Part 1, Section 1: Soil Management
SOIL COMPACTION
Soil compaction is the reduction of soil volume due to external factors. The risk of soil compaction is greater today than in the past due to an increase in the size of farm equipment.
Soil compaction reduces soil productivity. Research in tilled soils showed average first-year yield losses due to severe compaction of approximately 15 percent. Yield loss in the first year after compaction was mostly due to residual effects of surface compaction. In this summary of many studies in different countries, yield losses decreased to approximately 3 percent ten years after the compaction event (in the absence of recompaction). The final yield loss was assumed to be a result of subsoil compaction and considered a permanent yield reduction.
Besides reducing yields, soil compaction also reduces soil health and environmental quality:
- Compacted soil is dense and has low porosity. Compaction preferentially compresses large pores, which are very important for water and air movement in the soil. Infiltration is then reduced and erosion is increased.
- Compaction causes an increase in the soil’s penetration resistance. There is little root penetration in soil above 300 psi (pounds per square inch), except if there are cracks and macropores in the soil that can be followed by plant roots.
- More energy is expended when tilling compacted soil.
- Compacted soil is a harsher environment for soil organisms, especially earthworms, to live in.
- Compaction affects nutrient uptake. Denitrification rates can increase in compacted soil due to limited aeration. Manure ammonia volatilization losses have been found to increase when liquid manure is surface applied to compacted soils because of reduced infiltration. Phosphorus and potassium uptake can be reduced if root growth is inhibited.
Compaction is caused by wheel or foot traffic on the soil and by soil tillage. Soil is most compactable at a moisture content approximating field capacity (24 hours after a soaking rain). If the soil is saturated, it is difficult to compact because water fills pores. Rutting and slipping during trafficking of saturated soil, however, will destroy soil structure at these high water contents. We distinguish between surface and subsoil compaction (Figure 1.1-5).
Surface compaction is caused by contact pressure (expressed in psi). A pick-up truck tire can cause as much surface compaction as a manure spreader at the same contact pressure. Contact pressure is approximately similar to tire pressure in flexible tires.
Subsoil compaction is caused by axle load (expressed in tons). The higher the axle (or wheel) load, the deeper the stress will be transmitted into the soil.
Plow pans are caused just below the tillage tool, if that layer of the soil has a moisture content conducive to compaction at the time of tillage. The moldboard plow is renowned for causing plow pans, but the disk plow or harrow and chisel plow have also been found to cause plow pans.
Not all compaction is caused by humans. Some glaciated soils have been compacted by glaciers in the past and are still compacted at depth. Other soils have fragipans, which are naturally compacted subsoils high in silt content. Some sandy coastal plain soils have such poor structure in the subsoil that root growth is negatively affected.
An understanding of the causes of soil compaction makes it possible to develop management strategies that either avoid or correct its effects. It is easier to avoid compaction because correction strategies can be costly and will likely not correct the problem entirely
The aim of compaction management should be to avoid subsoil compaction altogether, and to limit surface compaction as much as possible. Soil compaction is not likely to cause much damage if traffic is limited to dry soil conditions (i.e., drier than the plastic limit, see above). If soil is moist, however, the following points are important:
- To avoid subsoil compaction, reduce axle load to at least below 10 tons by
- reducing load;
- increasing number of axles
- To avoid surface compaction, reduce contact pressure (should be no higher than 35 psi) by
- reducing tire pressures to minimal allowable pressures;
- using flotation tires;
- using tracks or duals to replace singles;
- using radial-ply instead of bias-ply tires;
- installing larger diameter tires to increase length of footprint;
- properly ballasting tractor for each field operation.
Reduce the number of passes over the field and limit the area of the field that is impacted by traffic by increasing swath width of spreading and spraying equip¬ment and reducing width of tracks.
- To avoid plow pans
- do not drive a tractor wheel in the furrow;
- use no-tillage;
- use a chisel instead of moldboard plow;
- use a field cultivator instead of disk harrow.
A producer can make the soil more resistant to compaction by increasing its organic matter content and by building a soil ecosystem that has a permanent macropore system. There is now much interest in using cover crops with root systems that serve to reduce or correct the effects of soil compaction.
Correcting soil compaction may be necessary if options to avoid compaction have not been successful. Tillage is an option for reducing compaction, but tillage costs money and has negative effects for soil quality and erosion. Subsoiling is one form of tillage that can reduce compaction. Contrary to popular opinion, subsoiling does not always increase yield.
Answering the following questions will help determine whether and which tillage method should be used to correct compaction:
- Is tillage needed to correct soil compaction?
- Which tillage tool is suited to address the problem?
- What are the best settings to correct soil compaction?
Whether tillage is needed or not depends on the soil type and the severity of compaction. Some coastal plain soils (found in the southeastern U.S.) are known to respond to annual subsoiling with a resulting yield boost, but most other soils do not automatically respond with a yield increase.
On most soil types, the degree of soil compaction should be diagnosed with a penetrometer. The penetrometer is not useful in stony or shaly soils. The severity of compaction also needs to be assessed visually (for example, do you see ruts?) And by analyzing field traffic history:
- If field traffic has been limited to dry soil conditions, tillage is probably not needed to alleviate compaction.
- If field traffic has occurred while an area was at field capacity, the area of the field impacted needs to be assessed.
- If less than 50 percent of the field was impacted by tires or tracks and there are no deep ruts, it probably does not pay to do tillage.
- If more than 50 percent of the field was impacted, however, it may be beneficial to do tillage to alleviate compaction.
- If large stones are present, it may not be beneficial to do tillage because equipment may break and many stones may have to be removed after the tillage operation.
- It may be possible to limit tillage to high-traffic areas such as field borders.
If the diagnosis has determined that tillage is justified, it becomes necessary to select the best tillage tool to ameliorate compaction. Leaving 30 percent residue cover after planting is recommended to reduce erosion and increase soil quality, so the tillage tool selected should not reduce residue cover below this level.
The moldboard plow is not recommended for ameliorating soil compaction because it buries most residue and can actually lead to the formation of a plow pan. Chisel plows are better suited than moldboard plows for alleviating compaction if they can penetrate the compacted soil, but the tension springs on the chisel plows are often not heavy enough to penetrate the compacted layer. In this situation, subsoilers are often used.
Traditional subsoiler shanks are heavy, wide, and have curved shanks and large points. These subsoilers were designed to cause maximum fracturing and disturbance of the soil. At the same time, however, they bury most residue and leave a rough surface that necessitates secondary tillage. Clearly, residue conservation and reduction of secondary tillage were not considerations at the time these subsoilers were designed.
Modern subsoilers are designed differently. They usually have narrow shanks, coulters to cut through residue in front of the shank, and some type of attachment to leave the soil in a condition that is ready to be planted. These subsoilers leave most residues at the soil surface and do not create much surface disturbance. They vary in subsoil disturbance according to their design.
Cover crops are increasingly used to correct compaction. These cover crops are planted in the fall and grow when soil moisture contents are high and soil is easy to penetrate. Some cover crops have a taproot that can create channels into the subsoil. Other cover crops have a massive, fine root system that intermeshes with soil particles, stabilizing aggregation and creating many small channels. Roots of summer crops can take advantage of the channels created by the cover crop roots at a time when soil moisture content is typically lower than in the winter. More research is needed to further substantiate the benefits of cover crops for soil compaction alleviation and to enable better recommendations for cover crop selection and management for this purpose.