Subsoil compaction is a serious soil conservation issue and a long-term threat to soil productivity. Most subsoil compaction occurs when the soil is wet and field equipment weights exceed 10 tons per axle. Kinds of equipment most likely to have loads in this range are combines, loaded grain carts, and slurry tankers. Plowing with a tractor wheel in the furrow will pack soil below the depth reached by normal tillage operations and can be another source of subsoil compaction.
The plant response to subsoil compaction, as with surface compaction, depends on the crop, soil conditions, and the climatic conditions in a particular year. If plants are already stressed for water, subsoil compaction may add to the stress by limiting the growth of plant roots to additional water. If plants are growing in soils that have aeration problems due to high water content, subsoil compaction will slow drainage and could result in an anaerobic root environment that limits nutrient uptake.
Figure 10. Relative corn yields over 12 years with a one-time soil compaction of 20 ton/axle (from Voorhees et al., 1986).
Simply put, subsoil compaction can affect:
Research studies conducted in northern latitudes show that the effect of severe subsoil compaction may affect crop yields for years. Research results from Lamberton and Waseca, Minnesota, Uppsala, Sweden and Quebec, Canada, show a similar trend of initially lower yields following compaction with axle loads of 10 tons or more. The effect decreased over time, and yields on compacted soil approach the yields on non-packed soil after two to seven years, depending on the soil and climate.
While these studies show a gradual, natural alleviation of subsoil compaction, the data from Waseca suggests that there is sufficient "residual" subsoil compaction to reduce crop yields in years where there are environmental stresses. Figure 10 shows that corn yields were back to normal within 5 years after the compaction was created. However, in 1988, 1990, and 1993 yields were reduced. In 1988, growing season precipitation was the lowest on recorded history while in 1990 and 1993, the region received above average rainfall (167% and 175% of the long-term average).
This study illustrates that a one-time compaction event can lead to reduced crop yields 12
years later. Under normal farming operations, heavy equipment is used every year. Thus, subsoil
compaction resulting from farming practices may be permanent.
There are four strategies commonly used in dealing with compaction; 1) avoidance, 2) alleviation, 3) controlled traffic, and 4) acceptance.
Avoidance is the most desirable where it is physically and economically possible. The old adage of "stay off the field until it's fit to work" still applies. However, the possible severe economic repercussions of delaying planting, harvesting, or other operations may outweigh compaction damage or loss. The dilemma the farmer faces in a wet spring or fall is not easy to resolve.
While large, heavy machinery is often blamed for soil compaction problems, it also offers opportunity to minimize compaction. Larger capacity machinery means fewer wheel tracks across the field because of wider working width. If wheel track spacing can be standardized among different pieces of equipment, soil compaction problems can be minimized.
Tracks vs. Tires
Tracks, as an alternative for tires, are not new in agriculture. Tracks accounted for 6-10% of all tractor sales between the years of 1925-1966. However, in recent years, the change from steel to rubber tracks, improved ride-ability, increased traction, and research citing that tracks create less surface compaction than tires have increased the popularity of tracks.
Tractors equipped with either tracks or tires can create surface compaction. The question is "Which one creates the least amount of compaction"? The answer: both radial tires and tracks will result in similar surface compaction if the radial tires are properly inflated.
Tractors weighing less than 10 tons an axle usually keep compaction in the top 6-8 inches, which can be alleviated by tillage. By and large, even the biggest tractors weigh less than 10 tons an axle. However, combines and grain carts weigh much more and whether equipped with tracks or tires, they can create compaction as deep as 3 feet.
In general, contact pressure largely determines the potential for compaction in the plow layer, while total axle load determines the potential for subsoil compaction. This is important when comparing tracks and tires for compaction effects and depth.
Tracks exert a ground pressure of approximately 5-8 psi depending on track width, length, and tractor weight. Radial tires exert a pressure of 1-2 pounds higher than their inflation pressure. For example, if a radial tire is inflated to 6 psi, the tire exerts a pressure of 7-8 psi on the soil. However, bias tires inflated to only 6-8 psi cannot operate efficiently and easily wear-out with such low tire pressures, consequently they have to be inflated to 20-25 psi.
Research has shown that tractors equipped with either tracks or radial tires create compaction in the top 5-8 inches, however, compaction effects were negligible below that depth. But what effect do tracks have on subsurface compaction when used in conjunction with heavy field equipment, such as grain carts or combines? Keep in mind that depth of compaction is a result of total axle weight and the role of ground contact pressure is secondary. Whether the equipment uses tracks or tires, the total axle load is nearly the same. Tracks will improve traction and ride-ability, but a 25-ton per axle grain cart will still create subsurface compaction.
Figure 11. Alleviation of wheel traffic compaction by tillage and overwintering.
At times, potentially damaging compaction is unavoidable. What can be done about it? There are two ways of alleviating and lessening the damage caused by compaction, 1.) Attempt to remove the compaction or 2.) Attempt to reduce the adverse effects of the compaction.
Moldboard tillage of the compacted depth has been effective in removing surface compaction in studies at Lamberton (Bauder et al., 1981). Wheel traffic during the growing season increased the bulk density to 1.55 g/cm3 in the surface foot of a Nicollet clay loam (Figure 11). Moldboard plowing this compacted soil in the fall (D) reduced the bulk density in the top foot to similar values measured in the soil without wheel traffic (E).
Where no tillage was done, freezing and thawing over winter reduced the bulk density only slightly and only above the 6-inch depth, (B). In this study, chisel plow and disk treatments (C) were less effective than the moldboard plow in removing surface compaction in one over-winter period. These results confirm that freezing and thawing alone may not remove compaction.
One way to reduce the adverse effects of compaction is to apply fertilizer in a way that
increases the availability. Such measures may include row/band application of phosphorus or
potassium. Split application of nitrogen or other practices that minimize the loss of nitrogen
by denitrification may also alleviate compaction problems.
In the Midwest, research results evaluating the effects of subsoiling have shown few positive yield responses to subsoiling. When they do occur, they are variable and relatively small.
There is a real dif-ficulty in accurately predicting the effects on crop yield from subsoiling a given field because of differences in soils, the level of subsoil compaction, the soil water content, subsequent traffic, uncertainties of future weather conditions, and differences in the crop grown and in tillage methods.
In a Waseca study, subsoiling to a depth of 16 inches failed to increase yields on the 20-ton per axle treatments for either corn or soybeans and decreased corn yield 11 bu/a in one of the two years.
One strong possibility for the lack of response to subsoiling is that the detrimental effects caused by compaction were no longer limiting crop yield. This could occur either because differences in crop or climatic conditions, such as rainfall, that may alleviate the effects of surface compaction.
Another possible explanation is that the subsoiling failed to effectively remove the compaction because of unfavorable soil moisture conditions at the time of subsoiling. If the soil moisture is too high, subsoiling will be ineffective. The subsoiling and associated traffic can reduce the soil macro-porosity, further restricting the drainage of water through the soil.
Another reason cited for the failure of subsoiling to remove a compacted layer is that subsequent wheel traffic may have reintroduced the compacted layer, thus negating the effects of sub-soiling. A loosened subsoil will have very little bearing capacity, meaning it can't support much weight. An ordinary 2-wheel drive tractor may be sufficiently heavy enough to re-compact the subsoil. Controlled traffic becomes even more important.
To increase the probability of obtaining beneficial effects from subsoiling, the following steps should be considered:
In a normal year, as much as 90% of the field may be tracked by equipment (Figure 12). The philosophy behind controlled traffic is to restrict the amount of soil traveled on by using the same wheel tracks. Seventy to 90 percent of the total plow layer compaction occurs on the first trip across the field. By controlling traffic, the tracked area will have a slightly deeper compaction but the soil between the tracks will not be compacted (Figure ).
Corn and soybean farmers who use global positioning systems (GPS), ridge till, strip till, or no-till can confine traffic between certain rows and avoid compacting the row area. This requires proper matching of all machines including combines, grain carts, and manure-handling equipment to confine the compaction to the same between-row areas.
There are occasional reports of adverse effects on plant growth where the wheel tracks are on both sides of the row, but even then the damage is confined to certain rows. Benefits to controlled traffic, using permanent compacted lanes, are improved tractor efficiency and floatation, less powerful machinery needed, and improved timeliness of operations.
Figure 12. Field coverage by normal annual field operations. Source: Management Strategies to Minimize and Reduce Soil Compaction. 1999. University of Nebraska, G89-896-A.
Figure 13. Field coverage in a controlled traffic situation. Source: Management Strategies to Minimize and Reduce Soil Compaction. 1999. University of Nebraska, G89-896-A.
Acceptance is waiting for the detrimental effects to be removed by natural forces. However, this may not be practical if there is compaction below the plow layer. The deeper the compaction and higher the clay content, the longer it will persist.
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