Minnesota soybean growers who deal with iron deficiency chlorosis (IDC) have seen them many times. Those green soybeans that appear in the wheel tracks across areas of yellow soybeans have always been a mystery. They are usually associated with the last pass of the field cultivator. They’ve been a mystery with no good explanation for the cause for several years.

These green tracks are important because an explanation for the cause might lead to development of management practices that would reduce the severity of IDC. So, a research project was established in 2004 to focus on developing an explanation for the cause of the observation. To conduct this project, green wheel tracks were found in fields of soybean producers in Redwood and Yellow Medicine counties. In the Yellow Medicine county field, green soybeans were evident over a recently installed tile line. So, that location was also included in the research.

In an effort to get an explanation, several soil and plant samples were collected where the soybeans in the wheel track were green. The same measurements were taken where the yellow soybeans were next to those that were green. These measurements included:

• soil moisture at depths of 0 to 3, 3 to 6, 6 to 9, and 9 to 12 inches
• bulk density (a measure of compaction) at depths of 3 to 6, 6 to 9, and 9 to 12 inches
• weight of whole plants
• concentration of iron in the plants
• uptake of iron
• weight of roots in the upper portion of the root zone (0 to 6 inches)
• carbon dioxide production at a depth of 0 to 4 inches

The measurements listed above were taken in mid-July. The measurement of carbon dioxide production continued into early August. The results for the locations in Redwood and Yellow Medicine counties are listed in Tables 1 and 2 (below) respectively.

After reviewing the information collected, we conclude that we still don’t have a good explanation for the green soybeans growing over the wheel tracks. As might be expected, plant weight was reduced where the soybeans were yellow. Root growth is usually associated with the above ground growth of the plant and roots were larger where the soybeans were green.

There was some variability among sites; but, soil moisture content was not substantially different where the soybean plants were green. Bulk density, measured as grams per cubic centimeter (g/cc) is a reflection of soil compaction. Bulk density can have a substantial impact on root growth and subsequent nutrient uptake. At the locations used in this study, bulk density was variable, but not directly related to either plant or root growth.

Soil properties (pH, P, K, soluble salt, carbonates) were not related to the growth of green soybeans. At any location, these properties were nearly the same where both green and yellow soybeans were growing.

Considering all locations, there were consistent differences in the concentration of carbon dioxide at the 0 to 6 inch depth of the root zone. In all cases, concentrations of this gas were substantially higher where the green soybeans were growing. These higher concentrations could be the result of larger plants growing or they could be the cause of the green soybeans. The data collected in 2004 do not provide an explanation for the dilemma.

This project will not end with the information gathered in 2004. Based on the information gathered, we intend to make additional measurements in 2005. So, we will be looking for green wheel tracks. We need help to identify these fields. So, if green wheel tracks appear where iron deficiency chlorosis is a problem, please call (612) 625-6210 or send an e-mail (rehmx001@umn.edu).

Table 1. Measurement of soil properties and plant growth where green soybeans in wheel tracks were observed in RedwoodCounty in 2004.

Table 2. Measurement of soil properties and plant growth where green soybeans were observed in Yellow Medicine County in 2004.

*At this location, the green soybeans were growing over a recently installed tile line.

[printer-friendly pdf version of this article]