Indiana Soils:Evaluation and Conservation Online Manual

VI. SOIL EROSION AND COMPACTION

Soil compaction is becoming of increasing concern to Indiana agriculture. More and more Hoosier farmers are seeing the adverse effects of compaction over the entire range of soil textures—from sands to clays.

Compaction is the moving of soil particles closer together by external forces, such as falling rain, livestock traffic, and the weight of farm equipment. Compaction increases the bulk density (weight per volume) and decreases the pore space between soil particles of soil. With less porosity, water, air, and roots move through the soil with more difficulty. Restricted root growth affects crop growth and yield directly and restricted water and air movement limits yield by reducing the effectiveness of subsurface drainage.

Subsections:Why is soil compaction important? | The effects of soil compaction | Examining fields for soil correction | Correcting soil compaction

Why is Soil Compaction Important

When discussing soil compaction, some would be tempted to ask, “Why is it important?” It is a good question because it has never been thought to be of much significance in the Midwest or really anywhere outside the southeastern United States. There are some physical factors that lead to soil compaction as well as some management practices that can create compaction problems.

Tillage Under Wet Conditions

For some time, the amount of land farmed by one person has increased. Large fields are created from small fields. Because of natural soil variability, large fields have a better chance than small fields of including poorly drained soils. The number of days suitable for field work has not changed, however. The end result is that the poorly drained soils in a field are often worked under wet conditions when they are especially subject to compaction. There is also a yield advantage for earlier corn planting, so many farmers plant corn early regardless of soil moisture conditions.

Both the size of farm operation and the desire for early planting have encouraged tillage and traffic on soils that are much wetter than in the past. This has led to much of the soil compaction in the Midwest and elsewhere. Compaction can occur at anytime there is traffic on wet soils, so we must be concerned not only with the effects of spring tillage but also with wet harvest conditions in the fall.

Unfortunately there is no data currently available to consider the trade off between early planting and potential problems from soil compaction. This is a difficult problem because the effects are greatly influenced by weather and to a lesser extent by other management practices. It is clear from research already completed that the complexity of the problem will not lead to a single answer but one that varies with conditions.

Size of Equipment

The total load on an axle is a very important factor in determining the severity of soil compaction. The trend has been to larger equipment both for tillage and harvest. This equipment has much potential to compact the soil but only if it is used under wet conditions. This is an important point because too often equipment is blamed for the results of misuse. More work is needed to define the equipment size as well as soil and moisture conditions that can cause problems.

Types of Implements

A disc harrow can cause compaction. Because a disc blade is thin, all the weight of the implement is on a very small area, much like the blade of an ice skate. Problems occur when a disc harrow is used at the same depth year after year. Problems can also develop when a disc harrow is used on wet soil following plowing. Under drier conditions there is little or no effect. Excessive tillage with any kind of implement can break down soil structure and lead to problems. Compaction problems may not be observed every year or in all moisture conditions.

Lack of Rotations

The economic pressures that cause farmers to work on wet soils have also led to a reduction in the amount of land that is in sod-based rotations. This has not been without a price. The good soil structure built in soils under rotation is lost with intensive row cropping. This is somewhat surprising because of the high tonnage of residue returned to the soil in corn production. For some reason the corn residue is not as effective as the forage crops in improving soil structure.

It may not be realistic to suggest a return to sod-based rotations on all soils; however, the lack of sod-based rotations does explain why soil compaction is a concern on soils where it was not thought to be a problem in the past.

Soil compaction shows no consistent symptoms in plants. Crop effects can appear in a variety of different forms, which can be easily confused with other problems. Comparing symptoms between compacted and non-compacted plots is one of the most interesting aspects of soil compaction. Different climatic conditions cause changes in the effects of soil compaction on plants. Usually plant height is affected. Much of what we in Indiana call “tall corn-short corn syndrome” is often related to soil compaction. Wheel traffic patterns show up in a field as rows of shorter corn as compared to taller rows where the tires never traveled.

Soil compaction can result in slower germination, and in some cases, lower plant population. This can easily appear, at first thought, to be related to seed quality, but it might be related to compaction.

Another common problem related to soil compaction is nutrient deficiency symptoms. Often corn in compacted plots will have the classic symptoms of nitrogen deficiency even with more than adequate nitrogen applied. In particularly severe examples of soil compaction the plants can exhibit multiple deficiency symptoms. In a situation where soil compaction is a factor, nutrient deficiency may be blamed, but the cause of the problem may not be totally related to soil fertility.

Recent work in a greenhouse experiment has shown that there can be a relationship between apparent herbicide injury and soil compaction. It is possible that damage diagnosed in the past as herbicide injury in some fields was at least partly caused by soil compaction rather than excess chemicals.

One of the best ways to assess the effects of soil compaction is to study plant rooting patterns. Root growth is quite responsive to changes in a soil’s density or porosity. Thus, deformed or flattened roots are a good indication of subsoil compaction. The roots grow down until they encounter a compacted zone, then become deformed or flattened if unable to penetrate.

Moderate compaction does not completely block root penetration, but rather limits the number of roots that make it through to the subsoil. This can affect crop yield, but not necessarily. If adequate moisture and nutrients are available above the compacted layer, yield may not be affected. But in situations of moisture stress, significant yield reductions can and do occur.

To get an assessment of compaction and root growth patterns, first dig a small pit, approximately 25 inches. deep. Leave one side free of shovel marks. The shovel can compact soil on the pit face. Take a large knife and press it into the soil profile every 2 in. or so from the top down. Note zones where the soil resists penetration by the knife. These are normally zones of compacted soil. While you are examining the soil with the knife, look for horizons that have plate-like structure; this is another clue to compacted soil. Next carefully examine the root systems for their tendency to be mostly shallow and spreading instead of conical and deep. Also look for roots growing in cracks in the compacted soil. If you find poor-appearing plants with shallow roots over subsoils that break out in horizontal layers, or plates with roots growing between the plates, you most likely have a compaction problem.

There are a few principles dealing with soil compaction that have been demonstrated in Midwestern research plots. First, the effects of soil compaction can last more than one year. Secondly, in the humid Midwest the effects of soil compaction for both surface and subsoil are reduced with time.

The third principle that experiments have shown is that the effects of soil compaction may not be observed every year. With ideal weather conditions, very good yields can be obtained on both compacted and non-compacted soils. But for years with less than ideal moisture, either too much or too little, yield reductions will occur on the compacted soil.

Several approaches to the problem are possible. Soil compaction is likely to occur with modern methods of farming. Considering some options and beginning to monitor fields for soil compaction are good first steps.

One approach, after observing soil compaction in a field, is to do nothing and let nature take its course to slowly reduce soil compaction. Crop rotations that add organic matter or additions of manure or sewage sludge fit this method well. This may be quite useful in moderate cases of soil compaction. It is important to identify soil compaction as the source of crop problems rather than something else. It is possible to spend time and money on a cure that time would have improved anyway. For this method, you must be willing to wait, accept a degree of potential yield loss and, of course, not add further compaction.

Deep tillage in relatively dry conditions to break up compacted layers might be a faster method of reducing the effects of soil compaction. Indiana studies show that this can be done. However, work with deep tillage as an annual practice has not proven beneficial. Once the compacted layer is broken up, there appears to be no further benefit to continued subsoiling. Another concern is that tillage or traffic under wet conditions after subsoiling may re-compact the soil even deeper.

There may be other practices that, over time, can improve the soil structure. Improving drainage is important so that tillage can be done under better conditions. Where possible, controlling traffic will help concentrate wheel traffic in lanes through the field other than where the crop is to be grown. Reducing the number of trips made to produce a crop can not only save money but also lower the risk that a particular trip must be made when the soil is susceptible to compaction.