Most soybean producers in western Minnesota have had them in their field at one time or another in the past several years. No, it’s not aphids or soybean rust. They have experienced the green wheel tracks in portions of the field where iron deficiency chlorosis (IDC) is a problem. These tracks are usually associated with the last pass with the field cultivator. These tracks were much more obvious in 2005 compared to 2004. In many fields, the tracks were obvious throughout the growing season.

There has not been a good explanation for the cause or origin of these tracks. There have been several theories. Since the tracks were so obvious in 2005, it seemed that a good explanation for their existence might possibly lead to development of management practices that could be used to reduce the severity of IDC.

In searching for an explanation, soil and plant samples were collected in early July from 8 fields where the green wheel tracks were very obvious. The results from one field south of Benson are summarized in Figure 1 and Table 1 that follow. These results are typical of measurements taken from other sites. There is no attempt to summarize all of the data collected from the 8 sites.

A penetrometer was used to measure bulk density. The results are shown in Figure 1. The values are reported as a come index reported as PSI (pounds per square inch). A higher index indicates a higher bulk density or more soil compaction. In the field south of Benson, compaction beneath the wheel track compared to the non-trafficked area started to increase at a depth of about 8 inches and continued to a depth of about 21 inches. The presence of compaction below the wheel track is not surprising. The presence of compaction at a depth of nearly 20 inches was not expected.

Soil samples were collected at the same time and analyzed for soybean cyst nematode. None were detected. For this field, at least, the yellow soybeans could only be attributed to IDC.

The other measurements that were taken are summarized in the table that follows (Table 1).

Table 1. Summary of soil properties from a soybean field where green wheel tracks were obvious. 2005.

There was no substantial difference in soil moisture to a depth of 9 inches. Even though the soil under the wheel track was more compacted, there was no difference in soil moisture content. Soil test values for pH, phosphorus and potassium were nearly identical for both areas. There is no logical reason to expect any differences.

Severity of IDC is frequently associated with soluble salts and/or free calcium carbonates. In this field, the values were nearly the same for both the wheel tracked and non-wheel tracked areas.

For this field, measured NO₃-N to a depth of 12 inches was higher in the non-wheel tracked area. The nitrogen concentration in the whole plant tissue was also higher in the plants showing severe IDC. This higher concentration of NO₃-N in the soil is considered to be important because NO₃-N affects the metabolism of iron in the soybean plant.

The relationship between green plants and NO₃-N in the soil at this site was consistent for all 8 sites. Therefore, it is possible that a link between a soil property and the severity of IDC may have been discovered. This relationship would help to explain the fact that soybeans tend to be green in IDC areas when there is competition form a small grain crop or other soybeans. It certainly appears that this relationship needs further testing in the future.

The amount of carbon dioxide (CO₂) in the top 4 inches of soil was also measured. The concentration was nearly twice as high where the soybeans were yellow. The significance of this observation is not obvious at this time.

The results from this site south of Benson are representative of the results gathered from the remaining 7 locations. It does appear that very useful information was collected in 2005. Management practices that will evolve from these measurements will be evaluated in the field in 2006.

Figure 1. Cone index values to 25 inches under wheel tracks and no wheel tracks in a soybean field south of Benson. 2005.

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