A frequent question of zone-till producers has been the depth to which they should adjust the shanks on their zone-tillage machine when used for sugarbeet production. If the shank is operated too deep, we needlessly waste input (energy) costs and limit the implement’s field capacity. The deeper the shank is operated, the more clods that are often created and the more difficult it is to completely close the shank mark deep in the soil. But if operated too shallow, any undisturbed soil compaction may limit root development or water and nutrient movement in the soil, thus restricting yield potential.
The objective of the study described here was to evaluate different zone-tillage shank operating depths and, in turn, to provide information on sugarbeet yield response for the benefit of beet producers.
The study was conducted in 2008, 2009 and 2010 at the University of Nebraska Panhandle Research & Extension Center near Scottsbluff. The soil type at this location is a fine sandy loam with 1.0% organic matter and 8.0 pH. Corn was the previous crop for all three study years, with the stalks shredded, disked twice and moldboard plowed (to a depth of 11-12 inches) in the spring of each study year.
The fields were then roller harrowed twice to firm the soil. Granular fertilizer (rates based on soil test) was applied between roller-harrow operations. These operations, it should be noted, were made using a tractor equipped with flotation tires and when the soil was relatively dry — the intent being to minimize surface-applied soil compaction.
Four zone-tillage shank depths (0, 5, 10 and 15 inches) were applied, as were three soil compaction levels (“none,” “moderate” compaction and “high” compaction) after the second roller-harrow operation. The compaction levels were applied at the soil surface, meaning the degree of compaction would be expected to decrease with distance from the surface.
The “moderate” soil compaction treatment consisted of one pass over the entire compaction plot area with an empty tandem-axle truck (weight of 21,000 pounds). The “high” compaction treatment consisted of a pass of the same truck but with a near-rated maximum load, for a total gross weight of 52,000 pounds. Both truck passes were made when the soil was relatively dry. Following the compaction treatments, the entire plot area (including the “no compaction”) was tilled to a 2.0-inch depth to achieve uniform planting seed depth.
The zone-tillage machine was a six-row (22-inch row spacing) three-point mounted Schlagel Till-N-Plant model, which used a 0.75-inch wide parabolic-shaped shank. Field speed was 3.5 mph.
Shank depth was adjusted to maintain correct operating functions for the advance coulter, the wavy closing coulters behind the shank, and for the rear rolling baskets. The machine was not used on the “0” depth plots.
Following the zone-tillage pass, all plots received one pass with a machine with individual rows of two rolling baskets (no shanks or disks). Its purpose was to break up any big clods left by the zone-tillage machine and to firm the seedbed, as the study intent was to focus on shank depth, not seedbed irregularities.
After each year’s plot planting (to the same Roundup Ready® pelleted variety), the field was sprinkler irrigated as needed for high emergence. It also was roughened with a rotary hoe between rows as needed to prevent wind erosion, but was not cultivated. Weeds were controlled with three applications of glyphosate, Quadris® was applied to help control rhizoctonia, and the plots were sprinkler irrigated for the duration of the crop season.
Along with plant stand counts made when emergence was considered complete, compaction effects were measured with a soil cone penetrometer. Maximum resistance measurements were taken with this instrument at four soil depth ranges: 0-3, 3-8, 8-13 and 13-18 inches. Beet root shapes were visually rated at harvest.
The accompanying tables (see next page) summarize the results in terms of plant stand, root yield and sugar per acre.
Plant response to soil compaction and to tillage systems is notorious for having high variability. This study was no exception. But there are several clear messages from this study regarding (1) surface-applied soil compaction and sugarbeet yield, and (2) zone-tillage shank depth and beet yield:
• The sugarbeet plants in the high-compaction, “0” shank depth plots were very small and unhealthy until mid-July. Based on the early growth, it was very surprising that root yield ended up as high as it did.
• Even moderate surface-applied soil compaction disfigured beet root shape and reduced yield. High soil compaction had an even bigger effect.
• In treatments with moderate or high surface-applied compaction, beet yield was increased by using a shank depth of 5 inches, compared to no zone tillage (“0” depth). Increasing the shank depth to 10 inches further increased yield in high compaction plots.
• A shank depth of 15 inches did not increase beet yield, compared to the 10-inch depth, for any of the three soil compaction levels.
• Zone tillage did not completely eliminate sugarbeet root yield reduction caused by intentional moderate and high surface-applied soil compaction. This might suggest running the shanks between old rows instead of in the old rows when zone tillage follows zone tillage and if soil compaction is present.
• Generally, soil compaction with a soil cone penetrometer resistance of greater than about 400 psi began to reduce beet root yield.
• Each field should be examined for the presence and depth of any soil compaction prior to the zone-tillage operation. In most situations, a shank depth of about 10 inches will be a good compromise of several factors.
By John Smith*
John Smith is an agricultural engineer at the University of Nebraska Panhandle Research & Extension Center, Scottsbluff.