Soil Salt Management and Impact of Tile Drainage (Site 1)

Introduction:
Soil salt issues are classified as either saline or sodic. Predominate salts in eastern SD soils are calcium (Ca+2), magnesium (Mg+2), and sodium (Na+1). Soils with high total salts are defined ‘saline’ and soils with too much sodium (Na) are ‘sodic’.

Soils with high salts have poor seed germination. Abundant presence of sodium salts provides an additional management issue. Soils with high sodium concentration have poor seed germination and are prone to dispersion and drain issues. In a dispersed soil, water infiltration is slowed.

If the soil texture contains high amounts of clay in addition to the sodium, water infiltration may be further inhibited. Sodic or sodium affected soils erode easily due to poor soil structure and water infiltration.

Soil Testing: Soil samples were analyzed for salinity by measuring soil electrical conductivity (EC, unit are mS/cm). For eastern SD soils, if the soil EC is > 4.0, the soil contains a high salt concentration.

Two indexes are used to measure Sodicity, sodium absorption ratio (SAR) or exchangeable sodium percent (ESP). Research at both South Dakota and North Dakota suggest that SAR or ESP values > 5.0 are soils with high sodium contents that are prone to dispersion.

Methods: Soil samples were collected in June 2016 from 2 points in the field where salt issues were apparent on surface soils. Soil samples were collected at points shown in Figure 1. Samples were collected from tile and non‐tiles areas.

Twenty cores were collected in a 20 foot radius around the point. Samples were
collected over 4 depths, 0‐3, 3‐6, 6‐12, and 12‐24 inches. The soil samples was analyzed for (1) EC, (2) parts per million (PPM) sodium, and (3) sodium absorption ratio (SAR).

Soils were prepared for lab analysis at South Dakota State University. Soil analysis was completed at North Dakota State University. Cation concentrations analyzed at North Dakota State using inductively‐coupled plasma spectrometry (Optima 5300V, PerkinElmer, Waltham, MA) or an atomic absorption spectrometer (Model 200A, Buck Scientific, East Norwalk, CT).

Soil test results for Tile North and No Tile South are presented in Table 1.

Results: Soil test analysis of these two field points suggest that the soil at the North Tile Point is classified as a ‘saline‐sodic’ and the soil at the South No‐Tile point is saline, borderline sodic.

Both sample sites have high salt concentrations over the 0‐24 inch sampling depth as indicated by EC > 4.0 μS/cm. The average soil EC over all depths and sites at the North Tile point is higher (13.1) than at the South No‐Tile point (7.7).

The soil sodium content is very high for all sampling depths at the North Tile point; the average SAR values across all four sites and depths is 28.5. SAR > 5.0 suggests that problems associated with too much sodium is apparent and documented in Figure 1B.

The South No Tile point has high salt content. The salt content is borderline high on sodium. SAR values > 5.0 suggest that problems associated with sodium such as dispersed surfaces become apparent. The other concern with the location of this point the salts associated with the soil sample location.

Soil Analytics: In the ‘Supplemental Soil Information’ section at the end of this report Ranslo –Harriet loam soil series is mapped at North Tile Point site shown in Figure 1B. Both the Ranslo and Harriet soils has elevated sodium concentrations near the surface; 9 inches for Ranslo and 2 inches for Harriet (Figure 4A and B). The elevated sodium in the test is most likely associated with sodic horizon as presented by ‘Btn’ horizon (Figure 4A and B).

In Figure 1A, the South No Tile point appears to be located in an area of the field where salt affected soils are not mapped. Soils to the east of the Dry Run Creek are mapped as ‘Great Bend’ and ‘Zell’; neither soils have ‘Bn’ horizons = high sodium (Figure 3B). Concern arises over managing water in these non‐salt affected soils. Is salt laden ground water moving into adjacent non‐salt soils and slowly expanding the problem over time?

Objectives: The objective of this study is to monitor these sites on annual basis. The functionality of the tile to remove salts will be evaluated through annual soil sampling over the course of 3 years. The non‐tile adjacent sites will serve as a comparison to monitor how well tile works to remove salts from the upper horizons of these soil profiles. The 2016 soil data is year one of a 3‐year study.

Subsequent years will be compared to the 2016 baseline data to create a database regarding if agriculture drain tile removes salts accumulated in upper soil horizons. Further information regarding soil characteristics at this site is provided under the section ‘Supplement Soil Information’.

Supplemental Soil Information: Imagery from this section was collected from ‘Soil Web’ https://casoilresource.lawr.ucdavis.edu/gmap/ and ‘Web Soil Survey’, https://websoilsurvey.sc.egov.usda.gov/App/WebSoilSurvey.aspx.

Soil surveys are intended to describe the type of soils found in a general area. Soil series boundary lines between different soils are not always so obvious that the lines can be plotted with high precision on a map. Often part of one soil series or soil complex is commonly included in the delineation of an adjacent different mapping units. Many soil surveys were completed over 50 years ago. It is also reasonable to assume that soil series boundaries have changed over time with land management changes.

Soil Map Information: Soils mapped by NRCS Soil Survey are summarized in Figure 2A and B. The soil series mapped at the North Tile Point is a Ranslo‐Harriet 0‐2% slope. Soils mapped at the South No‐Tile Point is mapped as a non‐salt affected soils, Great Bend‐Zell (Figure 5A and B). However, no crops grow at this south tile point so this soil is (1) either mapped incorrectly and is a Ranslo‐Harriet series or (2) was a Great Bend‐Zell that is being encroached upon with salt laden ground water.

The soils in Spink County, SD have long been known to contain excessive salts. Dr. Fred Westin (1970) did an extensive analysis of soil present when the ‘Oahe Irrigation Project’ was proposed in the late 1960s and 1970s. The central and north central parts of the county is a nearly level plain. The plain is a former bed of an extensive, shallow, glacial lake known as Lake Dakota (Figure 6).

The more level or flatter areas of this lake, lost water over time by evaporation more than runoff. The standing water of the central areas of this old lake plain favored settling out of finer materials like clay and salts. This is part of the reason why in the central area of the Lake Dakota plain, soils with clay, salt, or sodium horizons are more common as compared to counties to the east which were not covered by the glacial Lake Dakota.

Soil Series Descriptions: Soil series descriptions represent an ‘average’ soil of that series. Horizon depths and thicknesses will vary some. However if the soil is mapped to have a ‘Btn’ horizon, this horizon should be present and easily identified in the soil series.

Ranslo Soil Series: The Ranslo soil series comprises approximately 45% of the soils in this map unit (Figure 3A). The land capability class (LCC) is 4s. The LCC classifications range from 1 to 8. Soils with no limitations for use are classified as a 1. Typically farmland is classified as 1‐4 based on characteristics like slope, top soil depth (horizon A), drainage, texture, salt content, etc. The lower case ‘s’ is assigned based on soil limitations most likely associated with the salts native in the soil profile. The Ranslo is classified as ‘Not Prime Farmland.’

The soil is characterized as 0‐2% slopes that are occasionally flooded. This soil is characterized by a shallow ‘A’ horizon, 0‐4 inches; an ‘E’ horizon 4‐9 in.; and horizons that contain sodium (Bn), accumulated clay (Bt), lime (Bk) or other salts (Bz) (Figure 4A). The B horizons with salts extend from the 9 to 42 inches (Figure 4A). Calcium carbonate has a low soil water solubility and is typically not a salt of concern in a saline area (Table 2). Sodium chloride or sulfate salts are more soluble in water and mapped in the horizons at the site. Presences of the Ranslo soil series may indicate that sodium management issues can become problematic.

Harriet Soil Series: The second predominant soil series is Harriet (Figure 4B). The soil has loam texture and is classified as ‘strongly’ saline. The LCC for this soil is a 6s. The ‘s’ indicates that soil characteristics limit its use as farmland. This soil has horizons (2‐5, Figure 4B) labeled ‘Btn’, ‘Btnz’, ‘Bz1’, and ‘2Bnz2’ starting approximately 2 inches from the surface and extending 36 inches to a maximum depth of 38 inches. The lowercase letter ‘n’ in horizons 2‐3 indicates sodium salt accumulations. The term used by soilscientists to describe this horizon is a ‘natric’ horizon (‘Bn’). These horizons also have accumulated clay(Bt). Water infiltration of a soil horizon with the ‘Btn’ designation may be slowed from (1) dispersion due to sodium and (2) fine texture from clay.

Horizons 4‐5 are labeled with ‘z’ descriptor. A ‘Bz’ horizon had accumulation of salts more soluble than gypsum (CaSO4). A ‘y’ descriptor indicates an accumulation of gypsum, ‘By’. Presence of gypsum in the soil horizons is not mapped at this site. However, due to the accuracy of the soil survey maps and the age, presence or absence of gypsum could be identified with soil tests and physical examination of the soil horizons.

The Harriet is classified as ‘Not Prime Farmland.’ This soil has high salts including sodium and may potentially have management issues associated with sodium.

ACKNOWLEDGEMENTS:

Prepared by Cheryl Reese, SDSU Plant Science Department

Funding for this project is provided by SDSU Extension and the South Dakota Soybean Research & Promotion Council. On-farm research involves teaching, research and extension in partnership with funding agencies and local producers.

The author would also like to thank the producers who participated in this project.