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ISSUE 4  May 25, 2000

 

FACTORS AFFECTING SOYBEAN CHLOROSIS

    As we near the first true leaf stage of our earliest soybeans, it is good to revisit what factors are important in promoting chlorotic or yellow soybeans. First, there may be some differences in opinions among growers on what is really chlorotic and what is just nitrogen deficient. Since I came on board in 1994, there have been some fields with a pale yellow tint early in the season. Some of the color differences are due to variety. Some varieties just naturally have a stronger green color than others. Another problem is poor nodulation and otherwise poor N nutrition. High carbonate and high salt levels have been seen in
outside research and in-state research to reduce nodulation, so sometimes the pale yellow color is not really "chlorosis", but a moderate N deficiency.

    True chlorosis is not just a pale color, but a shades of yellowness from subtle to bright yellow between the veins of the leaf. Chlorosis associated with iron deficiency does not appear until the first trifoliate leaves appear in the plant. Prior to this, iron stored in the seed by the previous soybean crop is mobile inside the soybean plant. But after the trifoliates emerge, the physiology of iron mobility changes and iron is not mobile in the plant thereafter and must be continually supplied. So plants with only monofoliate leaves appear nice and green and then if other factors are present can turn yellow overnight as trifoliates emerge.

    The factors contributing to chlorosis are many. The first important and undisputed factor is the presence of free carbonates, particularly bicarbonate, which is a product of the combination of water and carbonate minerals in the soil. Bicarbonate solubility is low, and contrary to logic increases in concentration as the soil becomes wet. So in dry years and dry periods, chlorosis can be relatively minor, because bicarbonate levels interfering with plant uptake of iron are low. As the soil becomes more moister, bicarbonate concentrations increase and the plants become less able to take up iron.

    The phenomena of wheel tracks being yellow or green compared to the rest of the field is probably the result of soil wetness under the tracks. In dry conditions, the soil tends to be more moist under tracks because of capillary pull of water into the track similar to the press wheels of a planter pulling moisture into the seed zone. More moisture, more bicarbonate, more chlorosis. In wet years, wheel tracks and compacted areas tend to dry first, thence less moisture, less bicarbonate, green tracks.

    The following factors are stress factors that make the plants less able to take up iron in adverse circumstances; cool weather and soluble salts. Cool weather slows down plant systems, including iron uptake and plants tend to increase in chlorosis during cool periods when soils are moist and carbonates are present. Soluble salts have also been shown to be a major factor in contributing to the presence of chlorosis with supposedly iron chlorosis tolerant varieties. Often, carbonate levels and soil moisture levels are similar, but the presence or absence of salts has been the difference between whether chlorosis appears or not.

    Generally, salts in our studies have been more active in sandy soils than in soils with more clay. Sandy soils with salt levels of over 0.3 mmohs/cm (a mmoh is a milli moh, which is a term in conductivity related to an inverse ohm, a measure of resistance to current) are at risk of chlorosis when carbonates are present, while loam soils are probably not at great risk unless salts are greater than 0.4 mmohs/cm, and heavier soils greater than 0.6 mmohs/cm. The following chart is my first attempt to try to provide a guide to risk based on our observations to date. Varieties will move into and out of categories depending on moisture levels.

Chart of susceptibility to chlorosis and subsequent herbicide interaction based
on observations during 1998-1999 in the Red River Valley. Assumes carbonates are present.

Soluble
Salts

Sandy loam
organic matter

Loam

Silty clay/clay loam
mmohs/cm

<3%

>3%

<0.3

Low

Low

Low

Low

0.3-0.4

Med.

Low

Low

Low

0.4-0.6

High

Med.

Med.

Low

0.6-1.0

V. H.

High

High

Med

1.0-1.5

V.H.

V.H

V.H.

High

>1.5

------Not suitable------

Fair yields possible if under 2.0 with very tolerant variety

Where Med. represent medium risk and V.H. represents Very High risk.

Dr. Dave Franzen
NDSU Extension Soil Specialist

 


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