Soil Salt Management and Impact of Tile Drainage (Site 7)

Site Location:
In June 2016, four sites were soil sampled in a field tiled drained and another field that was not tile drained. The fields are located in Brown County, SD. The soil
samples locations are illustrated on Figure 1A. The no‐tile field was directly N of the tiled field across the road (Figure 1 & B).

Introduction:

Soil Test Data:
Soil salt issues are classified as either saline or sodic. Soils with high total salts are defined ‘saline’. Soil samples are analyzed for salinity by measuring soil electrical conductivity (EC). Units may be recorded as (a) dS/M; (b) mmhos/cm; or (c) mS/cm. The units are all equal. 1 dS/M = 1 mmhos/cm = 1 mS/cm. For eastern SD soils, if the soil EC is > 4.0, 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’ or ‘natric’. 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 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.

Methodology:

Soil samples were collected in June 2016 from 4 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 tiled and non‐tiled 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 Corn Tile 1, Corn Tile 2, Soybean Tile SW and No Tile Reference are presented in Table 1.

Results:

Soil test results for Corn Tile 1, Corn Tile 2, Soybean Tile SW and No Tile
Reference are presented in Table 1.

The soil at all four sites sampled in this field are classified as a ‘saline‐sodic’ soil (Table 1). The EC is > 4.0 for all sampling sites. The SAR is > 5.0 at all sampling sites. The soil has characteristics of a sodic soil with a dispersed surface horizon as shown in Figure 1B. In the ‘Supplemental Soil Information’ section of this report, the ‘G472A’ Aberdeen Nahon silty clay loam soil series is mapped at the north points, Corn Tile 1 (CT1), Corn Tile 2 (CT2), and No Tile Reference (NTR). At the Soybean Tile SW (STSW) point, the ‘G851A’ Nahon Aberdeen Exline silt loam
soil series is mapped. Each of these soil series contains horizons with salts (Fig. 4A-C).

Source of Salts:

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 carries the salt signatures of this old ocean bed and is high in salt. As the glaciers moved over the landscape, parts of this shale was 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. Hence the reason for sporadic nature of soils with salts found in landscapes in this area.

Many soils in Brown 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. Most of the eastern 2/3 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.

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 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.’

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 four sampling sites is Beotia‐Winship silt loams, 0 to 2 percent slopes.

The estimated major soil series at the north central three points is an Aberdeen‐Nahon (Figure 3A). In this soil series map unit, the Aberdeen (53%) and Nahon (33%) are the dominant soil series found in this map unit. At the SW Soybean Tile point, the Nahon‐Aberdeen‐Exline soil is mapped. In this soil map unit, the Exline soil (a soil with salt horizons) is found 19% of the time.

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.

Aberdeen Soil Series:

The Aberdeen soil series comprises approximately 53% of the soils in the North Central field area and 28% in the SW soybean site (Figures 2 & 3). The land capability class (LCC) is 3s. 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. This soil very deep soil formed in glacial lacustrine (lacustrine = lake) sediments on lake plains. Horizons 3‐6 are concerns as these contain water‐soluble salts that can move upward in the soil profile and reside at the surface (Figure 4A). The upper case letter ‘B’ indicates that this is the subsoil.

The lower case letter ‘Bt’ indicates that clay is accumulated in the profile. Horizons with high sodium are termed ‘natric’ horizons ‘Bn’; ‘By’ indicates gypsum or calcium sulfate is present; ‘Bz’ indicates other soluble salts such as magnesium chloride or sulfate are present. These soils have lime accumulations as
indicated by the ‘Bk’. Calcium carbonate has a low soil water solubility and is typically not a salt of concern in a saline area (Table 2). An average representation of this soil would have clay and sodium horizons starting at about 11 inches below the surface with salt affected horizons extending to a maximum depth of 38 inches.

Nahon Soil Series:
The second predominant soil series is Nahon (Figure 4B). This soil series consists of very deep and somewhat poorly drained soils formed in clayey glaciolacustrine (glacio= glacier and lacustrine = lake) sediments on lake plains. The LCC for this soil is a 4s. The lower case ‘s’ is assigned based on soil limitations most likely associated with the salts native in the soil profile. This soil is also described as have in a claypan. A claypan is dense, compact, slowly permeable layer in the subsoil having a much higher clay content than the overlying material, from which it is separated by a sharply defined boundary. Clay pans are usually hard when dry, and plastic and sticky when wet. They limit or slow the downward movement of water through the soil.

As with the closely related Aberdeen soils, soil horizons 3‐6 contain horizons with excessive salts. Unique to this horizon as compared to the Aberdeen soils series, gypsum is not mapped in the lower soil horizons; no ‘By’ horizon. ‘Bt’ horizons are present in this soil as well indicating that clay is accumulated. Water infiltration of a soil horizon with the ‘Bt’ designation may be slowed from fine texture from clay. This soil is classified as prime farmland.

Exline Soil Series:
The Exline soil is mapped at the SW soybean tile point and estimated to make up 19% of the soil series found in this map unit. These soils are very deep, somewhat poorly drained or moderately well drained soils formed in lacustrine (lake) and alluvial deposits on lake plains and terraces. These soils have very slow permeability. Of the three soils found in this area, the sodium horizon (‘Bn’) is found closest to the
surface (3 inches) in this soil series. The potential presence of sodium extents 16 inches, to a bottom average depth of 19 inches (horizons 3‐5). The LCC of the Exline soil is a 6s, suggesting that use of this soil for vegetative production is limited by the high sodium and clay content.

Harmony Soil Series:

The Harmony soil series is present in both of these mapping units to a low degree, 3‐4%. If present, the Harmony soil has the best LCC, 2s for vegetative production. The soil is classified as a ‘clayey’ soil most likely because of ‘Bt’ horizons 5 and 6 (Figure 4D). This soil does not contain any salts in the horizons with the exception of calcium carbonate or lime, ‘Bk’ horizon 6. However, as previously mentioned in
this report, the lime water solubility is very low and presence of a ‘Bk’ horizon is not associated with saline soils (Table 2).

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 like to thank the producers who participated in this study.