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, contributing to 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 is actually the opposite because higher nitrate levels make iron chlorosis symptoms worse. Photo by Dorivar Ruiz Diaz, K-State Research and Extension.
Fertilization strategies for iron chlorosis
Research in Kansas evaluated multiple factors to manage Fe deficiency, including fertilization strategies with Fe fertilizer applied with the seed and foliar, and in combination with variety selection. These studies were established in soils prone to develop Fe deficiency, with soil pH above 7.4.
Figure 2. Soybean response to seed-applied chelated iron fertilizer EDDHA Fe (6%). Photos by Dorivar Ruiz Diaz, K-State Research and Extension.
Greenness. The seed-applied Fe 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 was greater than the response to foliar Fe applications, suggesting less benefit from foliar applied Fe fertilizer. The variety most susceptible to Fe chlorosis greened up in response to the seed-applied Fe 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-applied Fe treatment.
Yield. Both the tolerant and susceptible varieties also had a good yield response to the seed-applied Fe fertilizer, and no significant yield response to the foliar Fe chelate treatments (Figure 3). Average yield increase was approximately 10 bushel per acre, while yield increase in the tolerant variety was approximately 15-20 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 3. Average chlorophyll meter reading and yield. Under these conditions favorable to iron chlorosis, an iron chelate fertilizer improved greenness readings and yield.
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