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Use chisel points to minimize the footprint of tillage point
By Daniel Davidson
DTN Agronomist
OMAHA (DTN)--About a week ago I did a story on spring tillage decisions. This prompted John Kappahan of Minnesota to call in. Kappahan said today most tillage tools come with chisel points rather than the more traditional sweeps and he wondered why. He asked about the advantages and disadvantages of sweeps and chisel points, how residue would be handled and what the final condition of the soil surface would be.
Kappahan is correct--more tillage tools are being sold with chisel points today than with sweeps.
A number of characteristics influence residue, surface rough and soil fineness including cutting edges, cutting widths, angles of approach, draft force requirements, critical depth, soil throws, soil disturbances and residue management. Understanding the basic design characteristics of tillage points and operation can assist in the selection and management of tillage equipment.
If Kappahan is concerned about compaction, he should use chisel points to minimize the footprint of the tillage point. If he wants to retain residue and is concerned about compaction, he should adopt a flat-chisel design. If he wants to bury residue and prevent compaction, he should use a twisted-chisel design. If he wants to maintain as much residue as possible and compaction is not a concern, he should use a sweep design.
Sharpness of the cutting edge of the point is important because a dull edge creates more smear and compaction. Research has shown dull-edged points generate increased upward vertical force and downward force and high sliding pressures along the length of the cutting edge, resulting in the formation of layers that are thin, smeared and compacted. These thin layers harden upon drying and lead to the formation of tillage pans.
Farmers should use sharp tillage points with wear-resistant coatings for maximum cutting efficiency and minimum shear.
A cutting edge effect can become quite significant with sweeps when extended over the full length of the point. Imagine having 6- to 12-inch offset sweeps spaced 15 inches apart. This can create a compaction layer across the width of the tillage tool. With narrower chisel points that have only a 2-inch face, this smearing action is localized only at the bottom of the actual furrow.
The point rake angle, or angle of approach, is an important design characteristic. Steeper rake or face angles result in greater horsepower requirements, reduced penetration ability, shallower critical depth, more draft and vertical uplift of the soil and increased lateral soil throw. Lateral soil throw contributes to the amount of soil disturbance, surface roughness, residue incorporation and the energy required pulling the tillage tool through the soil.
Typically, doubling the speed can quadruple the distance soil is thrown. Operating at too high of ground speeds has a large influence on soil disturbance and residue incorporation.
The draft force influences fuel use, tractor size and work rates and the economics of the tillage operation. Tillage depth is the most significant factor influencing draft followed by forward speed. A common rule is doubling the tillage depth will quadruple the draft power requirements. Along with doubling ground speed, it will quadruple draft resistance because of increased soil strength under higher loading rates and greater soil inertia force. Growers who increase tillage depth and ground speed will increase power requirements and fuel costs.
Points working below their critical depth often exhibit shiny, polished surfaces because of high sliding pressures involved. This results in the formation of thin compaction layers. Deeper critical depths can be achieved with greater point width, lower angle of approach, sandy rather than clayey texture and dry-brittle rather than wet-plastic soil condition. A higher ground speed is also likely to increase the critical depth due to increasing soil strength effects associated with higher loading rates, particularly in clayey soils.
The distance the soil is thrown influences soil-residue mixing, residue incorporation, fertilizer and herbicide incorporation, weed seed stimulation and soil moisture dilution and evaporation. The greater the ground speed, vertical uplift, along with the use of wings or twisted points will increase throw.
The amount of soil disturbance is determined by uplift forces, ground speed and using a twisted-point design. Minimum soil disturbance loosens soil below the seed zone providing an optimum environment for the root system. Excessive soil disturbance destroys soil structure, buries soil residue and leaves the soil exposed and at risk of erosion.
The type of tillage point along with its design and ground speed has a big effect on the amount of residue that remains on the soil surface after tillage. Today growers want to leave crop residue on the soil surface. The goals usually are greater than 30 percent residue coverage in minimum tillage and greater than 60 percent residue coverage in no-till.
Sweeps have a lower angle of attack, are more horizontal and can be less aggressive in disturbing the soil while chisel points having a steep angle of attack are blunter and more aggressive in disturbing the soil. Chisel points that are twisted are more aggressive than straight points. Sweeps with a lower angle of attack are much less aggressive than straight chisels and straight chisels are much less aggressive than twisted chisels.
--Sweeps 6-12 inches: 70 to 80 percent for corn, 50 to 60 percent for soybeans;
--Straight chisel points: 50 to 60 percent for corn, 30 to 40 percent for soybeans;
--Twisted chisel points: 30 to 50 percent for corn, 20 to 30 percent for soybeans.
If the goal is to maintain as much crop residue on the soil surface as possible, planning for residue cover begins at harvest. Ensure ample residues are spread evenly over the field by the combine. Then reduce the number of unnecessary tillage passes. Every tillage pass buries more crop residue. Select a tillage tool configured with straight points or sweeps on chisel plows instead of twisted points. Twisted points can bury 20 percent more residue. If you are working heavy clay soils, be aware that sweeps can contribute to compaction. Set tillage tools to work at shallower levels and reduce ground speed.
Date: 3/22/04
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