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Poor crop growth and development?: Identifying and remediating saline and sodic soils in NW MN

This article was written by UMN Extension crops educator Angie Peltier and NDSU Extension soil health specialist Naeem Kalwar.

It is always a treat to look at crops in the fertile Red River Valley (RRV) during the growing season; wondering when they were planted, if they received a bit more/less rain than your own crops – endless speculation…… Something else you may notice in fields farmed by others or in your own fields are areas in which crops are either slow to emerge or never emerge at all (Figure 1).

A photograph that shows a parcel of land on which nothing grows except a single salt-tolerant weed species.

Figure 1. Saline and sodic soil in which only specific weed species can grow. Photo credit: Naeem Kalwar, NDSU Extension soil health specialist.

Saline and sodic soils can both be found in the RRV and may be responsible, either individually or together, for some of this poor crop growth (Figure 2).  Our high water tables and wind can also provide challenging conditions for crop growth and will also be discussed in greater detail here. 

Map of saline, sodic and saline/sodic soils in northwest Minnesota.

Figure 2. This map highlights those areas of NW MN in which saline (red), sodic (green) or saline/sodic soils (blue) can be found in the top 3 feet (39 inches to be exact) of soil.  This map makes it appear as if the eastern RRV has nothing but saline and sodic soils, but NRCS’s Brandon DeFoe cautions that in most cases only a couple percent of each dot may be impacted.  Map credit: Brandon DeFoe, MN NRCS.

Saline soils

Salinity is caused by an excess of water soluble salts in the soil.  Table salt is one example of a salt. It is made up of the elements sodium and chloride, and is soluble {breaks into its component Na+ (sodium) and Cl- (chloride) ions} in water. 

Too many salts dissolved in soil water negatively affects plant growth, particularly that of germinating seeds and seedlings. A plant root’s cells have permeable membranes that allow the plant to take up soil water. Through the process called osmosis, water tends to flow through membranes from lower to higher salt concentrations. In soils that are not saline, water flows into plant cells because there are more salt ions inside than outside the roots. In saline soil, higher soil salt concentrations mean that plants grown in saline soil tend to have slower water uptake and can sometimes appear to be drought stressed, even when plenty of soil water is present. An NDSU greenhouse study of soybean revealed that even low salt concentrations can have a negative effect on both root mass and leaf area (Butcher et al. 2016).  This means that soil that remains unshaded by smaller plant leaves will continue to wick dissolved salts from lower down toward the soil surface; it also means there are fewer roots to draw dissolved salts down and away from the soil surface.  In legume crops, saline soils can also cause poor development of nitrogen-supplying root nodules. 

Fields with areas of saline soil are not uncommon in the RRV (Figure 2). To determine whether the unproductive areas of your field are saline, multiple soil cores should be collected down to 3 feet, separating depths from one another into 0 to 1 ft, 1 to 2 ft and 2 to 3 foot increments. Combine cores with same depth range from cores from other depth ranges. All of the depths (subsamples) should be analyzed individually for electrical conductivity (EC) using the saturated paste method of analysis.  For classification purposes, soils are considered saline when the EC is 4.0 dS/m or higher. However, saturated paste EC values much lower than 4.0 dS/m can adversely affect growth and yields of sensitive crops such as soybeans.

Sodic soils

Clay and organic matter (humus) in soils have negative charges, called cation exchange (CEC) sites. This means that positive charged ions like calcium, magnesium, potassium, ammonium, hydrogen and sodium are attracted to them. For classification purposes, sodic soils have sodium ions bound to 15% (or more) of a soil’s cation exchange sites. However, depending upon soil texture and chemistry, concentrations of sodium ions much lower than 15% attracted to cation exchange sites can still adversely affect soils. 

Soil aggregates are a combination of inorganic (sand, silt and clay particles, ions) and organic (living and dead bacteria, fungi, root tissues and their exudates) materials held together to create larger pieces. A well aggregated soil has plenty of larger and smaller soil pores that readily allow water infiltration and gas exchange in the soil profile.  Sodic soils have a drastic, negative effect on soil structure by causing soil aggregates to disintegrate. Sodic soils damage soil structure, leading to fewer and mostly smaller soil pores for water and gas infiltration. Soil that is sodic can look very different from non-sodic soil just a few feet away. Sodic soils are very dense, with no discernible soil aggregates or pores and particles sintered together.  Sodic soils have very poor water and air infiltration and often cause soil crusting and poor root growth, nodulation and nitrogen fixation in legumes (Singh et al. 1993). In addition, due to the poor soil water infiltration, salinity issues get worse as salts accumulate instead of leaching. 

Poor water and air infiltration in sodic soils is exacerbated when there is a higher magnesium than calcium content in the soil, because under these conditions pore spaces shrink as soil swells. 

There are two soil tests that estimate the sodium percentage of soils and can be used to determine whether a soil is sodic: the sodium adsorption ratio (SAR) and exchangeable sodium percentage (ESP). Soils are considered to be sodic if it has an SAR of 13 or higher or ESP of 15 or higher.

The role of water and topography on salinity and sodicity

A popular saying among people that live (and farm) in the RRV is that the land is so darn flat that you can see your dog run away for days. This flat topography, along with a combination of few remaining shelter-belts and aggressive tillage mean that few hills and shelter belts and little crop residue disturbs or slows the RRV’s high winds, resulting in drier topsoil and dissolved salts being wicked up closer to the soil surface. While living roots tend to draw water (and dissolved salts) down further into the soil profile, the combination of bare soil and high winds means that water (and dissolved salts) can get wicked up to the soil surface from lower in the soil profile. 

An additional complication is our high water tables that leave ground water close to the soil surface. In some instances, there is very little distinction between the ditches that border cropped land and the cropped land. Water and dissolved salts from ditches and field areas with a shallow water table can actually be wicked up and into adjacent areas of crop fields (think headlands) through capillary action, making the area affected larger and the salinity at the soil surface worse over time (Skarie et al. 1986). As water evaporates, it leaves salts behind. According to Franzen et al., water and dissolved salts can move up to 8 feet toward the soil surface in some of the finer-textured soils in our region, but these forces can only move water and dissolved salts up 2 feet toward the soil surface in sandier soils.  

Saline/Sodic soils

Some soils can be both saline and sodic at once, providing a whole cornucopia of management challenges.  In the order of operations, remediating the sodicity by incorporating a calcium amendment into the top foot or 6 inches of soil will make the salinity issue a bit worse in the near-term.  

Remediating sodicity followed by practices that reduce salinity near the soil surface by lowering the water table, if possible, can improve a soil’s productivity and expand the list of crops one might be able to grow on this now more productive soil.

Remediating saline soils

Depending upon how high the soil test results estimate your salt concentration to be, you may be able to plant some of the more salt tolerant crops.  Barley, sugarbeet, oats and rye are the most salt tolerant crops grown in northwest Minnesota; dry beans and soybean are among the least salt tolerant.

Improving water infiltration and drainage can increase the movement of salt dissolved in water away from the soil surface lower in the soil profile, where they will not affect germinating seeds and seedlings attempting to establish. 

Anything that one can do to facilitate drainage of water down into the soil profile or reduce the chance of dissolved salts getting wicked up into a field can reduce plant symptoms of salinity. For example, installing drainage tile (*see a cautionary note regarding sodicity and tiling*) or planting a deep-rooted, perennial crop such as alfalfa or perennial salt-tolerant grasses on field edges between the ditch and bulk of the field can intercept water containing dissolved salts before it is wicked up farther into the field, spreading over time. Some of the Farm Bill conservation programs administered through your local NRCS may be able to help offset the expense of establishing a perennial crop. Grazing, haying or mowing these perennial crops can result in positive returns on land that had before been costing more than it was producing. 

The NDSU soil health team has developed a laundry list of additional practices that one can adopt to help remediate saline soils, including: managing the depth of the water table, performing only shallow tillage, managing saline seeps, continuous cropping or maintaining crop residue to reduce soil surface water evaporation, reducing salinity related to nearby ditches and pattern tiling (Franzen, 2019).

Remediating sodic soils

In order to remediate sodic soil, the sodium ions that are bound to clay and organic matter and are responsible for causing poor soil structure will need to be displaced by calcium ions. The SAR or ESP and CEC test results and lab recommendations can help one to determine just how much gypsum (or any other amendment that can directly or indirectly add free calcium and is suitable for the soil) to add and incorporate into the soil to displace sodium. Once this has taken place, the sodium ions (Na+) will still be present and in the soil but no longer bound to clay particles – resulting in a soil that is now saline! This means that even once a person adds these expensive amendments to solve their sodicity problem, additional remediation steps will be needed to reduce the concentration of dissolved salts at the soil surface (see Remediating saline soils. above). 

Improving soil structure can be helped along by increasing the additions of organic matter in the soil by growing crops and cover crops that are salt-tolerant and spreading manure.

*Installing subsurface drainage tile is an investment that most farmers expect to immediately improve a parcel of land’s productivity. However, installing drainage tile on sodic soils before remediating sodicity will provide less than ideal results, as both the sodicity-caused poor soil structure and poor water and air infiltration mean than drainage water will not have an accessible route to drainage tile. This is one of the reasons that soil health specialists recommend soil testing for EC and SAR (saturated paste method) or ESP BEFORE deciding whether to tile a parcel.* 

Sometimes, particularly when dealing with highly sodic soils for which remediation is cost-prohibitive, options other than remediation might be pursued, including attempting to grow salt-tolerant perennial grasses or enrolling the parcel – or part of the parcel- in one of the USDA-NRCS conservation programs.  

For more information

The three-part “Making Every Acre Pay” series of webinars that were held this January, specifically addresses how to identify, remediate or do something differently to unproductive field areas due to salinity and sodicity. 

Part 1: Soil Health 101 & Remediation: features NDSU Extension soil health specialist at the Langdon Research Extension Center, Naeem Kalwar as he educates us about unproductive soils in NW MN, salinity and sodicity and how to remediate these soils.



Part 2: Dollars, Cents & Yield Maps: features both NDSU Extension soil health specialist at the Langdon Research Extension Center, Naeem Kalwar and Tanner Bruse, Minnesota ag and conservation programs manager for Pheasants Forever who discusses his organization’s programs working with farmers to objectively analyze their farm fields using yield maps.


Part 3: More Options for Under-performing Acres: features Naeem Kalwar, NDSU Extension soil health specialist at the Langdon Research Extension Center, Tanner Bruse, Minnesota ag and conservation programs manager for Pheasants Forever, and Alan Lepp, assistant Minnesota conservationist in field operations for USDA-NRCS as they talk about how some of the Farm Bill conservation programs could be used to try something different on those under-performing acres.

 

For more information about how best to plan to sample unproductive soils to get to the bottom of poor crop growth, consult an NDSU Extension article titled, “Soil Testing Unproductive Areas” (Kalwar et al. 2016).

Literature Cited. 




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