NDSU Crop and Pest Report
Soils


ISSUE 8  June 19, 2003

YELLOW SOYBEANS

The bright yellow color shown by many soybean fields in eastern North Dakota is most likely due to iron deficiency chlorosis. We have seen chlorosis as a common problem in the state for many years. It is caused primarily by the carbonate minerals in the soil. When the soil is wet, solubility of the carbonates (bicarbonate primarily) increases and inhibits the plants ability to take up iron. This of course occurs under all plants, but some plants, primarily soybeans, but also to a lesser degree dry beans and flax, have lower tolerance to the condition than others.

Expression of chlorosis and its severity are related to several factors, some environmental and some management. Chlorosis will not be seen when soil pH is under 7. When pH is over 7, then the amount of carbonates present is important. The wetter the soils are, the greater the chlorosis severity will be. Also, soluble salts influences the ability of soybeans to overcome this problem. The more severely chlorotic areas most likely will have higher levels of salts than other less yellow areas of the field. During wet periods, often wheel tracks are greener, because these areas tend to dry up faster than the rest of the field. In dry periods, wheel tracks are often more moist (capillary pull water, like press wheels on seeders), so wheel tracks are more yellow. Cool conditions also increase chlorosis severity.

Management can contribute to the problem or help to relieve it. Low seeding rates/emergence, especially in solid-seeded soybeans are most affected by chlorosis. Planting soybeans in rows, especially at higher seeding rates is most helpful in avoiding serious chlorosis. Also, when soybeans are in rows, they can be cultivated, further drying the soil and relieving chlorosis.

During the peak chlorosis time, avoid harsh contact herbicides and those with soil residual and systemic behavior. In some of our herbicide interaction with chlorotic soybean studies these herbicide options have had particularly negative effects on soybean yield. Having presented that, it is also the case that severe weed infestations can also have a more devastating yield effect than anything a herbicide can dish out, so the grower must weigh the effect of the weeds against any effect of the herbicide. But if there is a choice of products that can control the weeds present, choose one with gentle temporary effects rather than a harsher one or one whose gift keeps on giving.

If a field is chlorotic from fencerow to fencerow, two things may be wrong. First, the wrong variety may have been selected for the field. There is considerable difference in variety tolerance to our local soil causal factors (carbonates and salts). Some varieties may do quite well under some conditions, while another might just die. Checking with Dr. Goos’s chlorosis tolerance list published through the North Dakota Soybean Council would be a good place to start for next years soybean variety needs. See website:

http://www.soilsci.ndsu.nodak.edu/yellowsoybeans/

The second thing that might be wrong is the salt levels within the field. If the field is consistently higher in soil EC than 1.5 mmho/cm, then growing a yield of soybeans higher than 25 bushel/acre would be quite a feat. Check the salt (EC) level to determine if something else might do better in that field in the future.

It would be great if I could confidently recommend an iron treatment to take care of this problem. The unfortunate reality is that although many iron sources have been tested, there is no guarantee that a foliar treatment would do more than temporarily green up the plants. In some studies, several bushel increases have been noted, while in others the results are no response.

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Yellow soybeans - but not chlorosis

If soybeans do not have that bright yellow color associated with chlorosis and during closer inspection the yellowing is not interveinal, as in chlorosis, but rather includes the veins also, the problem may be low N levels in the plant. We have had cool, wet conditions for several weeks. It is possible that in some fields nodulation has not been very good yet. An inspection of soybean roots at 2-3 trifoliates should show some nodulation. If not, the plants might be at risk for N deficiency later in the season. Revisit the field in another week or so, and if the condition persists, topdressing or sidedressing may be necessary. A study out of the Carrington center last year showed that under N deficiency, later season topdressing improved yields. However, I would not be hasty with the application. Yield decisions in soybean are made later in the season and it is possible especially in fields that have grown soybeans before that nodulation will still take place even if initial infection is delayed. In first year soybeans, I would be more nervous.

Dave Franzen
NDSU Extension Soil Specialist
(701) 231-8884
dfranzen@ndsuext.nodak.edu


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