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AY-221

Soils

Purdue University
Cooperative Extension Service
West Lafayette, IN 47907

Soil compaction is becoming of increasing concern to Indiana agriculture. For many years,it was thought to be mostly a "southern U.S. problem" and of little importance in the Midwest. Today, however, more and more Hoosier farmers are seeing the adverse effects of compaction and over the entire range of soil types--from sands to heavy clays.

Significant problems are showing up on medium-textured soils, such as silt loam--a texture, found throughout most of the state. Observations in central Indiana indicate that the somewhat poorly drained soils of the low swells compact worse than the brown better-drained soils or even the black, poorly drained depressional soils.

This publication deals with the scope and nature of soil compaction and its remedies. Discussed are what compaction is and what it does, why it's becoming more prevalent, how to identify it and what to do about it.

What is Compaction and What Does it do?

Compaction is the moving of soil particles closer together by external forces such as falling rain or livestock and implement traffic. This can result in the following types of problems.

* Compaction increases the resistance of soil to penetration by tillage equipment and plant roots. This makes it more difficult to prepare a deep, well-shattered layer that forms an ideal seed bed, and this can adversely affect plant growth--and yield.

* Compaction increases the bulk density (weight per volume) of soil. A higher bulk density means less porosity (space between soil particles), which affects aeration. The finer-textured silts and clays have more total pore space than coarse-textured sands although coarse soils have more large pores that promote rapid water movement. Within a given soil, however, the greater the compaction (higher bulk density), the less porosity and the greater the difficulty for water and root movement.

* Compaction can restrict soil drainage enough to cause significant planting delays. This has been illustrated on a compaction plot at Purdue's Agronomy Farm. In one small area of a poorly drained but well-tiled field, the subsoil was purposely compacted by removing the plow layer, driving back, and forth over the exposed subsoil, then replacing the plow layer. A rain occurred a short time later. Long after the rest of the field was ready to plant, water was still standing in the compacted plot. This occurs commonly on the end rows in more level and naturally wetter fields.

What Soils Can Have Compaction Problems?

They all can! Of Course, we're probably more familiar with the compaction problems that occur in heavy clays. If these soils are tilled wet, there is a breaking down of structure called "pudding." This happens because the soil's structural aggregates (groupings of sand,silt and clay into larger particles) are very weak in a wet condition. When this wet soil is tilled, these aggregates are broken down and reformed into clods. Look for worst compaction tendencies on the somewhat poorly-drained soils as classified on Indiana's soil surveys.

Some would argue that sandy soils do not compact. They point out that sands are easy to work, dry out quickly and present few problems in terms of traction across the field.

But sandy soils do compact. In fact, the reason why roads are normally built with a sand and gravel base is that, once compacted, they stay that way. Sandy soils tend to remain compacted because the natural processes of swell shrink and freeze-thaw have little effect on them.

Why Are Compaction Problems Increasing?

There are four basic reasons for the increase in soil compaction problems being reported in Indiana--bigger equipment, earlier planting, lack of meadows in crop systems and overuse of tillage equipment.

* Field equipment is getting bigger, heavier and more adaptable to varying field conditions. The increased size and weight has to some extent, contributed to soil compaction. But the real "problem" is that this newer equipment with its bigger tires is being used to work wet soils which older equipment could not have worked. And wet soils are most susceptible to compaction.

* Most farmers want to plant early to insure maximum crop yield potential. This increases the possibility of working soil in a wet condition on at least part of the fields.

* A third reason for increased evidence of compaction is that few farmers use rotations that include a meadow crop. Such rotations reduced the susceptibility of soils to compaction, because they (1) required less intensive tillage, (2) included less clean tilled crops, (3) added organic matter in the form of large root systems which improved soil structure, and (4) introduced different root systems. While a rooting system of itself does not prevent or eliminate compaction, variations in the rooting patterns tend to maintain porosity in a non-compacted soil.

* Overuse of the disc as a tillage implement is another cause of compaction problems in Indiana. The disc has the lowest horsepower requirement for moving the greatest amount of soil; but it is also a compacting tool.

* Disc blades place tremendous force on the subsoil at their point of contact, much like ice skates do on ice. The total weight of an ice skater is concentrated in such a small area that the pressure melts the ice under the skate blade; the skater is actually gliding on a very thin layer of water. Similarly,the edges of disc blades exert such pressure so as to cause compaction in soils. It is very important, therefore, to know when and when not to use a disc.

How Can Soil Compaction Be Determined?

There are several ways to identify soil compaction in the field. They vary in amount of effort required, cost and degree of accuracy. Regardless of the one you choose, plan to repeat it at various locations throughout the field and from year to year; and consider having your findings confirmed by a soil conservationist or extension agent.

Knife Blade Penetration Method

This is a relatively simple procedure. First, dig a hole about 2 feet in diameter and 2 feet deep. Dig on just three sides, so one face of the hole is essentially free of shovel marks. (A shovel can be as much of a compactor as a disc blade.)

Then press a knife blade into the unblemished side at 2-3 inch intervals from top to bottom of the hole. If you encounter a soil layer where knife penetration is more difficult than the zone below it, this is evidence of soil compaction. How compact it is relative to the uncompacted zones or "normal" soil and how it may affect rooting and crop growth are both subjective judgments.

Of course, resistance to penetration depends greatly on soil moisture content. Therefore, this method is more reliable under drier soil conditions. In fact, if done in the spring when the soil is moist, it may be impossible to distinguish compacted layers from the non-compacted ones.

Soil Sampling Tube Method

The soil sampling tube is an excellent tool for identifying compaction problems immediately be low the tilled layer, if the soil is relatively dry. Like the knife blade method, this method "measures" compaction by the resistance encountered as you push down through the soil.

An obvious advantage of the soil sampling tube is that it's fast and easy to use, and is already in use by farmers who do their own soil sampling. The main disadvantage is that, as with the knife blade method, the resistance encountered (thus degree of compaction) depends on the operator's opinion--a highly subjective measure.

Penetrometer Measurement

Soil scientists, in doing soils research, use both the knife blade and soil sampling tube methods, but can also measure the amount of resistance encountered with an instrument called a penetrometer. This is a gauge which records the force needed to penetrate a soil layer. It provides specific values rather than having to rely on subjective judgment.

Penetrometers may be cone or pocket types. These instruments provide the soil scientist with useful data, but readings still require adjustment for soil moisture and careful interpretation before they can have great practical meaning.

Plant Rooting Pattern Assessment

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 it.

Moderate compaction does not completely block root penetration, but rather limits the number of roots that make it through to the deeper 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 accurate assessment of compaction on root growth patterns, be sure to examine plants at a number of locations in the field. At the same time,examine root systems for their tendency to be mostly shallow and spreading instead of conical and deep. If you find poor-appearing plants with shallow roots over subsoils which break out in horizontal layers or plates and have roots adhering to their upper surfaces, you'll know that you have compaction problems.

How Can Compaction Problems Be Corrected?

It is clear that no one remedy will correct all types of soil compaction problems. But there are "principles" that can be followed to minimize compaction or perhaps, in some cases, to eliminate it. Here are the major ones to consider.

Use Less Secondary Tillage

One sure way to help destroy natural soil structure is to over-till. Each pass over the field with a disc, cultivator, harrow, cultipacker or combination of tillage tools designed to prepare a finer seedbed further breaks down the soil's structural aggregates. The result is that the soil becomes more susceptible to implement compaction, and surface crusts are more likely to form after rains.

The less secondary tillage done, the less breakdown of structural aggregates and the less susceptible the soil will be to compaction.

Add Organic Matter to the Soil

Generally, organic matter promotes formation of good soil structure. It helps bind the aggregates together so they are not easily broken down by secondary tillage or by traffic if the soils are worked when wet. This is why those who farm prairie soils rich in organic matter are less apt to encounter compaction problems than those farming soils formed under forest vegetation.

There are several ways to add organic matter to the soil--(l ) by retaining previous crop residues, (2) by growing more small grains with grass-like root systems, (3) by growing a green manure crop in rotation or (4) by applying animal manures, sludges or other waste products. All these aid in improving soil structure as well as providing nutrients and organic matter.

Subsoil with a "V" Chisel

This has been an effective practice in the southern United States. Normally, farmers try to chisel at the driest point during the year to get the greatest "shattering action" of the compacted subsoil.

If soil is wet, it will merely mold around the chisel and not shatter the compacted zone as desired. In Indiana tillage trials, subsoiling did not shatter plowsoles where soils were too wet. And sometimes plowsoles and upper clayey subsoils were too resistant for a normal chisel plow to penetrate effectively.

Although there is no Indiana data to indicate a yield increase from deep subsoiling, most of this research was done in the 1950's when compaction problems were not as serious as now. If a compacted zone is present, the "V" chisel may be an effective way of eliminating it. Size of the chisel must match tractor horsepower to achieve the desired tillage depth. For satisfactory operation, 25- 30 horsepower per shank is needed.

Vary the Plow Depth

Another possible "anti-compaction" technique is to plow deeper than usual during a dry year. Then in succeeding years, vary the plow depth to keep from reforming the compacted layer.

Control the Traffic Pattern

This is another idea from the southern U.S. that deserves consideration in Indiana. Controlling the traffic pattern means restricting all equipment to particular "tracks" through the field, while leaving the rest of the field uncompacted for root growth.

This is essentially what is done in a cropping program when you plant in the same rows using matched equipment every year. The soil under the implement wheel tracks becomes thoroughly compacted, but the between-row areas without tracks receive little more compaction than what you do in preparing your seedbed for planting.

Such a practice is relatively untried in Indiana. In fact, with the mismatched equipment that is commonly used, 80 percent of a field may get at least one track in producing a corn crop. However, those using till-plant or no-till equipment not calling for any form of plowing are developing controlled traffic patterns, since crop rows and all wheeled traffic are in the same positions each year.

Summary

All the answers on how to deal with soil compaction are not yet available. So where problems exist, they will likely continue to limit crop yields.

However, our knowledge of the mechanics of soil compaction is growing, and certain principles as to correction and prevention have been established. Based on these, research programs are being developed to provide the answers we must still have.