By Donna Fleury
By Donna Fleury
Generally speaking, soil compaction may be caused by farming operations, equipment, tillage and livestock, and under some conditions may reduce crop yields. Soil compaction occurs when the force from wheel traffic, for example, pushes soil aggregates together, destroying soil structure and reducing water and air infiltration and root penetration. The level of compaction and impact on crop production depends on a number of factors, including the soil’s physical and chemical characteristics, climate and moisture conditions. “One of the big advantages we have in Western Canada is the type of clay soils, as well as the regular freeze-thaw cycles and wet-dry cycles,” explains Dr. Ross McKenzie, a research scientist in agronomy with Alberta Agriculture and Rural Development (AARD). “The types of clays we have on our prairie soils, which aren’t highly weathered like other areas of the world, allow moisture to move into the interlayers of the clay and cause the clay particles to swell. As the clay dries, the water in the interlayers is pulled back out and the soils shrink. The more clay in the soil, the more we will see definite cracks that can form in the soil. These two physical processes of freeze-thaw and wet-dry go a long ways to minimizing and correcting most soil compaction problems.”
Soil compaction can be a function of soil management or equipment. The shift to continuous cropping and direct seeding and the elimination of summerfallow have dramatically reduced cultivation compaction problems. “With frequent tillage, the soil structure is broken down and with little protective covering on the soil surface, spring rains can cause the soil particles to flow together and form a crust on the surface,” says McKenzie. “This restricts crop emergence and water penetration. However, the move to direct seeding has greatly reduced the problem of soil crusting. Direct seeding has greatly contributed to increased soil organic matter content, which in turn has improved soil structure and soil water infiltration rates into soil.”
Another potential problem induced by cultivation can be the use of sweeps for seeding. The shovels on the cultivator run along the soil at two or three inches deep, and if the soil is quite moist, the shovels put pressure right at the point of soil contact at the base of the cultivator shovel. At the depth of the cultivator, a “plow pan” or “hard pan” can develop. “This compaction problem has been dramatically reduced by the move to direct seeding and the use of disk or narrow-point openers on seeding equipment,” explains McKenzie. “The vast majority of irrigation farms still cultivate and can see this type of ‘hard pan’ compaction problems from tillage. However, in dryland direct-seeded farming, the problem has been significantly reduced.”
Soil compaction can also be caused by wheel traffic in fields in certain conditions. Larger equipment, earlier season planting and other field operations on wetter soils are some of the main culprits. McKenzie notes that many farmers have gone to larger equipment with dual or triple wheels or tracked equipment, which spreads out the equipment weight across the soil surface to minimize the potential compaction. “Occasionally there can be a bit of compaction right where the wheel tracks are, especially if conditions are wet, but it doesn’t usually go down that far. Generally, soil compaction from wheel traffic is difficult to see, visually. Farmers need to carefully look for distinct patterns in crop growth that are the same width and spacing of their farm equipment.”
Quantifying the situation
One of the ways soil compaction can be measured is with a cone penetrometer, a tool that measures the force required to push a standardized steel rod with a cone shaped tip into the soil. A gauge measures the force exerted to push the probe into the soil. “I use a penetrometer regularly in farmers’ fields and different research plots across the province,” explains McKenzie. “Moisture content can make a huge difference in the force needed to push the probe to a depth of 36 inches. However, for most of our cultivated soils, as long as they are moist but not wet, we usually do not see any significant compaction problems, with the exception of solonetzic soils, which are a whole different ball game.”
Solonetzic soils have a higher combination of clay and sodium, which have a naturally hard compacted soil layer below the surface soil.
Where farmers can run into the biggest problem is with wet falls, such as the conditions in the fall of 2010 across much of the Prairies. Under very wet soil conditions, the weight of fully loaded combines and grain trucks can cause significant wheel traffic compaction. However, in most years fall conditions are dry and normally dry soils are not subject to significant soil compaction.
Unless fields are really rutted, McKenzie suggests that farmers go in and seed normally in the spring of 2011. “If fields are quite rutted, then you might want to go in and disc the rutted areas to level out the ruts, which will look after the top six or seven inches where ruts were created,” says McKenzie. “Watch those areas over the growing season and if some crop growth is stunted in those areas, then you may decide in the fall to go in with a deep ripper to fracture the soil. Although I don’t normally recommend this practice unless absolutely essential, it may be needed to do some correction.”
However, it is critically important to only undertake deep ripping when subsoil moisture is low and then go in with a machine like the Paraplow, first designed in the United Kingdom, for example, that will go to a depth of 20 to 24 inches. Ideally, it is most effective to use a ripper that will lift the soil up, shift it over an inch or two, then drop the soil to cause fracturing to reduce the bulk density.
Some past research conducted by Dr. Jack Carefoot at the Lethbridge Research Centre looked at compaction on irrigated land in rotations with potato and sugar beets, which use frequent cultivation and very heavy harvest and trucking equipment. “He used various treatments with different levels of moisture conditions in the field, including driving loaded gravel trucks over the treatments to induce different levels of compaction,” says McKenzie. “He basically found the soils can look after themselves and the freeze-thaw and wet-dry cycles corrected any problems. He also did not find any benefits to deep ripping the soils for compaction.”
With the exception of solonetzic soils, McKenzie does not think there are significant compaction problems on the majority of prairie soils under normal conditions, but concedes that under wet soil conditions, soil compaction can occur. Therefore, it is important to note recommended practices for minimizing compaction in field operations and methods to manage and correct compaction where required.
Field trials showed that between 70 and 80 percent of compaction occurs during the first pass over the field if conditions are wet, so selecting where and when to drive on a field can be important. Avoid field activities that have the potential to damage the soil, and where possible avoid conducting field operations on wet soils. Delaying a field operation for a short time to allow soils to dry can make a big different in minimizing compaction. Vary tillage depth from year to year to reduce the development of a plow pan layer. “Eliminate unnecessary field operations, especially on wet soils,” says McKenzie. “For example, unload combines on road approaches or headlands where possible.”
Use recommended tire size and type inflated to the proper pressure. Using tires with larger tire footprints such as radials or larger diameter tires, and tandem axels can reduce soil surface compaction. However, deep soil compaction, which is affected by total load and not by soil area contact, will not change. Equipment with tracks causes the least amount of compaction.
Controlled traffic farming (CTF) is a practice being implemented in different parts of the world, with Australia being considered one of the leaders (see related story on page 24). Some Alberta and western Canadian growers agronomy consulting companies and researchers are very interested in determining advantages of CTF farming in Alberta and Western Canada, and future research and field trials will help confirm the main benefits under western Canadian prairie soils and climate conditions.