Research ArticleResearch Section

Combining gypsum with establishment of perennial vegetation improved the restoration of a salt-affected soil in the Northern Great Plains

S. Westhoff, C. Reese, S.A. Clay, D. Bhattarai, D.R. Joshi, G. Reicks, J. Moriles-Miller, K. Osterloh and D. Clay
Journal of Soil and Water Conservation November 2024, 79 (6) 271-279; DOI: https://doi.org/10.2489/jswc.2024.00082

Abstract

Classical saline-sodic soil management approaches are to apply gypsum or elemental sulfur (S), improve soil drainage, and irrigate with enough water to leach soluble salts out of the soil. However, these protocols do not incorporate the soil health principles of limiting soil disturbance, keeping the soil covered, providing a living root, and increasing species diversity. The objective of this study was to determine if combining chemical amendments (gypsum and elemental S) with the soil health principles of limiting soil disturbance, providing soil cover, and establishing an active root improved soil health in a lowland, salt-affected soil. The soil health metrics of soil electrical conductivity (EC1:1), sodium concentration (mg Na+ kg−1 soil), percentage sodium (%Na+), Na:EC ratio, wet aggregate stability, and water infiltration were compared in an upland and lowland soil with vegetation treatment alone or vegetation plus chemical amendments. The vegetation experiment was established at two landscape positions as a randomized complete block design (RCBD) that was then broken into a split-plot design to include chemical amendments. The vegetation main plots were corn (Zea mays L.), two perennial grass mixtures (Mix 1 and Mix 2), and a no-vegetation control. The split-plots were chemical amendments (gypsum, elemental S, and no-amendment). Soil samples to a 15 cm depth were analyzed for EC1:1 and Na+ concentration in 2018, 2019, 2020, and 2022. Wet aggregate stability and water infiltration was measured in 2020 and 2022. In the saline-sodic lowland soil from 2020 to 2022, the application of gypsum plus vegetation decreased EC1:1 (1.71 ± 1.43 dS m−1) and Na+ (1,445 ± 578 ppm) concentration and increased (p = 0.05) water infiltration from near impermeable to 362 mm h−1. Elemental S plus vegetation did not produce a change in the measured soil health metrics from 2020 to 2022. These results were attributed to the combined effect of gypsum plus vegetation providing soil cover and physical channels for water and Na+ transport, whereas elemental S plus vegetation was unsuccessful due to limited microbial activity for S oxidation. These findings suggest that an important step in restoring soil health to saline-sodic areas is the establishment of perennial plants. The application of gypsum has the potential to add other benefits, whereas the addition of elemental S had no measurable impact.

Key words:

Globally, expansion of soil salinity and sodicity removes 1.5 million ha of land from agricultural production and reduces productivity by an additional 46 million ha each year (FAO 2020). However, solutions to this problem are region-specific and one management recommendation will not fit all soils. In South Dakota, North Dakota, and other regions of the North American Northern Great Plains (NGP), development of salt-affected soils has been attributed to unsorted glacial parent materials overlying marine sediments, increased annual rainfall, and the replacement of native tall and mixed grass prairie vegetation with annual row crops (Darton 1909; Hoogestraat and Stamm 2015). In the NGP, it is not uncommon for surface salinity/sodicity to be first noticed in low-elevation, closed drainage systems (Malo et al. 1974; Franzen 2003; Butcher et al. 2016; He et al. 2018). These salts contribute to poor seed germination, reduced plant growth, and increased soil dispersion, which can cause catastrophic erosion events (Carlson et al. 2019; Franzen et al. 2019) (figure 1a).

Figure 1
Figure 1

(a) An image highlighting gully formation and soil loss from a saline-sodic field in the study area (courtesy of Dr. Cheryl Reese, May 15, 2018). (b) Surface salt accumulation along the boundary between perennial vegetation and tilled field in the study area (courtesy of Dr. Dwarika Bhattarai, April 22, 2021). Soils in the upland area were a Forman-Cresbard loam, 3% to 6% slope, and soils in the lowland area were a Cresbard-Cavour loam, 0% to 3% slope.

Common remediation approaches for annual row-crop production in saline-sodic zones are to improve soil drainage, apply chemical amendments, and remove salt from the root zone with excess water (Alcívar et al. 2018; Zhao et al. 2018; Franzen et al. 2019; Sundha et al. 2020). However, these techniques have had mixed results on saline-sodic soils in the NGP. For example, Kharel et al. (2018) and Birru et al. (2019) showed that the high levels of inherent lime and gypsum in NGP salt-affected surface soils limit the ability of supplemental gypsum and elemental sulfur (S) to increase yields and reduce electrical conductivity (EC) and percentage sodium (%Na+). An alternative approach is to establish perennial plants (phytoremediation) that cover the soil and increase the transport of surface salts to the subsurface (USDA NRCS 2013; Birru et al. 2019; Fiedler et al. 2021, 2022; Budak et al. 2022; Clay et al. 2024). The impacts of using a combination of perennial plant establishment and chemical amendments in the NGP are less understood. Therefore, our objective was to measure the impact of chemical amendments in conjunction with phytoremediation on changes to soil electrical conductivity (EC1:1), ammonium acetate extractable sodium (mg Na+ kg−1 soil), %Na+, the Na:EC ratio, aggregate stability, and water infiltration.

Materials and Methods

Site Description. The study location was near Carpenter, South Dakota (44°38′8.75″ N, 97°55′1.14″ W), and prior to initiating the experiment in 2018 the crop rotation was corn (Zea mays L.) followed by soybean (Glycine max [L.] Merr.). Baseline composite soil samples were taken in 2018 and initial soil properties are reported in table 1. At this study site, related research provided information on the soil microbial community structure, that reseeding with perennial grasses is a slow process, that soil respiration in this saline-sodic soil is very slow, and that nitrous oxide N2O emissions are very high (Jakubowski 2021; Fiedler et al. 2021, 2022; Clay et al. 2022; Neupane et al. 2024). In addition, related research in the study area showed that chemical amendments by themselves have a minimal impact on soil health improvements (Birru et al. 2019). However, these studies did not evaluate if chemical amendments combined with phytoremediation can accelerate soil health improvements.

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Table 1

Baseline 2018 soil chemical and physical characteristics for the nonsaline, nonsodic upland soil and the saline-sodic lowland soil at the study site (Fiedler et al. 2022).

The study area included a nonsaline, non-sodic upland soil and a saline-sodic lowland soil, hereafter referred to as upland soil and lowland soil, respectively. The upland soil was a Forman-Cresbard loam with 3% to 6% slope (fine-loamy, mixed, superactive, frigid Calcic Argiudoll; fine, smectitic, frigid Glossic Natrudoll) and the lowland soil was a Cresbard-Cavour loam with 0% to 3% slope (fine, smectitic, frigid Glossic Natrudoll; fine, smectitic, frigid Calcic Natrudoll) (Soil Survey Staff 2018). High erosion rates were observed in the lowland soil due to soil dispersion, little-to-no crop growth, limited soil cover, and near impermeable water infiltration rates (figure 1a and 1b).

The upland and lowland EC1:1 values for the 0 to 15 cm depth were 0.5 and 6.7 dS m−1, respectively, and the sodium adsorption ratio (SAR) values of the upland and lowland soils were 1.79 and 22.0, respectively (Fiedler et al. 2022). The soil organic matter (OM) was determined using the loss on ignition technique (Combs and Nathan 2011) and was 2.37 and 2.16 mg kg−1 in the upland and lowland soils, respectively. The bulk densities were 1.16 and 1.37 g cm−3 in the upland and lowland soils, respectively. The %Na+ surface soil values were 1.9% and 30.1% in the upland and lowland soil, respectively. The %Na+ values were determined with equation 1:

Embedded Image 1

where calcium (Ca2+), magnesium (Mg2+), potassium (K+), and Na+ were extracted with 1M ammonium acetate. The SAR value was based on concentrations of Ca2+, Mg2+, and Na+ in a saturated paste extract.

Experimental Treatments. In the fall of 2017, a split-plot randomized block design (RCBD) was established at two landscape positions. The main plots were no-vegetation control (nothing was planted); corn seeded in the spring of 2018, 2019, and 2020 at a rate of 79,000 seeds ha−1; perennial grass Mix 1 containing Certified First Strike slender wheatgrass (Elymus trachycaulus [Link] Gould ex Shinners) and Shoshone beardless wildrye (Leymus triticoides [Buckley] Pilg.); and perennial grass Mix 2 containing Certified First Strike slender wheatgrass, Garrison creeping meadow foxtail (Alopecurus arundinaceus Poir.), western wheatgrass (Agropyron smithii Rydb.), and AC Saltlander green wheatgrass (Elymus Hoffmannii K.B. Jensen & K.H. Asay). The perennial grass mixtures were dormant seeded at a 6 mm depth using a FLEX-II drill (Traux Company, Inc., New Hope, Minnesota) into 13 m strips on December 15, 2017, and overseeded with the same mixture on October 24, 2018, due to limited establishment in 2017. Seeding rates are reported in Fiedler (2020). Tillage was not used at the site and the perennial grass Mix 1 and Mix 2 were designed to follow the soil health principles of minimizing soil disturbance, keeping the soil covered, providing a living root, and increasing species diversity. Corn was planted on May 17, 2018, May 19, 2019 (Fiedler 2020), and May 29, 2020, at 79,000 seed ha−1. Corn was not planted in 2021 or 2022. In the lowland soil, perennial plants were allowed to establish after 2019 in the no-vegetation control and the corn plots (Clay et al. 2022).

In May of 2018, the main plots (vegetation) were split into a split-plot design. One of the three amendments (gypsum, elemental S, or no-amendment control) was randomly assigned to a split within a vegetation treatment (Freund et al. 2010). Gypsum was applied at a rate of 8.30 Mg ha−1, and elemental S was applied at a rate of 1.57 Mg ha−1 to the appropriate split-plot. Gypsum and elemental S rates were calculated following Clay et al. (2012). Amendments were lightly raked into the surface 2.5 cm of soil at the time of application.

Sample Collection. On June 29, 2018, June 13, 2019, June 15, 2020, and June 6, 2022, composite soil cores from the 0 to 15 cm depth were collected using a 2 cm diameter probe. Each composite sample contained 10 cores. Samples were analyzed for EC1:1 and ammonium acetate extractable Na+ as reported in Fiedler (2020). Soil samples from the 0 to 5 and 5 to 15 cm depths were collected for water stable aggregates on June 23, 2020. Soil samples from 0 to 15 cm depth were collected for water stable aggregates on June 6, 2022. Dry aggregates between 1 to 2 mm were analyzed for water stability via wet sieving using a protocol adapted from Yoder (1936). Water infiltration was measured in select plots on July 29, 2020, and August 3, 2023, using a single ring infiltrometer (USDA 1999).

Perennial plant biomass was measured on September 10, 2018, July 11, 2019, June 15, 2020, June 21, 2021, and June 15, 2022. Biomass was dried at 60°C. Further details on biomass sampling and processing are provided in Fiedler (2020). In the upland soil, corn grain and stover yield was measured following maturity (black layer) and corrected to 15.5% moisture in 2018. In 2019 and 2020, only aboveground biomass was measured. In the lowland soil, aboveground biomass was measured in 2018, 2019, and 2020. Corn results reported in table 2 represent aboveground biomass.

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Table 2

Annual biomass production of the vegetation treatments. Data for 2018 and 2019 from Clay et al. (2022). In the no-vegetation treatments, plant growth from adjacent plots was not prevented after 2019. Corn biomass in 2018 is grain plus stover, and stover-only in 2019 and 2020. Lowercase letters indicate significant differences.

Statistical Analysis. The statistical analysis was conducted in R Studio (R Core Team 2021) using analysis of variance (ANOVA) for a split-plot randomized block design from the library agricolae (de Mendiburu and Yaseen 2020). The upland and lowland soils were analyzed separately. Within-year treatment differences were determined using the Fisher’s Least Significant Difference (LSD) value at p ≤ 0.05 for the years 2019, 2020, and 2022.

To determine the temporal changes and associated 95% confidence intervals (CI) from 2020 to 2022, the 2022 measurements for EC1:1, Na+, and the Na:EC ratio for each plot were subtracted from the 2020 values (Δ = 2020 – 2022). A positive value from this assessment indicates that the EC1:1, Na+, or Na:EC decreased, and a negative value indicates an increase in that metric from 2020 to 2022. The change from 2020 to 2022 is significant when the 95% CI is less than the difference.

Results and Discussion

Climatic Conditions. The 30-year precipitation and temperature averages for this site were 604 mm and 6°C, respectively, and temperature and rainfall data are shown in figure 2. The number of frost-free days typically range from 120 to 150 days (Soil Survey Staff 2018). In 2019, South Dakota experienced an abnormally wet, cool year where rainfall was approximately 30% higher and temperatures were 1.4°C cooler than the 30-year average. High rainfall in 2019 provided an opportunity for excess water to percolate through the soil. However, Fiedler et al. (2022) reported that because hydraulic conductivity in the lowland soils was minimal, it is likely that percolating water had a minimal impact on reducing soil EC in 2019 and instead raised the salt-laden water table. Environmental conditions in 2018 and 2020 were overall warmer and drier than the 30-year average, while 2021 and 2022 were similar to the 30-year average.

Figure 2
Figure 2

Monthly precipitation (mm) and temperature (°C) for the study period. Data retrieved from the National Oceanic and Atmospheric Association NOWData portal using data from Clark, South Dakota, weather station ID CLARK NUMBER 2, SD US (44°52′54.84″ N, −97°44′3.12″ W) (National Weather Service 2023).

Vegetative Biomass. Biomass samples were collected from no-amendment control plots. In the upland soil, corn produced 10,833 kg ha−1 of aboveground biomass (grain + stover) in 2018 (table 2). However, in 2019, which was cool and abnormally wet, corn did not produce grain in the upland soil and only produced 2,003 kg of aboveground biomass ha−1. Comparatively, in 2019, perennial Mix 1 and Mix 2 produced 9,038 kg ha−1 and 10,663 kg ha−1 biomass in the upland soil, respectively. In 2020, corn produced 3,985 kg ha−1 of aboveground biomass. Corn was not planted in the upland soil in 2021 or in 2022. Plant species from the surrounding plots colonized the no-vegetation upland soil plots in 2019 and produced 4,043, 2,002, and 2,203 kg ha−1 in 2020, 2021, and 2022, respectively (table 2). The species that moved into the no-vegetation plots were a mixture of perennial grasses from adjacent plots and weed species, such as foxtail barley (Hordeum jubatum L.), from surrounding weedy areas (Clay et al. 2022).

In the lowland soil, the salt-tolerant perennial Mix 1 and Mix 2 produced more biomass than corn, except in the establishment year of 2018 (table 2). Total aboveground biomass in 2018 and 2019 were 2,245 kg ha−1 and 1,489 kg ha−1, respectively (Clay et al. 2022). Corn experienced a crop failure in the lowland soil in 2020, and corn was not planted in 2021 or 2022. In the lowland soil, the no-vegetation and perennial grass treatments produced similar amounts of biomass in 2020, 2021, and 2022, respectively.

Electrical Conductivity. In the upland soil, the application of gypsum increased the EC1:1 in the 0 to 15 cm depth from 0.5 dS m−1 in 2018 to 0.89 dS m−1 in 2019, which was higher than the 2019 no-amendment control or S treatment (p < 0.001) (table 3 and figure 3a). These findings suggest that a blanket gypsum application could increase the EC1:1 to levels that may reduce yields in sensitive crops (Carlson et al. 2019). In the 2020 upland soil treatment, gypsum (0.42 dS m−1) and S (0.39 dS m−1) increased (p < 0.05) the EC1:1 relative to the no-amendment treatments (0.26 dS m−1), and in 2022 the amendments did not influence EC1:1.

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Table 3

Electrical conductivity (EC1:1), sodium (Na+) concentration, and Na:EC ratio in surface soil (0 to 15 cm) collected from upland and lowland soils in 2018 (baseline), 2019, 2020, and 2022 as affected by the combined impact of plants and chemical amendments.

Figure 3
Figure 3

Annual soil test results by landscape position. Lowercase letters indicate significance between amendment treatments within years at α = 0.05. The dotted horizontal line in each graph represents the 2018 baseline value for that soil property. (a and b) electrical conductivity (EC), (c and d) sodium concentration (Na+), and (e and f) Na+ to EC1:1 ratio.

In the lowland soil, the application of gypsum or S did not impact EC1:1 in 2019 or 2020 (table 3 and figure 3b). In 2022, when compared with the no-amendment treatment, gypsum reduced soil EC1:1 from 6.49 to 4.54 dS m−1, and S did not reduce EC1:1. These findings are consistent with Sharma et al. (1974), who reported that gypsum reduced EC, but also reported that remediation may require 13 to 15 years to produce benefit.

Sodium Concentration. In the upland soil, the Na+ concentration was not affected by the amendment treatments (table 3 and figure 3c). Different results were observed in the lowland soil in 2022, where Na+ was lower in the gypsum (1,741 mg kg−1) than in the S treatment (2,867 mg kg−1) (p = 0.03). Interestingly, the no-amendment treatment had Na+ concentrations (2,466 mg Na+ kg−1) that were between the gypsum and S treatments (table 3 and figure 3d).

Sodium to Electrical Conductivity Ratio. Because Na+ and EC are soil chemical properties that have opposite effects on soil dispersion, a higher Na+ to EC1:1 ratio indicates an increased clay dispersion risk (Essington 2000; He et al. 2013; Budak et al. 2022). Budak et al. (2022) reported that high rainfall when combined with a subsoil Na:EC ratio >600 can slow the movement of water into the tile line. However, because different clays disperse at different Na:EC ratios, guidelines are site-specific (He et al. 2013).

The need to maintain a relatively high EC1:1 value to prevent soil dispersion must be balanced with the effect of EC1:1 on plant growth. Crop yields and EC1:1 are negatively correlated to each other, and, depending on the crop species, yield losses can occur at EC1:1 values as low as 0.47 dS m−1 (Carlson et al. 2019). Prior studies have highlighted the serious risk of crop failure if the balance between soil and plant health is not appropriately established (Birru et al. 2019). These studies suggest that preventing dispersion is a balancing act between elevating EC to a level that prevents dispersion but not so high as to adversely impact crop production.

In the upland soil, gypsum and S had lower Na:EC ratios than the no-amendment control treatment in 2019 and 2020 (table 3 and figure 3e).

In 2019, gypsum and S reduced the upland Na:EC ratio from 259 in the no-amendment treatment to 149 and 165, respectively (p = 0.01). In 2020, gypsum and S reduced the upland Na:EC ratio from 351 in the no-amendment treatment to 166 and 153, respectively (p < 0.001). In 2022, chemical amendments did not influence upland EC1:1 or Na+, but S did reduce the Na:EC ratio from 244 in the no-amendment treatment to 139 (p = 0.01) (table 3). In the lowland soil, the amendments did not reduce the Na:EC ratio in 2019, 2020, and 2022 (table 3 and figure 3f).

Temporal Changes in Electrical Conductivity, Sodium Concentration, and Sodium to Electrical Conductivity Ratio from 2020 to 2022. Differences in EC1:1, Na+, and the Na:EC ratio from 2020 to 2022 were determined to assess the temporal effect of chemical amendments on the soil properties (table 4). Positive values indicate that EC1:1 decreased from 2020 to 2022, and negative values indicate an increase over this period. In the upland soil, from 2020 to 2022 gypsum reduced EC1:1 by 0.22 ± 0.11 dS m−1. Similar temporal results were observed in the lowland soil, where gypsum reduced EC1:1 1.71 ± 1.43 dS m−1. These decreases in EC1:1 were attributed to increased water movement and salt transport out of the surface soil.

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Table 4

The combined impact of vegetation and chemical amendments on changes in the electrical conductivity (EC1:1), ammonium acetate extractable sodium (Na+), and the Na:EC ratio from 2020 to 2022. A positive ∆ value indicates that the soil property decreased from 2020 to 2022. The change from 2020 to 2022 was significant if the 95% confidence interval is less than the change from 2020 to 2022.

Microbial oxidation is required to oxidize elemental S and produce sulfuric acid (Al-Mayahi et al. 2023), which would lower soil pH and solubilize soil Ca2+ to replace Na+ on clay binding sites. However, any potential pH decrease from the elemental S would be buffered by calcium carbonate (CaCO3) contained in the soil and low microbial activity resulting from high EC and soil water contents (Zhao et al. 2022). The lack of differences in EC1:1 in the elemental S treatment was attributed to low microbial activity (Fielder et al. 2021) and a high soil pH buffering capacity.

From 2020 to 2022 in the upland soil, the chemical amendments had minimal impact on the ammonium acetate extractable Na+ concentrations. While all treatments exhibited a reduction in Na+, only the no-amendment treatment (42.3 ± 34.10 mg Na+ kg−1) was significant. In the lowland soil, gypsum reduced the EC1:1 and Na+ concentrations. These decreases were attributed to the impact of following soil health practices (providing cover and a living root) that increased water infiltration from near impermeable to 362 mm h−1 (p = 0.05). Increased water infiltration combined with Ca2+ replacing Na+ on clay exchange sites allowed Na+ to leach with the percolating rainwater. Associated with the Na+ decrease was a decrease in the Na:EC ratio. These findings suggest that gypsum plus vegetation was superior to elemental S plus vegetation at reducing EC1:1, Na+, and the Na:EC ratio from 2020 to 2022 in this lowland, salt-affected soil.

The importance of plants in the restoration of saline-sodic soils was previously reported by Halvorson (1984) in a study conducted in Montana. In this semiarid-to-arid region, vegetation and soil cover were used to manage the capillary movement of water and associated salts from the subsurface to the soil surface. Casas et al. (2020) showed the importance of active roots and soil cover in Argentina, as EC was decreased 37% when panicum (Panicum coloratum L.) was planted into a Typic Natraqualf.

Soil Aggregate Stability. In 2020, wet aggregate stability was lower in the 0 to 5 cm depth than the 5 to 15 cm depth (figure 4). Soils from the lowland soil had lower aggregate stability than upland soils. The average wet aggregate stability in the upland position was 94.61% (n = 48) and 95.61% (n = 48) in the 0 to 5 cm and 5 to 15 cm depths, respectively. The average wet aggregate stability in the lowland position was 77.02% (n = 14) and 85.20% (n = 19) in the 0 to 5 cm and 5 to 15 cm depth, respectively. Reduced wet aggregate stability in the lowland soils is consistent with the soils’ high Na+ concentration (Malo et al. 1974; He et al. 2018).

Figure 4
Figure 4

Water stable aggregate stability data for 2020. Upland soil series: Forman-Cresbard loam, 3% to 6% slope. Lowland soil series: Cresbard-Cavour loam, 0% to 3% slope.

In 2022, wet aggregate stability was higher and %Na+ was lower in the upland than the lowland soils (table 5). However, the chemical amendments did not increase wet aggregate stability in either soil. In the lowland soils, these results were expected because the Na:EC ratio and the %Na+ were similar between chemical amendments (tables 3 and 4).

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Table 5

The 2022 wet aggregate stability and percentage sodium (%Na+) in the upland and lowland landscape positions in surface soil (0 to 15 cm) as impacted by the chemical amendments (α = 0.05.)

Correlation between Elemental Concentrations and Aggregate Stability. Many of the soil properties measured in this study were positively or negatively correlated to each other (figure 5). Soil wet aggregate stability was negatively correlated to SO42–, Na+, EC1:1, %Na+, pH, Ca2+, and Na:EC, whereas wet aggregate stability was positively correlated to K+ concentration and organic matter. Many of the negative correlations to aggregate stability were the result of well-established relationships between soil chemical properties. For instance, high amounts of Na+ decrease wet aggregate stability and increase soil pH. The strong correlation among multiple factors makes it difficult to assess the impact of a single factor. Interestingly, Ca2+ was negatively correlated with wet aggregate stability, and K+ was positively correlated with wet aggregate stability. However, since Ca2+ is also positively correlated to Mg2+, Na+, and SO42–, it is impossible to decipher the true effect of Ca2+ on aggregate stability.

Figure 5
Figure 5

Correlation matrix displaying r values of measured variables. The data used in this analysis were from soil samples collected from the 0 to 15 cm depth in 2022. Aggregate stability = Agg. Stability; EC =electrical conductivity; Ca.ppm = calcium concentration; Mg.ppm = magnesium concentration; Na.ppm = sodium concentration; S.ppm = sulfate concentration; K.ppm = potassium concentration; Na.EC = Na:EC ratio; and OM = organic matter. All correlations in this table are significant at p < 0.001. Upland: Forman-Cresbard loam, 3% to 6% slope. Lowland: Cresbard-Cavour loam, 0% to 3% slope.

Effects of the Vegetation Treatments (Phytoremediation). By covering the soil and providing an active root system, plants produce channels for water flow, which can reduce the surface soil EC and Na+ concentration. Evidence for this is a decrease in the concentration of salts (white precipitates) on the soil surface (figure 1b). Associated with these decreases are often a reduced soil dispersion risk (lower Na:EC ratio) and improved aggregate stability. However, despite these improvements, increases in wet aggregate stability were not observed.

In a related study, Birru et. al. (2019) conducted a randomized complete block experiment on soils located in the backslope, footslope, and toeslopes. In this experiment, four amendments (no-treatment, CaCl2, CaSO4, and S) were applied and the soils were seeded to annual crops. The little improvement in chemical properties was attributed to the annual crops not providing sufficient soil cover. However, this present study highlights that decreases in EC, Na+, and the Na:EC ratio (table 4) can occur in the lowland soil by combining gypsum with perennial, salt-tolerant plants.

Summary and Conclusions

Increases in EC1:1 and Na+ in 2019 are attributed to excessive rainfall (figure 2) reducing the depth to the salt-laden water table and allowing for capillary rise to bring dissolved salts to the soil surface. From 2020 to 2022, rainfall was similar or slightly lower than the 30-year average. Slightly lower rainfall between 2020 and 2022 alongside establishment of deep-rooted perennial vegetation likely lowered the water table and the movement of dissolved ions out of the root zone. This conclusion is based on decreases in lowland soil EC1:1 values from 2020 to 2022. This work also showed that restoration is a multiyear process.

Based on the results of this study, producers could implement the use of gypsum alongside the establishment of salt-tolerant perennials to reduce soil EC1:1 and Na+ concentrations. The success of this approach is dependent on weather conditions and requires ample time to facilitate the necessary chemical reactions and plant establishment. Although salinity and sodicity were reduced at this study site, final EC1:1 and Na+ concentrations were still too high to support annual crops. This is an important consideration for producers as they create long-term management plans. Producers should consider perennial vegetation as a key component of sustainable management.

Future research should include additional applications of soil amendments, additional sampling, and potentially working with new plant species for phytoremediation. Salinity and sodicity consume more and more arable acres every year in South Dakota and in other regions of the NGP. Degraded and fragile soils pose serious financial risk for producers and serious environmental risk for surrounding ecosystems. Working toward a large-scale solution to reclaim, or at the very least stall, the expansion of these acres will be critical for the fate of food security in South Dakota and beyond.

Acknowledgements

Funding for this project was provided by the South Dakota Corn Utilization Council, USDA Agriculture and Food Research Initiative (SD00G656-16 and SD00H555-15), and USDA Natural Resources Conservation Service–Conservation Innovation Grants (69-3A75-285).

  • Received September 9, 2023.
  • Revision received July 17, 2024.
  • Accepted August 29, 2024.

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Combining gypsum with establishment of perennial vegetation improved the restoration of a salt-affected soil in the Northern Great Plains
S. Westhoff, C. Reese, S.A. Clay, D. Bhattarai, D.R. Joshi, G. Reicks, J. Moriles-Miller, K. Osterloh, D. Clay
Journal of Soil and Water Conservation Nov 2024, 79 (6) 271-279; DOI: 10.2489/jswc.2024.00082
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