Extension Responds: flood-august 2007
Managing Soil Compaction Following the Heavy Rains of 2007
Contact: Dick Wolkowski, Extension Soil Scientist, Department of Soil Science, University of Wisconsin
Portions of southwestern and southern Wisconsin received in excess of twelve inches of rain in early August of 2007. These storms not only saturated soils in the regions, but in many cases floodwater deposited sediment on crop fields. This event brings into sight a potential for soil compaction from subsequent vehicular traffic used for agricultural operations. The following article discusses the soil compaction process and steps producers can take to limit its effects.
Soil compaction is an inevitable consequence of modern agricultural systems, which like pests, disease, and weeds can be managed. Compaction occurs when a force is exerted on a soil that exceeds its bearing strength. As this force increases soil aggregates are re-arranged and if the force is great enough the aggregates are destroyed resulting in a massive, structure-less soil condition. Because the soil bulk density increases and porosity decreases (much of the loss in porosity being the larger pores that are essential for drainage and aeration) the soil tilth can be damaged to an extent that requires years to recover.
Compaction is affected by several factors, including the soil’s water, organic matter, and clay content. Arguably the factor that is most critical is the water content. Soils tend to be most easily compacted when the soil water content is near field capacity; the water content anticipated when the soil has been saturated and then allowed to drain for several days. Further drying will increase the bearing strength of the soil, but improvement in the soil condition will obviously be related to future weather conditions, crop water use, evaporation, and management.
As the growing season nears its end there is the potential for heavy vehicular traffic from operations such as silage, forage, and grain harvest; and tillage and manure application. These operations are likely to be conducted when soils are susceptible to compaction. Listed below are several key considerations that operators should consider to reduce the potential for soil compaction this fall.
- Avoid operations on wet soils.
Modern agricultural equipment such as 4WD and tracked equipment allows operations in extremely wet soil conditions where conventional tractors would become stuck. Both surface structural puddling and compaction of the subsurface are likely in these conditions. If the soil is allowed to dry, often for just a day or two, its bearing strength increases and the potential for compaction decreases. Exercise patience and wait for soil conditions to improve.
- Limit load weight.
Soil compaction increases as vehicle weight increases. Adding duals or increasing tire size does not compensate for increased axle load and actually spreads compaction over a greater volume of the soil. When axle loads approach 10 tons the effects of compaction can be found well below the depth of tillage; possibly to a depth of 24 inches. Avoid driving large loads on soils that are wet and easily compacted. For example partially load chopper boxes and manure tankers. While this decreases operational efficiency it will limit the extent of compaction. Also, manage axle load by properly ballasting the tractor, maintaining proper tire inflation pressure, and limit slippage.
- Control traffic.
Most of the effect of soil compaction occurs in the first pass, so decisions on where to drive on wet soils is important. If travel is required in these conditions confine it by staying within the same tracks and avoid cutting corners. “Chasing the combine” can increase harvest efficiency, but in the long run it could reduce productivity due to increased compaction. Whenever possible unload in the headlands or better yet outside the field. There is a farming concept called “Controlled Traffic Farming” that uses guidance systems to permanently establish and maintain wheel traffic lanes. Research has shown that this practice can increase yield up to 15%.
- Maintain soil quality.
Practices that improve soil quality will reduce the compactiblity of a soil. Avoid removing crop residue and add organic matter such as manure or other organic materials to build and strengthen soil structure. Rotate crops such as forages and wheat when possible with corn. Consider adopting a reduced tillage system such as strip-tillage or find other ways to limit tillage intensity. Maintain soil fertility by applying recommended fertilizer and lime, with special attention to potassium. Research has shown row-placed potassium can offset some of the yield loss from compaction.
- Alleviating compaction.
Once a soil is compacted and structure is destroyed time will be needed to re-establish soil tilth. Natural phenomena such as wetting/drying and freezing/thawing will help rebuild structure over time, but should not be expected to correct problems by the next season. Deep tillage or subsoiling can remove restrictive soil features. A four year on-farm study in Manitowoc County showed that a straight-shanked subsoiling tool was superior to a v-ripper. Before investing in subsoiling document the extent and depth of compaction with a penetrometer or by digging a small pit to examine the soil and root distribution. Consider leaving two to three un-tilled strips to confirm that the subsoiling was worth the time and expense.