Carbonate Testing/Salts and Iron Chlorosis

AGVISE Laboratories began offering routine Calcium Carbonate testing (Carbonates or CCE) this past summer. University Research in North Dakota has shown that high carbonates (>5.0%), along with high salts (>1.0 mmhos/cm), increases the risk of iron chlorosis in soybeans. In the cornbelt, growers know that soils with a high pH (>7.3) are more prone to iron chlorosis. Any soil with a pH >7.3 has some amount of carbonates present. The problem is that two soils may have the same pH, say 7.5, but have very different levels of carbonates. For example, one soil may have 0.5% carbonates and another may have 7% carbonates and they both have a soil pH of 7.5. In eastern North Dakota, the average soil pH is about 8.0. Most fields will not have a problem with iron chlorosis in soybeans, inspite of this high soil pH. In areas with high soil pH, it is more helpful to know the carbonate level and the soluble salt level. These two values, when used together, can help growers decide which fields to plant soybeans on and more importantly, which fields to avoid. Fields with a carbonate level <1% and a salt level <0.3 mmhos/cm would have a low risk of iron chlorosis. Fields with a carbonate level >5% and a salt level >1.0 mmhos/cm have a high risk of iron chlorosis.

In southern Minnesota, South Dakota and Iowa, where the common rotation is corn and soybeans, the carbonate test is being used in different ways. In these areas the carbonate and salt tests are being used to map the differences between areas of the same field. In some areas with severe iron chlorosis, the carbonate level is 18% or higher, while other areas have no chlorosis symptoms and a carbonate level of less than 5%.

An example of the variability of the carbonates and salts in a field that has been gird sampled is shown in the table. In the future, all soybean seed companies may rate their varieties on how they perform at different levels of carbonates and salts. This would enable growers to seed a more iron chlorosis tolerant variety through the high carbonate/high salt areas of the field and other varieties through the rest of the field. The result will be the highest yield possible from the entire field.

Carbonates & Field Variability
Field #1 (Grid Sampled)
Chippewa County, MN
Salts (0-6″)
Soil pH
5.5% 0.3 7.9
1.6% 0.4 7.8
2.0% 0.4 7.8
13.6% 1.4 7.5
5.7% 0.6 7.8
7.3% 1.5 7.6
8.6% 0.5 7.9
15.2% 0.5 7.9
11.2% 1.6 7.5
1.0% 0.4 7.8
6.8% 0.4 8.0
9.0% 0.3 7.9
0.3% 0.3 7.6
1.0% 0.3 7.9