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ISSUE 7   June 17, 1999

 

SOYBEAN CHLOROSIS

    With many soybeans emerged, it will soon be time to weather the annual chlorosis problem in the
region. Chlorosis is an interveinal yellowing which appears in the upper leaves of soybeans and sometimes
dry beans. The cause is perceived to be high pH by some, but the real causes are often deeper than that.

    Chlorosis is the result of lack of iron or manganese or both. In our area, it is generally assumed that iron
is the main problem, however, recent studies suggest that manganese may also be involved. In one study,
application of postemergence iron fertilizer increased, rather than decreased symptoms at one site. In
another study, plant manganese levels were more related to the degree of chlorosis than plant iron levels.

    Regardless of whether iron or manganese is the problem, or they just take turns, the mechanism for
each deficiency is similar. Chlorosis may be the result of low soil iron/manganese levels or the low relative
availability of soil iron/manganese reserves. High pH (pH higher than about 7) results in very low solubility
of iron/manganese in soil, so availability is reduced. Soybeans and other broadleaf crops have adapted to
meet this challenge by excreting acids from the roots to create an environment that overcomes simple
soil pH differences. This acid environment also makes suitable chemistry to allow the activity of a plant
enzyme that is also excreted and changes Iron +3 into Iron +2, making it a trillion times more soluble than
it normally exists in surface soils. So why the problem with chlorosis if this is all worked out?

    One of the major problems is bicarbonate levels. Bicarbonate is a product of solubility of calcium
or magnesium carbonates (lime) in our soils. In the central corn belt, high free carbonate levels seldom
exceed 2%, however in our region, free carbonate levels can exceed 40%, and are regularly 5-20%.
The reserves of carbonate are therefore very high and the resulting bicarbonate levels can also be very
high. Bicarbonate levels are important because they tend to neutralize the acidity excreted by the roots.
With root-zone pH higher due to the bicarbonate, the iron reduction mechanism is reduced, resulting in
lack of iron to the plants.

    Bicarbonate levels are also influenced by soil moisture. The wetter it is, the higher the levels of bicarbonate.
So in dry years, there is less chlorosis and in wet years there is more. Some producers also see differences
in wheel track patterns. In dry years, the wheel tracks are yellow (higher moisture-higher bicarbonate), while
in wet years, wheel tracks are greener (less moisture-less bicarbonate).

    The health of the root is also a factor. Cooler temperatures means less root growth and slower
metabolism and less resistance to fight the assault on the roots ability to acidify its environment. Recent
research at NDSU also has shown that the level of soluble salt is very important in determining whether
plant will be chlorotic. Higher salt levels may also influence root health and ability to overcome chlorosis.
Generally, the higher the salt level, the greater the chlorosis.

    What to do?

    Foliar sprays of a number of iron fertilizers have only been marginally effective in combating
chlorosis. In a recent and on-going study, a combination of seed-placed Iron-EDDHA chelate, followed
by a foliar spray of iron was effective in increasing yields. However, the mechanics of this process are
still being worked out. Iron-EDDHA formulations are probably not ready for widespread use as seed
treatments and in-furrow applications are being evaluated. For now, planting as tolerant varieties as
possible is the best approach. Varieties are being selected more in our region for our type of chlorosis,
so the varieties of the future may eliminate or minimize chlorosis as a problem.

    The herbicide connection-

    An on-going study is looking at the interaction between post-emergence soybean herbicides
and chlorosis-stressed soybeans. Unfortunately, one of the ironies of soybean production is that
the time that soybeans are stressed the most is the time for spraying postemergence herbicides.
According to last years results, there was as much as 19 bu/acre difference between the safest
herbicide and the least safe herbicide at one of the sites. In the first of a two year study, Pinnacle,
Cobra, Blazer and Storm were generally hardest on soybean yield when sprayed under chlorosis,
while Galaxy was easiest on the yield. In plots with relatively low salt levels, Raptor and Pursuit
were also relatively easy on yield, however, when salt levels were high, yields under Raptor and
Pursuit were relatively low. In sands, "low salt" was about 0.3 mmhos/cm, and "high salt" was
greater than 0.5 mmhos/cm. In loam textures and heavier, "low salt" was about 0.5 mmhos/cm
and "high salt" was greater than 0.8 mmhos/cm.

    When weed pressure is heavy, weeds must be controlled. When given a choice of herbicides
that all control similar weeds, choosing the one with normally the least visual effect on the soybeans
may be the best option. In soils with high salt levels, choosing a short-lived, not-so-systemic herbicide
also seems to be a good option.

Dr. Dave Franzen
NDSU Extension Soil Specialist

 

SUGARBEET CROP UPDATE

    Heavy rainfall in many sugarbeet areas is adversely affecting crop stands in the lower parts of
sugarbeet fields. Lower plant populations will result in lower average yield per acre. Weed control
continues with micro-rates; aerially in wet areas and by ground rigs in dry areas. There are reports
of resistant Kochia. Lanceleaf sage is a serious problem in the northern areas of the Red River Valley.
Cutworm has also been reported in some areas. In the Hillsboro area, the cutworms found were in the
second larval instar stage. Fields should be scouted regularly and control when there is about 4-5%
seedling damage. Sugarbeet root maggot adult number was high in the northern end of the valley.
Some fields with beets smaller than 10-14 leaf stage may need a post spray to prevent economic
damage.

Mohamed Khan
NDSU Sugarbeet Specialist
mkhan@ndsuext.nodak.edu


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