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Soil test results provide clues about struggling soybeans in 2021 - Part 5

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 and here to read the second, here to read the third and here for the fourth

Another cause of poor water infiltration and gas exchange

The type of clay minerals that exist in the Red River Valley have a tendency to shrink and swell more than other types of clay minerals. When soils are wet or thawed, these clay layers swell, and when dry and frozen, they tend to shrink. The swelling property is even more pronounced when there is more magnesium than calcium in a soil. That happens due to the bigger hydrated ionic size of magnesium ions versus calcium, which when sandwiched between clay sheets, let more water seep through resulting in excessive swelling.

Kalwar, an NDSU soil health specialist stationed in Langdon, ND, explains that it is both the numerical differences themselves and the magnitude of the differences in calcium and magnesium content that influences the severity of swelling. To not end up with a major swelling issue that negatively affects pore space, water infiltration and gas exchange, “Ideally, calcium content should be at least 2.5 times higher than magnesium content and 5 or more times higher than sodium,” says Kalwar.  In the field area with poor soybean growth, the 0 to 1 ft and 1 to 2 ft soil samples had 2.2 times and 2.1 times more magnesium compared to calcium, respectively. These results suggest that in that area of the field swelled clay particles likely significantly reduce the size of pore spaces, reducing the soil’s capacity for water infiltration and gas exchange (Figure 6). 

Figure 6. Soil element content of 0-1, 1-2 and 2-3 ft soil samples collected from areas of poor and good soybean growth in a Norman County, MN field (n=1). *The field areas with soil that has at least twice the magnesium content than calcium content is likely to sustain more swelling than normal.

The results of this soil elemental analysis reveal that the field area that had poorly growing soybeans may benefit from the addition of calcium by amending the soil with calcium sulfate or calcium chloride. High chloride concentrations can negatively impact soybean growth, development and yield and so in these alkaline soils calcium sulfate (gypsum) would be the better choice. In soils that have an acidic pH, calcium carbonate would be a better choice. Unfortunately, a couple of more tests should have been done on these samples, including determining the soil’s cation exchange capacity. This measurement would be needed to determine the rate of gypsum with which to amend the soil.

SCN egg densities.

It is human nature to try and find some positive aspect in any situation. Soybean cyst nematode population densities as estimated by egg counts revealed that when soil conditions are too tough for soybean to grow, conditions are too tough for SCN to build up large population densities. From the area of the field in which plants were growing well, the SCN population density would be considered high (11,350 eggs/100 cc) and from the area of the field in which plants were growing poorly, the SCN density would be considered low (900 eggs/100 cc).


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

Rengasamy, P. and Marchuk, A, 2011. Cation Ratio of Structural Stability. Soil Research Journal. 49:280-285.


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