Soybean is one of the most susceptible field crops to iron chlorosis (yellowing), and this problem is not uncommon in Kansas. Iron is a catalyst in the production of chlorophyll, so a deficiency of iron (Fe) displays as a yellowish or pale color in the leaves. Iron is an immobile nutrient in the plant so symptoms first appear on the youngest leaves.
Iron chlorosis is usually caused by a combination of stresses rather than a simple deficiency of available soil Fe. Some of the soil chemical factors that play a role in Fe chlorosis include high pH, high carbonate levels, high salinity (EC), low available iron (DTPA-Fe), and high soil nitrate levels. Other factors that play a role include variety susceptibility and the presence of soybean cyst nematodes and root rotting fungi. Given all these factors, Fe chlorosis is a complex problem and not one that can be determined solely on the basis of a soil Fe test.
One of the factors that can be involved in the development of Fe chlorosis in soybeans is high levels of soil nitrate. Iron is taken up in the ferric form (Fe+3), then is immediately converted within the plant into the ferrous form (Fe+2) (existing in the chlorophyll). High concentrations of nitrate-N seem to inhibit this conversion of Fe+3 to Fe+2 in the plant, creating Fe deficiencies. It is important remember that high soil nitrate levels alone will not cause iron chlorosis in soybeans, but is simply one additional factor that will magnify the problem.
Figure 1. Wheel tracks are noticeable with greener plants in this field of soybeans with iron chlorosis. Soil nitrate levels in these wheel tracks are much lower than the rest of the field due to some soil compaction and the subsequent N loss by denitrification. Usually where soil nitrate levels are lower, plants are not as green. But in the case of iron chlorosis, it’s actually the opposite. That’s because higher nitrate levels make iron chlorosis symptoms worse. Photo by Dorivar Ruiz Diaz, K-State Research and Extension.
Fertilization strategies for iron chlorosis
In 2009-10, we conducted tests at eight locations in Kansas with seed coating treatments and foliar Fe treatments to correct Fe deficiency symptoms. We used two varieties, one with good tolerance to Fe chlorosis and one that was susceptible to Fe chlorosis, and locations were under irrigated conditions.
The seed coating treatment was approximately 0.3 lb/acre of actual Fe (chelated EDDHA Fe -6%). The foliar treatments were 0.1 lb/acre EDDHA Fe (6%) and 0.1 lb/acre HEDTA Fe (4.5%). There was an untreated check included. Soil pH at these locations varied from 7.9 to 8.4.
Figure 2. Soybean response to seed coating with chelated iron fertilizer. Photos by Dorivar Ruiz Diaz, K-State Research and Extension.
Greenness. The seed coating treatment had a significant effect in improving the greenness of the foliage, as shown by the chlorophyll meter reading results (Figure 3). Overall, the greening response to the seed coating was greater than the response to foliar Fe applications. The variety most susceptible to Fe chlorosis greened up in response to the seed coating much more than the variety more tolerant to Fe chlorosis, even though there is also increase in greenness with the tolerant variety. This indicates that the tolerant variety stayed greener during the growing season but still showed additional benefit from the seed coating treatment. The seed treatment also increased plant height by an average of about 5 inches for both varieties (data not shown).
Figure 3. Chlorophyll meter reading after foliar Fe application. Higher values are correlated with greener plant leaves. Under these conditions favorable to iron chlorosis, an iron chelate seed coating improved greenness readings.
Yield. Both the tolerant and susceptible variety also had a good yield response to the Fe chelate seed coating, and no significant yield response to the foliar Fe chelate treatments (Figure 4). Yield increase due to the seed coating treatment in the susceptible variety was approximately 10 bushel per acre, while yield increase in the tolerant variety was approximately 20 bushels per acre. Previous studies suggested that tolerant varieties tend to utilize Fe fertilizer sources more efficiently, which would explain these results in plant response observed in our study.
Figure 4. Average yield for the “tolerant” variety without seed coating treatment was 44 bu/acre, and with seed coating treatment was 63 bu/acre. Average yield for the “susceptible” variety without seed coating treatment was 47 bu/acre, and with seed coating treatment was 58 bu/acre.
Summary
- Fe deficiency potential cannot be explained well by any single soil parameter.
- Foliar Fe treatments to soybeans with Fe chlorosis seem to increase the “greenness” effectively but results suggest inconsistent yield response.
- An iron chelate seed coating provides significant yield increases to soybeans under conditions favorable to Fe chlorosis. Another alternative to seed coating is in-furrow application of chelated Fe fertilizer. Seed contact with the fertilizer source seems to be particularly important for reducing Fe chlorosis symptoms.
- If Fe chlorosis has been a common problem in the past, producers should select a soybean variety that is tolerant to Fe chlorosis. It may also pay to use a chelated Fe fertilizer in-furrow, or an iron chelate seed coating.
- Producers should avoid excessive application of nitrogen fertilizer to the crop that precedes soybeans in the rotation. In fields with some risk of iron chlorosis, the high levels of soil nitrate may be a complicating factor.
This study was supported by the Kansas Soybean Commission.
Additional information can be found in: https://store.extension.iastate.edu/product/Micronutrients-for-Soybean-Production-in-the-North-Central-Region
Dorivar Ruiz Diaz, Nutrient Management Specialist
ruizdiaz@ksu.edu
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