Soil test results provide clues about struggling soybeans in 2021 - Part 2

This article was written by Angie Peltier, UMN Extension crops educator and Naeem Kalwar, NDSU Extension soil health specialist. This is the second article in a series dedicated to this topic. Click here to read the first article in this series.

Why calcium soil carbonate equivalent content is important to measure

While many in northwest Minnesota struggle with soybean production due to high soil calcium carbonate content (resulting in high or alkaline pH), folks elsewhere in the state actually need to amend their soils with it (as ag lime) to raise their soil’s pH. In soils with a pH closer to neutral (7.0) crops are unlikely to suffer from iron, manganese, boron, copper or zinc toxicity. Each of these elements is more water soluble in acidic soils and in those conditions can be more easily taken up the plant through transpiration. Calcium carbonate equivalent (CCE) is a measure of a soil’s buffering capacity, or ability to neutralize an acid.

Soybean roots exude organic acids into the soil. These acids tend to make some soil nutrients (like iron) that are less water soluble in the more calcium carbonate rich soils of western Minnesota, soluble in water so that they can be taken up by the plant. However, soils with a high CCE content have a high buffering capacity. The organic acids that soybeans exude therefore tend to get neutralized before solubilizing iron - leading to foliar symptoms of iron deficiency chlorosis (IDC).

The soil samples collected from the Norman County farm had very high CCE values; values ranged from 7.7 to 35.9% (Figure 2). AgVise Laboratories, a soil testing company with locations in Minnesota and North Dakota, advises their clients that (depending upon the electrical conductivity measurements of a soil), soybean production may not be recommended when tests reveal CCE values greater than 5.0. 

Figure 2. Percent calcium carbonate equivalent (CCE, a measure of a soil’s buffering capacity) of 0-1, 1-2 and 2-3 ft composite soil samples collected from areas of poor and good soybean growth in a Norman County, MN field (n = 1). 

 

Reference

Franzen, D., Gasch, C., Augustin, C., DeSutter, T., Kalwar, N. and Wick, A. 2019. Managing saline soils in North Dakota. NDSU Extension publication SF1087.