Carbonate Testing and Iron Chlorosis

Iron chlorosis in soybeans can be a big problem in some fields, especially when the weather turns wet and cool in early June. In the past, we didn’t know which fields would have chlorosis or how bad it could get. We learned the hard way by planting soybeans and hoping we didn’t have a problem. We now have soil tests that can help us predict which fields are more likely to have severe chlorosis, and which fields are unlikely to have iron chlorosis. Research at NDSU and the U of M has shown that soils with a high level of carbonate and soluble salts are more prone to iron chlorosis in soybeans.

In 2001, AGVISE sponsored an iron chlorosis education project with the help of many customers. In this project, customers went to soybean fields which had iron chlorosis symptoms and collected soil samples. One sample was collected from the center of the worst chlorosis, a second sample was taken where the chlorosis became mild, and a third sample was collected out into the field where there was no chlorosis present (nice green beans). These topsoil samples were tested for carbonates, salts and pH. Data was collected from sites in 98 fields in MN, ND and SD.

All of this information was sent to University researchers in MN, SD and ND. We hope this information can be used along with University research to develop a model that will help predict the risk of getting iron chlorosis based on the carbonate and salt level in the soil. Iron chlorosis is a complex plant reaction to carbonates, soluble salts and many other factors. We knew the model would not be perfect, but it could be very useful to growers. The table that was developed from chlorosis observations in the field and testing the carbonate and salt level in soil from 98 soybean fields is shown. In the 98 field sites that had severe iron chlorosis symptoms, the table would have predicted 81% of the sites to have a high, very high or extremely high risk of iron chlorosis based on the carbonate and salt level in the soil. In the 98 field sites that had no chlorosis (nice green beans), the table would have predicted 73% of the sites to have a low to moderate risk of iron chlorosis. While this is not perfect, it will provide good guidance for growers when choosing which fields to plant soybeans on. This information is also valuable when growers are choosing which variety to plant. If the carbonate and salt levels are high, the grower will want to choose a variety that has a high resistance to iron chlorosis. Fall 2001 soil test results show us the areas where high carbonates and high salts are most likely to occur. Two maps showing the percentage of fields testing high in carbonates and salts are provided to give you an idea of the percentage of fields in your area that test high in carbonates and salts.

%Soil Samples with Salts greater than 1.0 mmhos/cm

% Soil Samples with Carbonate greater than 5.0%

Risk of Iron Chlorosis in Soybeans
Based on carbonate and salt level
Soluble Salts
Risk of
Iron Chlorosis
0-2.5% <0.5 Low
0-2.5% 0.51-1.0 Moderate
0-2.5% >1.0 High
2.6-5.0% 0-.25 Low
2.6-5.0% .26-.50 Moderate
2.6-5.0% .51-1.0 High
2.6-5.0% >1.0 Very High
>5.0% 0-.25 Moderate
>5.0% .26-.50 High
>5.0% .51-1.0 Very High
>5.0% >1.0 Extreme