Soil salinity is a critical soil health issue in northeast North Dakota as well as in the rest of the state. Saline soils will have excessive levels of soluble salts in the soil water, which are a combination of positively and negatively charged ions (for example, table salt; Na+Cl-). High levels of ions (positive and negative) from soluble salts restrict normal water uptake by plant roots, even when the soils are visibly wet, resulting in drought-stressed plants (osmotic effect).
The recent wet cycle that started in 1992 led to shallow groundwater depths resulting in moderate to severe salinity issues throughout North Dakota. In order to push the excess salts out of the topsoil, a lower groundwater depth is important along with a decent amount of rain. However, lower groundwater depths under drier weather may not result in lower salinity levels as there will not be enough rain to push the salts into deeper depths. In addition, increased evaporation combined with transpiration versus lower precipitation will result in increased wicking up (capillary rise) of groundwater toward the topsoil resulting in higher salt levels. Tiling, however, has been perceived by some stakeholders as the one-stop solution. Tile the saline areas and salts will disappear in few years.
It is estimated that at least 15 percent of the crop land in our state, which is about 39 million acres, is affected by excessive salt levels. That is about 6 million acres.
By spending $82 to $187 per acre as an average direct cost of planting soybeans, spring wheat, canola and corn, every year, producers lose millions of dollars on saline-sodic areas that either produce poor stands or do not produce anything.
The basic test needed to determine the levels of soluble salts (soil salinity) is Electrical Conductivity (EC). Soil EC is a measure of the concentration of ions from water-soluble salts in soils, and the test results are indicative of soil salinity. EC is the ability of a material to conduct an electrical current, and it commonly is expressed as dS/m or millimhos/centimeter (mmhos/cm). One dS/m = 1 mmhos/cm. Soil EC is measured by passing an electrical current through the soil solution and it is inversely proportional to the electrical resistance in the soil solution. Water-soluble salts in the solution enhance the transfer of electric current (electric conductance). EC results will also vary depending upon the method type. Generally, EC values determined through the saturated paste extract method are double or more than the values obtained through the 1-to-1 method.
For classification purposes, a soil is considered as saline if the saturated paste extract electrical conductivity (EC) equals or exceeds 4 deciSiemens per meter (dS/m). However, in terms of crop production, even an EC of less than 4 dS/m can result in considerable yield loss, for example in the case of soybeans.
Remediation of soil salinity requires improving soil water infiltration, lowering the groundwater depths and reducing evaporation by establishing a suitable vegetative cover under decent rain to push the excessive salts into deeper depths.
An effective way to lower groundwater depths is to install a field tile drainage system. Since tiles are generally 3-4-feet below the surface, the efficiency of a tile drainage system depends upon the permeability of soil layers above the tiles. This requires analyzing soils for salts and Na+ causing sodicity. In contrast to saline soils, sodic soils are highly saturated with Na+ ions at the soil cation exchange sites (negative charges of clay and humus particles that attract positively charged chemical ions). High Na+ levels compared to Ca2+ in combination with low salt levels can promote “soil dispersion,” which is the opposite of flocculation. Soil dispersion causes the breakdown of soil aggregates, resulting in poor soil structure (low “tilth” qualities). Due to the poor soil structure, sodic soils have dense soil layers, resulting in very slow permeability of water through the soil profile. In cases of high Na+ levels causing sodicity, not adding Ca2+ can render tiling ineffective.
Depending upon soil texture, the average cost of tiling ranges from $800 to $1400 per acre. Landowners who are planning to tile should protect their investments worth thousands of dollars by checking the potential fields for salinity, sodicity and consider the 5-10 year weather forecast before making the final decision.
Challenges for landowners considering tiling could be:
- What if soil sodicity levels are high in the fields they would like to tile?
- In cases of high sodicity levels, what should they do first, tile or apply the amendments?
In the next issue I will discuss the unique research project that was started at the Langdon Research Extension Center, which was financed by private sources. Results from that project will also be outlined that indicate there are other methods to reduce salinity and sodicity levels in the soil, as the cover photo of this issue indicates.