Soil test results provide clues about struggling soybeans in 2021 - Part 3

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.

Are these soils considered saline?

Saline soils have high concentrations of various soluble salts. These salts come from the parent materials that formed the sand, silt and clay particles in our soils and are typically measured by determining the electrical conductivity (EC) of an equal volume of soil and water, with the higher the EC value, the greater the soluble salt concentration.  This method is often referred to as the saturated paste extract or 1:1, soil:water test.

Soluble salts can move deeper into the soil profile under optimum rain, water infiltration and lower groundwater depths. However, can also move shallower in the soil profile through capillary wicking and tillage. Too much soluble salt in the soil can negatively impact plant growth and development by interfering with water and nutrient uptake. In drought years, there is little rain to move salts deeper into the soil profile and poorly developed plant roots have stronger competition with salts for water. Additionally, the hot, dry weather and windy conditions common to the Northern Great Plains increases salts wicking up in the soil profile due to evaporation. Each of these factors means that in dry years, the areas of poor growing plants due to high soluble salt content can increase in size.

The 1:1, soil:water EC values in the soil composite samples collected from the Norman County farm from the area with poorly growing soybeans were as low as 0.78 dS/m to as high as 27.6 dS/m from the 2 to 3 feet deep and 0 to 1 foot deep samples, respectively (Figure 3).  NDSU Extension soil health and nutrient management specialists recently summarized the EC values at which crops grown in the region can begin to experience yield declines due to salinity (Franzen et al. 2019). At an EC as low as 0.2 dS/m, spring wheat and soybean can begin to experience yield losses. Dry edible bean yield begins to decline at an EC of 0.5 dS/m, corn, sunflower and alfalfa yields decline at an EC of 1.0 dS/m, barley’s at 1 to 2 dS/m, oats’ at 3 to 4 dS/m, canola’s at 2.5 dS/m, and rye’s 3.8 dS/m.

AgVise Laboratories, advises their clients that there is an extreme risk of iron deficiency chlorosis and that soybean production is not recommended in the instance that a soil test reveals CCE values greater than 5.0 and EC values greater than 1. This combination of values were found for all but one soil sample collected from this Norman County field (Figures 3 and 6).

Figure 3. Electrical conductivity (EC, a measure of soluble salt concentration) 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). Note that depending on soil texture, soybean can begin to lose yield potential in soils with an EC value as low as 0.2dS/m (Franzen et al. 2019).

These results were not a surprise however, as roads can stop natural water flow, holding water in neighboring ditches that then infiltrates into soil in the field. This water dissolves the salts in its path between the ditch and the field area where it is wicked toward the soil surface. This can increase the concentration of soluble salts closer to the ditch in sandier soil, further into the field in siltier soil, and further still into the field in clayey soil, due to the capillary action inherent to the soil type (Figure 4).

Figure 4. Saline soil development near shallow streams, road ditches and sewage lagoons. Figure was originally published as Figure 2a in “Managing Saline Soils in North Dakota” (Franzen et al. 2019).

What can be done to remediate salinty?

The expenses associated with tillage, planting, adding fertilizer, combined with both poor yields and high EC values from the poor field area suggest that the struggle to grow annual crops on this field area is currently not a profitable endeavor.  A better strategy, perhaps would be to grow perennial salt-tolerant grass species. The grass species' fibrous roots would help to trap that ditch water as it begins to infiltrate into the field, reducing the likelihood of the unproductive area increasing in size. 

For this purpose, Mr. Kalwar and his collaborators in North Dakota recommend direct seeding 8 lb total per acre (1.5 lb each) of tall, slender, western and green ('AC Saltlander') wheatgrass, Russian wildrye and a winter-hearty alfalfa (ex. '6472A').

Having pattern tiling installed in a field can also sometimes help to remedy salinity issues, provided that the water lost through tile lines has a way to drain relatively quickly from the ditch and not just get infiltrated back into the field (see Figure 4). Another important precaution: please have your saline soil also tested for sodicity before taking on a tiling project. It is recommended that before pattern tiling is installed on a sodic soil, that one first amend the soil to improve water infiltration. 

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.