Evaluation of Soils for Suitability for Tile Drainage Performance

Recent periods of excess precipitation have brought the groundwater level nearer to the soil surface in many farm fields throughout North Dakota. Groundwater near the surface reduces oxygen in these saturated soil layers, resulting in an unfavorable growth environment for plants.

In addition, the shallow groundwater can have high levels of water-soluble salts and sodium (Na+), leading to increased soil salinity and sodicity.

The higher levels of soluble salts are due to marine shale materials in glacially deposited parent materials where, in many places, the parent material is in contact with underlying Na+-rich shale bedrock. Weathering of the soil parent material and the underlying shale releases salts and Na+ into the groundwater.

In lower field areas (Figure 1), shallow groundwater depths (Figure 2) can bring excessive salts and Na+ to the surface (Figure 3) or they can accumulate below the surface in the plant rooting zone (Figure 4). Through time, water evaporates from the soil surface, leaving behind an accumulation of excess salts and Na+. Soils with groundwater depths within 6 feet of the surface (less than 2 meters) are highly susceptible to developing salinity or sodicity problems (Seelig, 2000).

The combination of shallow groundwater and high soil salt and Na+ results in moderate to severe crop yield losses. These factors provide the impetus for many farmers and landowners to install subsurface (tile) drainage systems.

During wet weather and with good soil water infiltration, tile can drain excess water in a timely manner. Properly functioning tile systems maintain groundwater depths at desired levels and allow for leaching and removal of water-soluble salts.

In addition, soils with good drainage show improved soil productivity and crop yields. Other advantages of drainage include lower crop production risks, more water and cropping management options, reduced seasonal wetness and improved timeliness of field operations.

On the other hand, the cost of installation and maintenance, wetland determination issues, outflow management, need for water in dry seasons and strained relationships with neighbors may be disadvantages associated with tile drainage.

Although tile drainage is usually successful, instances may occur where the tile functions properly when first installed, but within a few growing seasons, areas in fields may not drain as expected. This situation may develop because of changes in soil chemistry due to the removal of salts and resulting soil swelling and dispersion rather than improper installation of the tile drains. Due to the high cost of tile installation, poor subsurface drainage performance can have a significant economic impact on a farming operation.

The loss of subsurface drainage effectiveness may be due in part to tile being placed in or below a zone of sodic or saline-sodic subsoils or below restrictive clay layers in the soil profile (Figure 5). The sodic or saline-sodic characteristics are often not readily noted at the soil surface. Characteristics of saline, sodic and saline-sodic soils are shown in Table 1.

Table 1. Characteristics of saline, sodic and saline-sodic soils (from UDSA Handbook No. 60).

Tile installed in soils or subsoils that are sodic or saline-sodic often will function normally for a period of time after installation because these soils often contain divalent (2+ charged) calcium (Ca2+) and magnesium (Mg2+) salts that keep the soil in an adequately flocculated state (maintain soil structure) under natural moisture conditions. But when the soils are drained and excess water is removed, the divalent salts also are removed, with the water leaving the soil material above or around the tile line saturated with monovalent (1+ charged) sodium (Na+).

When this occurs, the soils lose their natural structure and become dispersed. This can cause sealing of the soil above and/or around the tile lines, resulting in ineffective drainage. In addition, if sodicity starts from the surface layers/topsoil, it will further reduce the soil water infiltration, resulting in increased runoff or rainwater standing in a low spot despite tile installation (Figures 5 and 6).

Reduced drainage performance is more likely to occur in fine textured (silty or clayey) soils and to a much lesser extent in coarser textured (sandy) soils or where subsurface layering of soils occurs (Figure 7). Once drainage performance is reduced, essentially little can be done economically to restore the effectiveness of the drainage system.

However, producers can take precautions prior to tile installation on soils where drainage performance may be affected. These precautions include: (1) examining the characteristics of the soil series (soil types) in the field under consideration for drainage, (2) evaluating the soil chemical characteristics for each of the soils mapped in the field, (3) evaluating the soil properties for suitability to install tile and (4) verifying soil types and chemical characteristics (items 1 and 2 above) by deep soil sampling and testing. Using these precautions can help avoid installation of tile in areas where poor subsurface drainage performance is likely.

The occurrence of a specific soil series, types or map units on a parcel of land under consideration for tiling can be obtained from a county soil survey map or online from the U.S. Department of Agriculture-Natural Resources Conservation Service’s Web Soil Survey (http://websoilsurvey.nrcs.usda.gov). The soil series listed in Table 2 and Table 3 are soils that are most likely to have dispersion problems when drained. 

Also, soils with the greatest probability of drainage problems are those soils with pH values greater than 8.5 in the surface or subsurface zones. A pH greater than 8.5 often indicates high sodium (Na+ ) saturation, which could lead to tile sealing when salts are leached out of the soil above the tile line. If any of the soils listed in Tables 2 and 3 occur in the parcel of land to be drained, the soil chemical characteristics need to be evaluated. 

Most drainage system designers and installers evaluate soil texture in a field as a part of the system design process. However, soil chemical characteristics are not normally part of this process. Preliminary evaluation of soil chemical characteristics can be accomplished by utilizing soil chemical data embedded in the Web Soil Survey. In addition, a soil drainage suitability rating for North Dakota soils is available in the Web Soil Survey.

The Web Soil Survey is an internet-based digital product provided by the USDA-Natural Resources Conservation Service at http://websoilsurvey.nrcs.usda.gov (Figure 8). Most soils in North Dakota can be evaluated from maps and information contained in the Web Soil Survey (Figure 9). The following illustrations are an example of how to access this information from a personal computer or other device with internet access.

Soil chemical properties can be evaluated utilizing the “Soil Data Explorer” tab in the Web Soil Survey. Once a parcel of land is selected (Figure 10), choosing the “Soil Chemical Properties” menu within “Soil Data Explorer” provides options for soil evaluation that will bring up a menu of several chemical characteristics that can be evaluated (Figure 11).

For this soil evaluation, the characteristic of interest is “Sodium Adsorption Ratio” (SAR). Clicking on “Sodium Adsorption Ratio” will bring up an interactive area where depths to be evaluated can be specified in inches or centimeters (cm). A general evaluation of SAR to a depth of 5 feet (150 cm)
can provide a realistic evaluation of soil chemical properties. However, for greater accuracy, the evaluation should be carried out for successive 1-foot increments to a minimum depth of 5 feet or at least 2 feet below the deepest depth of the drain line.

For each increment, a colored field map will appear over the photo base map showing the level of hazard related to each soil type (Figures 12a-d). A table with an interpretation and average electrical conductivity (EC) or SAR values accompanies the map and interpretative information. Within this evaluation, red, green and yellow generally indicate lower hazards, while blues indicate higher hazards. Dark blue indicates the highest hazard.

Figure 12. General evaluation of soil SAR in the 5-foot depth zone: (a) evaluation of dominant component, (b) evaluation of dominant condition, (c) evaluation of weighted average of all components and (d) evaluation of all soil components in each map unit. Note that for this field a, b and c are identical, but that may not be true for all land parcels. Evaluation of all components within each map unit provides the most stringent information regarding soil SAR conditions. In these images, dark blue indicates a high degree of SAR hazard. (See Table 4 for SAR ratings)

Maps for each depth increment can be printed for reference. The information contained in these evaluations is generalized for each soil series or map unit based on the total composition of the map unit. The data for each map unit is populated with chemical and physical property information that is aggregated from various soil laboratories and the National Soil Survey Laboratory.

This information may change from time to time as the database is updated. Also, colors indicating the degree of hazard may vary from county to county within the Web Soil Survey.

You have four options for evaluating the soil layers in each mapping unit. Because each soil mapping unit includes small areas of varying sizes of soils that may not be suitable for drainage, the worst-case scenario should be utilized to identify soils that can contribute to problems with subsurface drainage.

The four choices for soil map unit evaluation are: (1) evaluation of all soil components, (2) evaluation of dominant soil components, (3) evaluation of dominant soil condition and (4) evaluation of a weighted average of all soil components. Figures 12a-d illustrate the comparison of these four choices for a general evaluation of the 5-foot (150 cm) depth zone of an actual parcel of land with sodium-affected subsoils. The numerical ratings for the field shown in Figures 12a-d are shown in Table 4.

 As shown in Table 4, evaluating the soil map units on the basis of all components will give the ratings for the most limiting soils within the map unit and provide for identification of the highest risk (or worst case) scenarios.

Data embedded in the Web Soil Survey is based on typical characterization sites across the normal geographic range of the occurrence of a specific soil type. Due to natural variability, the soils in the parcel of interest may vary from the “typical” map unit of the soil designated when the soil survey was conducted.

In addition, detailed variability and small inclusions within a soil map unit are difficult to show at the scale of typical soil surveys. Thus, a more detailed survey of the field to be drained will be useful for determining whether problems with subsurface drainage may exist.

All soils with an SAR value of 6 to 12 should be sampled for detailed chemical characterization. A qualified professional soil scientist or classifier should be consulted when making these evaluations.

A soil drainage suitability interpretation has been incorporated into the Web Soil Survey. This rating evaluates all soils in a soil mapping unit using three criteria: installation, performance and tile water outflow quality. The information used for the evaluation of each criteria is shown in Table 5.

The suitability rating provides a scale of 0 to 1 for each of these criteria and provides a weighted rating based on the components of a soil map unit. rating near 0 indicates no limitations for subsurface drainage, while a rating near to 1 is very limited.

Soils with a rating greater than 0.15 for SAR performance should be subject to verification by soil sampling and testing. The SAR suitability ratings for soils based on the soil chemistry evaluation described above are shown in Table 6.

 This tool may give limitation ratings for multiple factors for each soil in a mapping unit. While most limiting factors are based on soil properties that do not change with drainage or management, soil SAR and EC factors are subject to change as soils are drained. However, other limitations identified by using this tool may respond to modifications in design and installation.

What the soil drainage suitability rating will not provide is a comprehensive site evaluation, determination of wetlands and flooding issues or soil productivity or design information, and it will not address social or environmental issues.

The soil drainage suitability rating is not designed to tell the landowner, land manager or tile installer that a field should or should not be drained. The rating is mainly designed to present information that a decision maker can use in making a decision whether drainage is a suitable option as a land treatment.

Once soil areas that have a moderate or severe SAR hazard are identified, these areas should be sampled to the proposed depth of the tile line in 1-foot increments. The soils should be sampled at a minimum of three locations within each soil map unit where an SAR hazard has been identified.

Samples from these locations can be composited into one sample for each depth increment. A minimum of one composite soil sample should be submitted for each five acres of a soil map unit in question.

Each soil depth increment should be analyzed for electrical conductivity (EC) and SAR using standard soil saturation paste extracts for the evaluation. This allows for making direct comparisons with the information contained in the Web Soil Survey database to verify the suitability for subsurface drainage.

If the soil analyses indicate that the SAR values are lower than the values shown in Table 6, then the soil is likely suitable for subsurface drainage. If soils are rated unsuitable for tile drainage, then alternatives such as leaving the soil area undrained or placing the area into permanent cover should be considered. Utilizing registered professional soil scientists or NRCS soil scientists can assist in making decisions about alternatives to subsurface drainage.

The ratings shown in Table 6 are only a guide for drainage suitability. Dispersion of the soils under subsurface drainage conditions depends on several factors, including the composition of soil minerals, soil texture, composition of shallow groundwater and composition of soil salts. You also should recognize that soils subject to dispersion may be localized or only be a small proportion of the soils in the field or land parcel to be drained.

Evaluating soils for subsurface drainage suitability prior to installation can reduce the incidence of poor tile performance and unrecoverable installation costs. If soils susceptible to poor drainage are identified prior to tile installation, then alternatives to drainage can be considered and implemented. Once soils disperse due to subsurface drainage, attempting to remediate the soils to near their original internal drainage condition is extremely difficult and costly.

Franzen, D., H. Kandel, C. Augustin and N. Kalwar. 2013. Groundwater and Its Effect on Crop Production. North Dakota State University, Fargo, N.D. www.ag.ndsu.edu/langdonrec/soil-health/groundwater-and-its-effect-on-crop-production

Franzen, D., A. Wick, C. Augustin and N. Kalwar. 2014. Saline and sodic soils. North Dakota State University Extension, Fargo, N.D. 8p. www.ndsu.edu/soilhealth/wp-content/uploads/2014/07/Saline-and-Sodic-Soils-2-2.pdf

Franzen, D., N. Kalwar, A. Wick and T. DeSutter. 2019. Sodicity and Remediation of Sodic soils in North Dakota. NDSU Extension publication SF1941. North Dakota State University, Fargo, N.D.

Kalwar, N., T. DeSutter, D. Franzen and C. Augustin. 2016. Soil Testing Unproductive Areas. NDSU Extension publication SF1809. North Dakota State University, Fargo, N.D.

Seelig, B.D. 2000. Salinity and sodicity in North Dakota Soils. NDSU Extension publication EB57. North Dakota State University, Fargo, N.D. 16p. www.ag.ndsu.edu/langdonrec/soil-health/salinity-and-sodicity-in-nd