Soil Salt Management and Impact of Tile Drainage (Site 2)

Introduction:

Soil Test Data: Soil salt issues are classified as either saline or sodic. Soils with high total salts are defined ‘saline’. Soil samples were analyzed for salinity by measuring soil electrical conductivity, EC. Units to measure electrical conductivity in soil from salts on soil test reports will be either (1) mS/cm, (2) dS/m= 1, or (3) mmho/cm. These units are equivalent; 1 mS/cm = 1 dS/m= 1 mmho/cm.

For eastern SD soils, if the soil EC is > 4.0 mS/cm (or dS/m or mmho/cm), the soil contains a high salt concentration. Predominate salts in eastern SD soils are calcium (Ca+2), magnesium (Mg+2), and sodium (Na+1). 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.

The term used to describe a soil with too much sodium is ‘sodic’. In a dispersed soil, water infiltration is slowed. If the soil texture contains high amounts of clay in addition to the sodium, water infiltration if further inhibited due to the fine texture of the clay soil. Sodic affected soils erode easily due to poor soil structure and water infiltration.

Soil testing labs use one of two indexes to measure sodium, 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 3 points in the field where salt issues were apparent on surface soils. Soil samples were collected at points shown in Figure 1. 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 three sample points are presented in Table 1.

Results: Soil test analysis of these three field points would suggest that the soil at this location is classified as a normal soil. The total salt concentration as measured by the soil EC is < 4.0 mS/cm at all the sample sites over the 0‐24 inch sampling depth. The soil sodium content is low for all sampling depths at all the sample points; the SAR is < 1.0 for all sites. In these sites, sodium does not appear to be a problem in this area.

Objective: 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 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.’

Discussion: Often the question arises as to why do the salts appear at random locations in a field? The answer to this questions is complex and not fully understood. However, exploration of the past geologic events in the area over the last 1,000,000 years provides clues.

The first theory is associated with glacial Lake Dakota (Figure 5). The east side of Brown County is a former bed of an extensive, shallow, glacial lake known as Lake Dakota (Figure 5). This lake existed up to 9000 years ago. As the climate changed and the glaciers melted, the water in the lake begin to drain or evaporate. 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. Therefore, eastern parts of Brown county that were once covered by the Lake Dakota plain have soils that have higher clay, salt, or sodium horizons (Figure 5). Dr. Fred Westin (1970) did an extensive analysis of soil present when the ‘Oahe Irrigation Project’ was proposed in the late 1960s and 1970s. Many soils in Spink and Brown County, SD have long been known to contain excessive salts since Dr. Westin’s survey in the 1970’s. The second thought or hypothesis remains to be tested.

This thought addresses soils with salts found in glacial till parent material. In Figure 6, the surface geologic deposits are mapped around the soil sample area. The Elm River formed and cut through the surface formations when the glaciers started to recede about 12,000 years ago.

Along the banks of Elm River, a formation ‘Kp’ is mapped (Figure 6). The ‘Kp’ represents the Pierre Shale. The Pierre Shale is the uppermost (close to the ground surface) geological bedrock formation in Brown County. This shale was deposited from 65‐145 MYA when a shallow inland ocean occupied this area. The Pierre shale carries the salt signatures of this old ocean bed and is high in salt.

Therefore, any groundwater that is in contact with the Pierre Shale will contain salts since many of these salts are easily dissolved in water (Table 2). As the glaciers moved over the landscape, parts of this shale were pulverized from the weight of the glacier and incorporated into the glacial till. The glacier acted as slow, massive, field cultivator that passed over the old ocean deposits at different depths; the mixing was not homogeneous or all the same.

As the ice melted, areas where salty shale minerals were mixed with till were deposited at a random pattern over the current landscape. This idea has been presented (but not yet scientifically evaluated) as one reason for sporadic nature of soils with salts found in glacial till landscapes in this area that ARE NOT ASSOCIATED with the old glacial Lake Dakota plain.

Supplement 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 all the sampled points is Rimlap‐Heil.

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

Rimlap Soil Series:
The Rimlap soil series comprises approximately 50% of the soils in this map unit (Figure 3). The land capability class (LCC) is 4w (Figure 4A). 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.

This soil is a very deep, poorly drained soil that formed in local alluvium (alluvium = deposited by water) overlying till (till = deposited when the glaciers melted) in depressions. Permeability is slow or very slow. This soil series is often found in closed depressions. These characteristics are associated with LCC labeling of a soil with ‘w’ or water limitations. The Rimlap is classified as ‘Not Prime Farmland.’

This soil is characterized by a shallow ‘A1 and A2’ horizons 1 and 2, 0‐6 inches (topsoil); an ‘E’ horizon 6‐10 in. (horizon 3); horizons (4‐6) that contain accumulated clay (= ‘Bt’) ‘Bt1’, ‘Bt2’, ‘Btk’ from 10‐39 inches; lime (‘Bk’) is present in horizons 6‐7, from 29 – 45 inches; horizons 8‐9 are labeled ‘C’ and 2C2 at 45‐102 inches (Figure 4A). An ‘E’ horizon has been significantly leached of clay, iron, and aluminum oxides.

The accumulated clay in the ‘Bt’ horizons below the ‘E’ may create a claypan that is denser than horizons above or below. Calcium carbonate or lime in the Bk has a low soil water solubility and is typically not a salt of concern in a saline area (Table 2). The ‘C1’ horizon is alluvium or water deposited through local flooding events over time. The ‘2C2’ is the glacial till deposited when the glaciers receded thousands of years ago.

The official description of the Rimlap soil also states that the soil may contain a ‘Bn’ or n = natric horizon. A natric horizon is typically characterized by a high concentration of sodium salts.

Heil Soil Series:
The second predominant soil series is Heil (Figure 4B). The soil has silty clay texture and formed in clayey, calcareous alluvium (alluvium = deposited by water). The LCC for this soil is a 6s. The ‘s’ indicates that soil characteristics limit its use as farmland. This soil has horizons 2‐3 (Figure 4B) labeled ‘Btn’ and Btng’ starting approximately 3 inches from the surface and extending 21 inches to a maximum depth of 24 inches.

The lowercase letter ‘n’ in horizons 2‐3 indicates sodium salt accumulations. The term used by soil scientists 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 5‐6 are labeled with ‘y’ descriptor. A ‘y’ descriptor indicates an accumulation of gypsum, ‘By’ (CaSO4). A ‘Bg’ or ‘Cg’ horizon is mapped from 24‐60 inches. The lower case letter ‘g’ defines the soil as a ‘gleyed soil’. A gleyed soil developed under conditions of poor drainage, resulting in reduction of iron and other elements and also in a typical grey/blue soil coloring.

The Heil 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.