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Chapter 2 - Systems and Equipment, No. 9, Winter 1988

Traffic Compaction Affects Productivity

Roger Veseth

Soil compaction is an increasing problem in the Northwest and needs greater attention by producers and agricultural researchers. Compaction decreases crop yield potential by restricting root growth and creating a less desirable root environment. Increased surface water runoff on compacted soil also reduces yield potential as well as accelerates topsoil erosion.

The increasing trend toward conservation tillage, which requires fewer tillage operations, has the potential of greatly reducing the possibility of traffic compaction of soils. This benefit of conservation tillage can be reduced, however, by the trend toward larger, heavier tractors in recent years.

John Hammel, University of Idaho soil scientist and STEEP researcher at Moscow, has focused much of his current research effort on the impacts that tractor weight and tillage practices have on soil physical properties and production potential. Hammel points out that, depending on implement widths, about 60 to 90 percent of the soil surface in the field may be traversed by tractor wheels during conventional tillage production of small grains. Additional compaction occurs from the combine, particularly if the soil is wet at harvest. Each track made by the equipment contributes to compaction of the soil.

Hammel explains that the average weight of a wheel type tractor in the early 1950's was 3 tons. Tractor weight increased to an average of 5 tons in the late 1960's, Weight of 4-wheel-drive tractors today can range between 10 to 20 tons.

In general, the greater the load applied to the soil surface, the greater the degree and depth of soil compaction. Compaction below the normal tillage zone is of particular concern because correction, if possible, is difficult and expensive, A deep subsoiling operation maybe required. In some cases, however, deep soil compaction may only be reduced by years of rooting action from deep-rooted plants.

Factors Influencing Compaction

The potential of a soil to compact depends on its physical properties, water content and applied load. Physical properties influencing compaction potential include texture, bulk density, organic matter content and structure. Generally, there is a greater potential for more serious compaction with soils that have finer textures, high porosity (low bulk density), low organic matter content and a weak, poorly-aggregated structure.

From a management standpoint, the two factors that control compaction are the water content of soil at the time of the field operation and the wheel-track load, When soil is dry, it is more resistant to compaction because of the high bonding or cohesive strength, high degree Of particle interlocking and frictional resistance to deformation.

As the soil water content increases, the water film weakens these cohesive bonds and lubricates the particles, resulting in an increased potential for compaction.

Compaction Impacts

The detrimental effects of soil compaction on productivity are numerous but often indirect, making it easy to overlook or mistaken as a different production-limiting factor. Hammel explains that as soil is compacted, the number of large soil pores is reduced and soil density is increased. The resultant soil condition restricts the movement of water and air through the soil and increases the soil strength or resistance of the soil to penetration by roots.

Physical resistance to root growth and an unfavorable root environment can lead to lower nutrient and water uptake, and consequently, severely stress the plant. Compaction also reduces internal drainage which causes wet, poorly aerated soils. Wet soils warm more slowly than well-drained soils, which can further reduce root growth rates. In addition, cool wet soils favor soilborne diseases such as Pythium root rot, Rhizoctonia root rot and takeall. Plants under stress are also much more susceptible to infection by root pathogens.

Soil strength and pore size are the primary factors affecting root penetration. Crops grow best when the soil pore distribution and geometry matches the root size and configuration. Roots with a small diameter tip can penetrate compacted soil more easily than large diameter roots. Consequently, crops with relatively large root tips, such as peas and beans, are more strongly affected by compaction than cereals. Of the cereals, barley tends to be one of the more susceptible to compaction.

Soil compaction is also believed to be an important contributor to the high rates of surface water runoff and soil erosion in the Northwest. Restricted rates of water infiltration into the soil and poor internal drainage due to compaction also result in a lower amount of "stored plant-available" precipitation for crop production because of increased evaporation and runoff.

Field Research Initiated

Little research information is available on the effects of traffic compaction in the Northwest. To help address this increasing problem, Hammel initiated a field study north of Moscow in April 1986. The soil at the site is a deep Palouse silt loam with 3.6 percent organic matter in the surface foot. This level of organic matter is commonly found on bottom land or gently-sloping areas in the Palouse region but is about twice or more the level typically found on slopes and ridges where water or tillage erosion has reduced topsoil depth.

Soils with greater amounts of organic matter tend to be better aggregated and have stronger bonding between particles. Consequently, a high organic matter content tends to stabilize soil and decrease the compactive effects of high traffic loads.

To avoid complications of existing soil compaction, a field site was selected where only light track-type tractors had been used in the past. The field had also been in an alfalfa-grass pasture for the previous 12 years. The field site has a slope of only 5 to 10 percent and showed little evidence of topsoil erosion.

Hammel points out that, because of the deep topsoil, high organic matter content and past cropping practices, the site is probably less susceptible (''more forgiving") to compaction than typical production fields in the region. The reader should keep this in mind when evaluating the preliminary results of this study.


The study focuses on four areas: (1) influence of high axle loads on soil properties (bulk density, soil strength and permeability to water); (2) effects of the resultant compaction on crop growth and yield; (3) duration and extent of compaction under the different axle loads; and (4) ability of winter rape to alleviate soil compaction through root action.

Research Variables

Three axle loads were selected: 5, 10 and 20 tons per axle. Large 4-wheel drive tractors today commonly have 8 to 10 tons per axle weight distributions. The 5 tons per axle load was used as the control for comparison with the 10 and 20 tons per axle in single-wheel treatments (no dual wheels). Load treatments were made with four consecutive passes, wheel track to wheel track. This arrangement was selected to simulate a potential spring-tillage traffic pattern. The traffic loads were applied only once at the start of the experiment, in the spring before seeding the 1986 spring crops.

At the time of compaction, the soil had been allowed to drain for 1 week after being saturated. Water content of the soil was near field capacity. This soil water content is typical for early-spring tillage conditions. Compaction impacts are being evaluated in two 3-year crop rotations: spring barley-spring pea-winter wheat and summer fallow-winter rape-winter wheat. In the first growing season of the study in 1986, spring barley and spring peas were planted and a partial season of summer fallow was conducted with the winter rape being planted in August. Conventional tillage methods are utilized in each rotation: fall moldboard plow/disk/harrow before seeding. Plow depth is approximately 8 inches.

Effects on Soil Properties

The evaluation of axle load effects on soil bulk density, soil strength and permeability to water was conducted immediately after the traffic loads were applied. Axle loads of 10 and 20 tons progressively increased soil bulk density to a depth of 36 inches (Fig. 1). The most dramatic increase in bulk density was at the 6-to 24-inch depth with the 20 tons per axle load. The increased bulk density results in higher soil strength at these deeper depths, well below the tillage zone.

Soil strength or soil impedance was measured as the pressure required to push a metal probe (penetrometer) with a 0.75-inch diameter cone tip into the soil (Fig. 2). The increased soil impedance with the higher axle loads closely reflected the pattern of increased bulk density. Depending on soil water content and other conditions, soil impedance values of about 150 lb/in2 can reduce the root growth rate of peas and other sensitive crops. Values in excess of 250 to 300 lb/in2 will restrict root growth of most crops.

Reduced root growth can limit a crop's ability to absorb water and nutrients during critical stages of growth or reproduction. The resultant stress, particularly during hot, dry periods, can reduce yield potential through loss of tillers or other yield components.

Water movement into the soil under a saturated condition (saturated hydraulic conductivity) was progressively reduced with increased axle load (Table 1). Under periods of heavy rain, rapid snowmelt or combinations of the two, considerably more surface water runoff could occur hot, dry periods, can reduce yield potential through loss of tillers or other yield components.

Water movement into the soil under a saturated condition (saturated hydraulic conductivity) was progressively reduced with increased axle load (Table 1). Under periods of heavy rain, rapid snowmelt or combinations of the two, considerably more surface water runoff could occur with the reduced water conductivity rates from the 10 and 20 tons per axle loads than with the 5 tons per axle load.

Fig. 1. Effect of soil compaction from three wheel-track Loads on bulk density of Palouse silt loam soil, 1988, Moscow, ID (Hammel, UI).

Fig. 2. Effect of soil compaction from three wheel-track Loads on resistance to a probe with 0.75-inch cone tip pushed into Palouse silt loam soil, 1986, Moscow, ID (Hammel, Ul).


Effects on Crop Yields

The effects of traffic compaction on crop yields varied considerably between 1986 and 1987. Hammel points out that growing season precipitation amount and timing greatly influences the impact of compaction on crop growth and yield. About 1.5 inches more precipitation occurred in both June and July of 1987 than 1986. Consequently, water stress associated with restricted root growth and rooting depth from compaction was not as important in 1987 as 1986. He emphasizes that the impacts of restricted root growth with compaction is most evident on drier years and may not be noticed in years of above-average precipitation or with timely rains.

Table 1. Effect of soil compaction from three wheel-track loads on water movement into a Palouse silt loam soil under saturated conditions (saturated hydraulic conductivity), Moscow, ID (Hammel, Ul).

Soil Depth

Axle Load (tons)




6 1.5 2.2 1.1
12 2.8 1.4 0.5
18 1.4 1.5 0.2
24 1.6 1.3 0.4

The difference in growing conditions between the two years resulted in the greatest difference in axle load response in spring peas (Table 2). In 1986, the 20 tons per axle load resulted in a significant 72 percent yield reduction compared to the 5 tons per axle control. Yields increased with the 20 tons per axle load in 1987, however. Hammel speculates that with soil compaction from the 20 tons per axle load, the pea crop had more of its active roots system restricted to a shallower depth in 1987 and was better able to take advantage of the timely June and July rains.

Spring barley yields were significantly reduced by compaction at the 20 tons per axle load in both years (Table 3). Stress-related loss of yield potential due to compaction may have occurred before the June and July rains.

An evaluation of soil water extraction in 1986 helps explain part of the yield decline in spring peas (Fig. 3) and spring barley (Fig. 4) under the 20 tons per axle load. In both crops, soil water extraction was reduced with depth indicating that compaction may have been restricting rooting depth. Nutrient uptake would also have been restricted, as well as water.

Hammel found no significant differences in yield between wheel-track loads in winter wheat and winter rape in 1987. If root growth was restricted by soil compaction, the yield boost from timely June and July rains apparently overshadowed any negative effects on yield.

Table 2. Effect of soil compaction from three wheel-track loads on yield of spring peas in 1988 and 1987, Moscow, ID (Hammel, Ul).

Axle Load


1986 1987

Yield 1


% of control

Yield 1


% of control
















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