Soil Compaction: Causes, Effects, and Control

As farm tractors and field equipment become larger and heavier, there is a growing concern about soil compaction. Soil compaction can be associated with a majority of field operations that are often performed when soils are wet and more susceptible to compaction. Heavy equipment and tillage implements can cause damage to the soil structure. Soil structure is important because it determines the ability of a soil to hold and conduct water, nutrients, and air necessary for plant root activity. Although much research has been conducted on soil compaction and its effects on yield, it is difficult to estimate an economic impact because fields vary in soil types, crop rotations, and weather conditions.

Figure 1. Effects of compaction on pore space.

WHAT IS SOIL COMPACTION?

Soil compaction occurs when soil particles are pressed together, reducing pore space between them (Figure 1). Heavily compacted soils contain few large pores and have a reduced rate of both water infiltration and drainage from the compacted layer. This occurs because large pores are the most effective in moving water through the soil when it is saturated. In addition, the exchange of gases slows down in compacted soils, causing an increase in the likelihood of aeration-related problems. Finally, while soil compaction increases soil strength-the ability of soil to resist being moved by an applied force-a compacted soil also means that roots must exert greater force to penetrate the compacted layer.

Soil compaction changes pore space size, distribution, and soil strength. One way to quantify the change is by measuring the bulk density. As the pore space is decreased within a soil, the bulk density is increased. Soils with a higher percentage of clay and silt, which naturally have more pore space, have a lower bulk density than sandier soils.

MYTHS ABOUT SOIL COMPACTION

There are two wide spread myths about compaction; 1) Freeze-thaw cycles will alleviate a majority of soil compaction created by machinery, and 2) What compaction "Mother Nature" does not take care of, deep tillage or subsoiling will alleviate.

Freeze-Thaw Cycles

Although soils in Minnesota are subject to annual freeze-thaw cycles and freeze to depths of 3 feet or more, only the top 2 to 5 inches will experience more than one freeze-thaw cycle per year. The belief that freeze-thaw cycles will loosen compacted soils may have developed years ago when compaction would have been relatively shallow because machinery weighed less and grass and legumes were grown in the rotation.

Research conducted in 1960 at Lamberton reported that nine years of cropping and annual freezing and thawing did not remove a compacted soil layer at the bottom of the plow furrow in a Nicollet clay loam (Voorhees, 1983). The 21/2 ton, 12-inch wide wheel used to compact the plow furrow exerted a pressure of about 110 pounds per square inch (psi) on the bottom of each plow furrow. The compacted soil drained more slowly and remained wetter after a rain. In this study, corn and alfalfa yields were not affected by the compaction. This was likely due to the timeliness and amount of rainfall. In other studies at Lamberton, compaction due to wheel tracks also persisted over winter at depths of 6 to 18 inches.

Deep Tillage/Subsoiling

While deep tillage (greater than 10 inches) is capable of shattering hard pans created by wheel traffic, it has not been proven to increase yield consistently or for long periods of time. In Midwestern studies where plots with established compaction were split with a deep tillage treatment (14-16 inches), corn yields were either unaffected or reduced slightly (10 bushels per acre) compared to the non-subsoiled plot. The one possible exception would be on an irrigated loamy sand. A lack of consistent positive yield response may be due to:

• Insufficient depth of subsoiling
• Ineffective removal of compacted layer due to high soil moisture levels
• Recompaction of loosened soil by subsequent wheel traffic
• Deterioration of some soil properties by the subsoiling operation
• Detrimental effects caused by compaction were no longer limiting crop yield

WHAT ARE THE CONSEQUENCES OF SOIL COMPACTION FOR PLANT GROWTH?

Soil compaction can have both desirable and undesirable effects on plant growth.

Figure 2. Nitrogen deficiency symptoms in corn.

Figure 3. Potassium deficiency symptoms in corn.

Figure 4. Effects of weather on crop yield response to compaction level (adapted from Soane et al., 1994).

Desirable Effects

Slightly compacted soil can speed up the rate of seed germination because it promotes good contact between the seed and soil. In addition, moderate compaction may reduce water loss from the soil due to evaporation and, therefore, prevent the soil around the growing seed from drying out. Corn planters have been designed specifically to provide moderate compaction with planter mounted packer wheels that follow seed placement.

A medium-textured soil, having a bulk density of 1.2 grams per cubic centimeter (74 pounds per cubic foot), is generally favorable for root growth. [Note: a soil bulk density of 1.2 grams per cubic centimeters is comparable to a non-tracked soil after a secondary tillage operation.] However, roots growing through a medium-textured soil with a bulk density near 1.2 grams per cubic centimeter will probably not have a high degree of branching or secondary root formation. In this case, a moderate amount of compaction can increase root branching and secondary root formation, allowing roots to more thoroughly explore the soil for nutrients. This is especially important for plant uptake of non-mobile nutrients such as phosphorus.

Undesirable Effects

Excessive soil compaction impedes root growth and therefore limits the amount of soil explored by roots. This, in turn, can decrease the plant's ability to take up nutrients and water. From the standpoint of crop production, the adverse effect of soil compaction on water flow and storage may be more serious than the direct effect of soil compaction on root growth.

In dry years, soil compaction can lead to stunted, drought stressed plants due to decreased root growth. Without timely rains and well-placed fertilizers, yield reductions will occur. Soil compaction in wet years decreases soil aeration. This results in increased denitrification (loss of nitrate-nitrogen to the atmosphere). There can also be a soil compaction induced nitrogen and potassium deficiency (see Figures 2 and 3). Plants need to spend energy to take up potassium. Reduced soil aeration affects root metabolism. There can also be increased risk of crop disease. All of these factors result in added stress to the crop and, ultimately, yield loss.

Research from North America and Europe indicates that crops respond to soil compaction as shown in Figure 4. In a dry year, at very low bulk densities, yields gradually increase with an increase in soil compaction. Soon, yields reach a maximum level at which soil compaction is also optimal for the specific soil, crop, and climatic conditions. However, as soil compaction continues to increase beyond optimum, yields begin to decline. With wet weather, yields are decreased with any increase in compaction.

WHAT CAUSES SOIL COMPACTION?

There are several forces, natural and man-induced, that compact a soil. This force can be great, such as from a tractor, combine or tillage implement, or it can come from something as small as a raindrop (Figure 5). Listed below are several types of soil compaction and their causes.

Raindrop impact - This is certainly a natural cause of compaction, and we see it as a soil crust (usually less than 1/2 inch thick at the soil surface) that may prevent seedling emergence. Rotary hoeing can often alleviate this problem.

Tillage operations - Continuous moldboard plowing or disking at the same depth will cause serious tillage pans (compacted layers) just below the depth of tillage in some soils. This tillage pan is generally relatively thin (1-2 inches thick), may not have a significant effect on crop production, and can be alleviated by varying depth of tillage over time or by special tillage operations.

Wheel traffic - This is without a doubt the major cause of soil compaction. With increasing farm size, the window of time in which to get these operations done in a timely manner is often limited. The weight of tractors has increased from less than 3 tons in the 1940's to approximately 20 tons today for the big four-wheel-drive units. This is of special concern because spring planting is often done before the soil is dry enough to support the heavy planting equipment.

Minimal Crop Rotation - The trend towards a limited crop rotation has had two effects: 1.) Limiting different rooting systems and their beneficial effects on breaking subsoil compaction, and 2.) Increased potential for compaction early in the cropping season, due to more tillage activity and field traffic.

Figure 5. Reduced root growth due to compaction from raindrop impact, tillage, and wheel tracks. Source: Compaction-Soil Management Series 2. University of Minnesota Extension Service, BU-7400

EFFECT OF SOIL MOISTURE AND AXLE LOAD ON DEPTH OF COMPACTION

Greater axle loads and wet soil conditions increases the depth of compaction in the soil profile. Compaction caused by heavy axle loads (greater than 10 tons per axle) on wet soils can extend to depths of two feet or more (Figure 6 and 7). Since this is well below the depth of normal tillage, the compaction is more likely to persist compared to shallow compaction that can be largely removed by tillage.

Figures 6 and 7. Depth of compaction as (6) axle load and (7) soil moisture increases (Adapted from Soehne, 1958).
(Tire pressure remained at 12 psi for all tire sizes)	(Tire size 11 x 28, load 1,650 lbs, pressure 12 psi)

A research study conducted in Lamberton, Minnesota (Voorhees et al., 1986) illustrates this effect. The Nicollet clay loam soil was compacted with 10 and 20-ton axle weights. When the soil was dry, most of the increase in bulk density was confined to the top foot with no detectable effect on the bulk density at the 18-inch depth. Under wet conditions the 20-ton axle load compacted the soil deeper than 18 inches. Similarly, under wet conditions on a Webster clay loam at Waseca the 20-ton axle load increased the bulk density to at least the 24-inch depth.

Total axle load, as well as contact pressure between the tire and soil, affects subsoil compaction. Historically, as equipment weight increases, tire size also increases. This avoids drastic increases in contact pressure (pounds per square inch (psi) of pressure exerted by the tire on the soil surface). Axle load for various field equipment are listed in Table 1.

DENSITY EFFECTS DUE TO TILLAGE

Figure 8. Soil resistance to penetration after 10 years of continuous tillage for four tillage system.

Compaction resulting from equipment weights of less than 10 tons per axle is generally restricted to the upper foot of the soil. Compaction in this zone can be largely removed by chisel or moldboard plowing the compacted layer. While wheel tracks are often the most obvious cause of surface compaction, they are by no means the only cause. Livestock and tillage equipment can also produce compaction.

Following ten years of a continuous corn tillage study on a clay loam soil at Waseca (Bauder et al., 1981), a dense compacted layer was detected just below the depth of tillage (4 inches) on the disc treatment (Figure 8). A cone penetrometer was used to register soil strength. The greatest force required to penetrate the top 12 inches was measured on the plot that was spring disked and had no other tillage. The light tandem disc penetrated only the top three inches of soil. Below the 4-inch depth, the resistance to penetration on the disc treatment equaled or was greater than that measured on the no-till treatment and both were considerably greater than the moldboard or chisel plow treatments in the 0- to 12-inch depth.

In tillage studies on a loamy sand at the Sand Plain Experimental Farm, at Becker, Minnesota, comparing no-till, chisel, moldboard and ridge till systems, and at a study on a silt loam at Arlington, Wisconsin, comparing chisel, moldboard and ridge till systems, the greatest resistance to penetration in the 0- to 12-inch depth occurred on the no-till treatments.

In both studies a plow pan was detected just below the depth of operation with the moldboard plow. The Wisconsin study found that with wheel traffic there was little difference in penetration resistance among tillage systems. Soil compaction from wheel traffic tends to mask the effects of tillage on penetration resistance.

PLANT RESPONSE TO SURFACE COMPACTION

The effect of compaction on plant growth and yield depends on the crop grown and the environmental conditions that crop encounters. In general, under dry conditions some compaction is beneficial, but under wet conditions compaction decreases yields. Response of various crops, including soybeans, corn, wheat, potatoes and sugar beets, to surface compaction has been studied in Minnesota and in surrounding states. The results of these studies will be discussed by crop.

SOYBEANS Research results in Minnesota indicate that the effect of surface compaction on soybean yields depends very strongly on weather conditions and on soil nutrient levels. The yield response from compacting the soil was greater during dry years and when the soil phosphorus tested low. During a 13-year (1973-1985) study on a clay loam soil at Lamberton (Voorhees et al., 1986), the effect of surface compaction on yield was related to the rainfall amounts. When May - August rainfall was less than 14 inches, soybean yields were greater in the tracked rows. However, if the rainfall was more than 14 inches, yields from the tracked beans were less than the tracked rows. Possible reasons for the increased yields on the "wheel-tracked" treatment during the dry seasons include increased uptake of phosphorus, increased root branching, and reduced water evaporation from the plow layer. This provides more favorable water conditions for nutrient uptake. CORN At Rosemount, Minnesota (Blake, 1958), axle loads of 4 tons in 1957 and 6 tons in 1958 were applied to the entire surface of a Waukegan silt loam soil prior to planting. Corn yields were reduced an average of 7.5% over the two-year period. Increased levels of soil fertility effectively overcame the yield loss due to compaction. The yield effects of increasing tire contact pressure on a clay soil in Quebec, Canada, were influenced by the May to September precipitation. When the precipitation was 21 inches, yield decreased as the compaction was increased. When the precipitation was 14 inches, corn yield first increased as compaction increased and then decreased as compaction was increased further. Thus, under dry conditions some compaction was beneficial but too much was detrimental to yield. Under wet conditions any amount of compaction decreased yields and the greater the contact pressure the lower the yields. Table 2. Timing effect of compaction on percent wheat protein in Grand Forks, ND. Treatment % Protein Non-compacted 13.6 Spring compacted 12.9 Fall and Spring compacted 12.7 WHEAT The effect of wheel traffic compaction on wheat yields on a clay loam at Morris depended on the April to June precipitation. Under dry conditions, the wheel-tracked area produced higher yields than the non-tracked area. However, as the precipitation increased, the non-compacted area resulted in higher yields than the compacted area. In 1980 (Voorhees et al., 1985), wheat yields of non-compacted, fall-compacted, spring-compacted, and fall- and spring-compacted treatments were not significantly different. However, percent protein was significantly lower on the spring-compacted (12.9%) and fall- and spring-compacted (12.7%) treatments compared to the non-compacted treatment (13.6%) (Table 2). In contrast, a 1982-1983 North Dakota study reported that neither wheat yield nor percent protein were significantly different between non-compacted and compacted treatments. European research has shown that compaction generally decreased wheat and barley yields under wet conditions, while moderate compaction had little effect or slightly increased yields during normal precipitation patterns and increased yields under dry conditions. In all cases, severe compaction decreased yields. POTATOES Because of the potato tubers growth pattern and its relatively confined root zone, soil compaction frequently has a detrimental effect on potato yields. There has been a good deal of research confirming lower potato yields due to compaction. Deliberate packing of the soil in the Red River Valley decreased potato yields 54%. In a two-year study at Grand Forks, Minnesota, compaction reduced potato yields an average of 21%. At Morris, Minnesota, potato yields were reduced 35% because wheel traffic restricted tuber development. Table 3. Effect of soil compaction on sugarbeet and total recoverable sugar yields, Grand Forks, ND (Giles, 1980,1981).   Soil Properties Probe Resistance (lbs/inch2) 1979* and 1980 Average Tons/Acre Beets Sugar Compacted 133 11.5 1.53 Non compacted 78 10.5 1.42 *Compaction increased yields because of a higher final stand (108 vs. 79 beets/100 ft. of row for compacted and non-compacted treatments respectively). SUGARBEETS The sugarbeet appears to be less sensitive to surface compaction if it does not interfere with plant emergence. The effects of surface compaction on yields in a North Dakota study are shown in Table 3. Although soil compaction increased beet sprangling, which causes problems at harvest, compaction had no effect on recoverable sugar or root yields in 1980. Compaction was created by repeated passes with a loaded, single-axle, dual-wheel truck (GVW 18000 lbs).

EROSION

Figure 9. Reduced water infiltration in wheel tracks.

Soil compaction in the surface layer can increase runoff, thus increasing soil and water losses.

However, when the compacted layer is tilled with a moldboard or chisel plow, the resulting rough, cloddy surface can decrease runoff and erosion. While it sounds contradictory, both effects are possible, depending on the soil and soil conditions encountered.

Field studies at Lamberton have shown greater surface roughness and cloddiness following moldboard plowing where the soil was tracked before tillage. Other studies show that greater surface roughness was caused by greater bulk density before tillage. On soils such as these, which have a relatively stable structure, greater surface roughness can increase infiltration, reduce runoff, and reduce erosion up to the point that runoff begins. Studies at Morris have shown that even after tillage, wheel tracks due to spring planting increased runoff and erosion.

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