| | The problem: Effects of wheel track compaction | The solution: Low ground pressure
Farmers frequently see wheel tracks in their crops. Sometimes the tracks can be seen only as the crop emerges, but often the evidence remains at harvest with lower yields along the tracks.
The tracks usually result from temporary soil compaction caused by heavy wheel loads on the fragile, moist soil surface in the spring. Wheel tracks also cause other problems often associated with compaction: soil pulverization, lumps that don't pack into the seed bed, and different planting depths in and out of the wheel tracks.
The improved soil structure of direct seeded fields seems to be more easily affected by wheel track problems. We don't know the cost of wheel track damage, but we do know it can be eliminated most of the time at little or no cost to the farm.
The Problem: Effects of Wheel Track Compaction
Wheel tracks of tractor and tow-between air tank
These wheel tracks can create several problems:
- A compacted soil layer forms that resists proper penetration of the planter ground openers that follow in the tracks. The wear rate of these openers is greater, which aggravates the problem. The main cause of this problem is too much tractor or air tank weight for the tire size.
- Excessive lumps are created by the ground openers following in the compacted tire tracks, causing poor seed germination and reduced crop emergence. The main causes of this problem are that the soil is too wet and the tractor wheel slip is too great.
- Drive wheel slippage aggravates soil compaction. Slippage also changes the seed bed characteristics by causing lumps when the soil is wet or by pulverizing the soil structure when the soil is dry. The main cause of this problem is that the tractor is undersized for the power required by the planter. If the tractor does have the correct power but slippage still occurs, then the tire configuration and/or tire pressure need to be adjusted.
Wheel tracks of trail-behind air tank and fertilizer wagon
Wheel loads following the seeder can compact the soil above the seed, reducing plant emergence. This problem is most likely to occur on heavy clay soils and in very wet conditions. Other causes are excessive air pressure in wagon tires, undersized tires and poorly structured soils that could benefit from increased straw residue cover.
Note: Sometimes the crop comes up first and best in the tire tracks. This may indicate the crop was planted too deeply or the seed bed was not packed adequately. In either case, the situation should be studied, so action can be taken next year.
Wheel tracks of combine and grain truck
Harvest equipment leaves wheel tracks that can cause a problem for direct seeders. If loaded grain trucks repeatedly follow the same path, the soil will become extremely compacted. Often, even by spring, ground openers of planters cannot penetrate deeply enough into these tracks. Since these trucks must have high air pressure in the tires, the only solution is to not drive the same path twice.
Heavily loaded combine tires often leave ruts or depressions that remain in the spring. Only planters that use individual depth control on each opener are unaffected.
The Solution: Low Ground Pressure
Wheel loads of tractor and tow-between air tank
To reduce soil compaction, lighten the wheel loads. Lower wheel loads reduce contact pressure between the tire and the soil. Contact pressure is the combination of the area of the tire contacting the soil and the load the tire carries.
Large tires and a light tractor (or tow-between air tank) load produce the lowest possible contact pressure and, thus, the lowest soil compaction. For proper tire and ballast (load) selection, see Ballasting Your Tractor for Performance, Alberta Farm Machinery Research Centre (AFMRC) Report 725.
Wheel loads of trail-behind unit
The method of reducing tire pressure for tractors also applies to trail-behind units and to tow-between air seeders. Trailing units may be air tanks or wagons carrying liquids or anhydrous ammonia. Reduce the tire pressure to the minimum required to carry the load. (Ask your equipment manufacturer or tire dealer for information on minimum tire pressure.)
For example: An anhydrous ammonia tank carrying 4000 L (1000 gal) weighs about 6400 kg (14000 lb). Each of the four tires must carry 1600 kg (3500 lb). A common tire size used on anhydrous ammonia wagons is 14:80R20. These tires have a maximum design load of 4100 kg (9040 lb) at an operating pressure of 840 kilopascals (120 pounds per square inch). Quite suitable for the front axle of a concrete mixer truck, but not to pussy-foot across your planted crop! To carry 1600 kg, only 210 kPa (30 psi) is necessary. Rarely will this load cause wheel track problems.
Tire tread design may help reduce the effect of wheel loads. Deep lugged tires place a greater portion of the wheel load into the soil below the seed bed. The action of a lugged tire may also help break up the tire imprint.
Proper selection of tires and air pressure and use of rubber tracks both help to reduce wheel track problems.
Traction
A tractor should be capable of pulling the direct seeder and its fully loaded wagons without excessive wheel slippage. Excessive slippage is defined, for this purpose, as an amount of wheel spin that causes damage to the soil resulting in a poor seed bed in the wheel tracks.
The wheel slippage that can be tolerated by a tractor pulling a direct seeder is far less than that for optimum fuel efficiency. No research has been done to specify what this slippage should be, but lower slip is better when soil conditions are not ideal for making a seed bed. The experience of many farmers shows a target range of 4 to 8 per cent. (See AFMRC Report 725 for a method to measure slip.)
A tractor operating at direct seeding speed (6 to 8 km/h), ballasted in the 100 lb/ power-take-off hp range and slipping only 4 to 8 per cent will not produce its maximum power output. In fact, at full throttle, the engine may only pull half way down to the rated engine rpm. If this is the case, you can gear up and throttle down to increase fuel efficiency.
Prepared by:
Murray Green , Alberta Agriculture, Food and Rural Development.
Source: Agdex 519-12. April 1997. |
|