall residue management treatments near Pendleton, OR, STEEP researcher Bob Ramig, USDA-ARS soil scientist at the Columbia Plateau Conservation Research Center, determined that about 2 inches more water was stored from August to March with standing cereal stubble than with fall burning in a wheat-fallow rotation. Where the stubble was flailed and left on the surface, soil water storage was reduced 0.3 inch compared to standing stubble.

In another study, standing stubble also resulted in 2 inches more stored soil water than fall plowing in a long-term, winter wheat-spring pea rotation study near Pendleton. Ramig measured a 20 percent average increase in pea yields from the additional soil water, plus a 5 percent increase in winter wheat yields.

Keith Saxton, USDA-ARS hydrologist, and other STEEP researchers at Pullman, WA, have also reported about 2 inches additional overwinter soil water storage with standing stubble compared to where the stubble was burned. Their study was conducted near Colfax in southeastern Washington between late November and early April over the past two winters.

Evaporative Water Loss Potential

Besides reducing surface water runoff, reducing evaporative water loss with surface residue is a major factor in increasing overwinter soil water storage. Standing stubble creates a unique microclimate at the soil surface. The "windbreak effect" of the stubble effectively reduces air movement over the soil surface, directly reducing evaporation rates. Daytime temperatures of the surface soil are also significantly cooler under standing stubble than with a bare soil. Thus standing stubble is Particularly effective in reducing evaporation.

Recent STEEP research shows that the reduction of daily freeze-thaw cycles with standing stubble also helps reduce evaporation losses. Joe Pikul, USDA-ARS soil scientist at Pendleton, measured 0.2 inch more evaporative water loss where the stubble was removed by burning than under standing stubble after one week of clear, cold nights and warm days in March 1982. Half of the increased evaporative water loss from the bare soil occurred during one 24 hour freeze-thaw cycle.

As the bare soil surface froze each night to about 0.5 inch depth, water was pulled from the soil below, toward the freezing front. During the day, water was evaporated from the thawing, saturated soil surface, which can have evaporation rates equal to that of a free-water surface. Soils under standing stubble did not freeze during the monitoring week.

Standing Stubble vs. Fall Chiseling

The striking difference in storage efficiency of winter precipitation between standing stubble and a residue-free surface resulting from fall burning or complete burial with tillage makes standing stubble an easy choice from a water storage perspective. However, the choice between standing stubble and fall chiseling is not always so easy. Several considerations are important to the decision-making process, including seeding methods, soil freezing, compacted soil, snow distribution and other variables.

How Will the Next Crop be Seeded?

No-Till Seeding – If the next crop will be seeded with a no-till drill, the decision between standing stubble and fall chiseling is already made. Most of the no-till drills will do the best job of seeding through undisturbed crop stubble where harvested residue was uniformly distributed. Once the stubble is unanchored by tillage, drill plugging problems can be severe, particularly with hoe or knife openers. Disk drills would have more difficulty cutting through the residue, resulting in straw '' hair pinning" in the seed row, uneven depth control, poor seed-soil contact and other problems.

Minimum Tillage or Conventional Seeding — If some tillage is required in seedbed preparation for the next crop, fall chiseling is an option to consider, but should not be an automatic choice. The immediate advantage is speeding up seedbed preparation time for a spring crop. From a soil water storage standpoint, the straight-shank chisel is one of the better fall tillage implements, especially where soils commonly freeze. It can leave the tallest stubble and greatest amount of residue on the surface. Chiseling will also leave a rough soil surface and effectively break up tillage compacted layers for increasing water infiltration.

Is Precipitation or Snowmelt on Frozen Soils a Common Occurrence?

Frozen soils are a major contributor to high runoff and erosion rates in much of the Pacific Northwest. Water infiltration into the soil is stopped or severely reduced by soil freezing. This creates the potential for extreme soil erosion of the thawing soil surface, as well as significant water loss from runoff and evaporation.

Standing Stubble Provides Best Frost Protection — STEEP researchers have demonstrated that surface residue, particularly standing stubble, is effective in reducing the frequency, depth and duration of soil freezing. Research by Pikul and USDA-ARS hydrologist John Zuzel, also at Pendleton, shows frost penetration under standing cereal stubble averages only 35 percent of the frost depth under a bare soil surface. The frost structure under standing stubble also allows more infiltration than the impermeable concrete-like frost layer that commonly forms with a bare soil surface. During periods of shallow frost depth or daily freeze-thaw cycles on a bare soil surface, soil under standing stubble commonly does not freeze. Soil frost depth under chiseled stubble is typically intermediate between standing stubble and a bare surface.

Chiseling Gives Rough Surface and Non-Uniform Frost Depth — Although frost penetration is typically deeper under chiseled stubble than standing stubble, there is an important difference in frost uniformity. Pikul and Zuzel point out that the boundary between frozen and unfrozen soil under standing stubble and under a bare, smooth surface is nearly a straight line. In chiseled stubble, however, frost penetration is not uniform. Residue mats, large clods and chisel marks all influence the depth of frost penetration. Frost is often discontinuous in less severe frost periods, maintaining channels for water infiltration into the unfrozen soil below.

Don McCOO1, USDA-ARS agricultural engineer, has been making residue management-runoff comparisons at the Palouse Conservation Research Station near Pullman over the past 7 years. He found a slight reduction in runoff with chiseled cereal stubble compared to stubble left largely undisturbed after winter wheat was no-till seeded with a disk drill. Chiseled stubble lost an average of 0.6 inch of water as runoff compared to 1.0 inch with no-till seeded standing stubble. Between 60 and 90 percent of the runoff occurred on frozen soils. MCCOO1 attributes the effective runoff control of chiseled stubble to increased water infiltration with the non-uniform, often discontinuous frost penetration, and rough, high residue surface.

The slow surface water movement and pending with rough chiseled stubble allows more time for water infiltration into the soil. This is one of the reasons cited by Pikul and Zuzel for a 1.2-inch increase in soil water storage with chiseled stubble compared to standing stubble after a single runoff event on unfrozen soil near Pendleton. Increased water infiltration through tillage compacted layers was also an advantage of chiseling.

Does the Soil Have a Tillage Compaction Pan ?

Where the moldboard plow has been a standard, primary tillage tool, compacted "plow pans" can severely reduce downward movement of water through the soil profile. Other tillage implements can also create compacted layers at shallower depth. Soil traffic compaction from increasingly larger, heavier equipment can be a problem as well.

After 52 years of a tillage-residue management study initiated in 1931 near Pendleton, a thin, compacted plow pan at about 8 inches depth was found to be the most restrictive layer determining soil water movement. Pikul and other researchers measured a saturated hydraulic conductivity of less than 1.5 inches per day through the plow pan compared to 6 inches per day below the plow pan.

Effect of Deep Chiseling – Chiseling to a depth of 10 inches to break up a plow pan can effectively increase the rate of water movement. In a field study on a Walla Walla silt loam soil near Pendleton, Zuzel showed that chiseling increased saturated hydraulic conductivity through the plow pan at an 8-inch depth from 1.2 inches per day to 6.7 inches per day. This value was similar to the underlying soil.

In a separate Pendleton field study where a plow pan was present at 8 inches, Pikul and Zuzel used a rainfall simulator to compare water infiltration rates between different tillage treatments. The field had a history of conventional tillage winter wheat-fallow with moldboard plowing the primary tillage operation. On unfrozen soil in the fall of 1984, they measured a 5-inch per day infiltration rate under standing stubble compared to 8 inches per day where the stubble was chiseled to a depth of 10 inches. When the soil was frozen to a depth of 4 to 5 inches later in the winter, infiltration rates decreased to 1.2 inches per day for standing stubble and 3.4 inches per day for chiseled stubble. Where winter wheat was conventionally seeded on fallow with almost no surface residue, water infiltration rate was 2.2 inches per day in unfrozen soil and zero, no water infiltration, when soil was frozen.

Impact of Tillage Compaction Pans on Water Storage — Research results indicate that compacted soil layers affect soil water storage potential in two main ways. First, reduced water infiltration rates can directly result in higher water loss from surface runoff and evaporation. Second, the restricted water ifiltration and drainage within the soil can create a temporary "perched watertable" above the compacted layer. When this wet soil layer freezes, it creates an impermeable, concrete-like frost layer which completely stops infiltration and further increases runoff and evaporation.

Is Snow Accumulation and Wind Redistribution Significant?

Any residue management practice that increases snow trapping can potentially increase overwinter soil water storage. Leaving cereal stubble standing overwinter is one of the most effective ways of trapping snow where wind redistributes the snow cover. Any tillage or residue management practice that reduces stubble height or uniformity reduces snow trapping potential.

As snow depth increases in the stubble, the insulation effect also increases. Frost depth is reduced, if soils are frozen at all, allowing increased water infiltration during snowmelt and winter rains. Evaporative water loss is also reduced.

Evaluate Individual Conditions

No best choice can be made between standing stubble and fall chiseling for all situations. The decision must take into account the method of seeding the next crop; frequency and depth of frozen soils and associated runoff events; evaporative water loss potential; presence of compacted soil layers; importance of snow and snow drifting, and other variables. Evaluate the local conditions to determine how to most efficiently store the yield-determining winter precipitation.

     
 

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