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Guideline S.3 Soil

Intent

To ensure the maintenance and restoration of healthy soils by documenting existing soil conditions, preserving and protecting benefits of existing soil, minimizing the impacts of construction, repairing soils to return to supportive conditions, and documenting soil maintenance practices to ensure ongoing optimal soil conditions.

Required Performance Criteria

Guidelines apply to all projects designated New Buildings and Major Renovations with site work site scope that includes an area of site disturbance that is greater than 3,000 s.f..

1. The soil conditions of the non-building area of the project site shall reference data from the NRCS WSS (nrcs.usda.gov) and shall be described. This description must include:
  1. A Site Soil Inventory Map (SSIM).
  2. A listing of which of the eight soil orders, 1,000 soil series, and seven slope classes that occur in Minnesota are present on the subject site based on the NRCS WSS.
  3. A listing of any soil limiting constraints for organic, wetland, or expansive clay soil’s (shrink/swell) for the proposed project site uses.
  4. A listing of which of the three stages (as defined by this B3 Guideline) currently apply to the state of the project site’s soils: natural, agricultural, or urban.
  5. A determination of whether the project is considered a greenfield (see definition below).
  6. A listing of specialized DNR defined native plant communities (NPCs).
  7. A mapping of any naturally occurring atypical soils (see definition below)
  8. Results from soil testing, at a minimum using the following rates for the following human soil stages: Natural: two soil tests per acre; Agricultural: three soil tests per acre; Urban: four soil tests per acre. Testing must be performed prior to construction and is recommended to be included in the geotechnical report. Additional testing at three and ten years after construction is recommended.
2. For projects developing on a greenfield site, the following should be submitted:
  1. A written rationale of the need to develop a greenfield site rather than a previously developed site.
  2. A plan for minimizing the disruption of existing, native, noninvasive vegetation.
3. Soil disturbance defined as grading, compacting, piling, tilling, scraping, storing, should be limited and the removal of soil within natural and agricultural human soil development spectrum areas should be ensured by the following and included in the Stormwater Pollution Protection Plan (SWPPP) for the project:
  1. Protecting intact soil with intact soil horizons using Site Soil Protection Zones (SSPZ); delineating exclusion barriers for these areas to ensure soil protection during construction.
  2. Soil in the following areas should not be disturbed:
    1. 40 ft. beyond the building perimeter.
    2. 15 ft. beyond the primary roadway curbs, parking lots, main utility branch trenches, or impervious areas.
    3. 5 ft. beyond walkways.
    4. Any area under or closer than 1 ft. of tree driplines per 1 in. of diameter at breast height (DBH) trunk diameter (e.g., 12-in. DBH will require tree protection fence at least 12 ft. from the trunk).
    5. Areas within any identified stormwater management features.
    6. Retaining walls within these areas may be employed as needed to maintain necessary grades.
  3. Trees shall be protected as individuals with the tree protection fence located outside the drip line, as defined above in C.2.iv, prior to site activities. Trees may be protected as groups if their canopies are within 10 ft. of each other, with tree fence protection zone distances as defined above in C.2.iv.
4. Soil management and erosion control plans should be created and implemented to protect the soil profile of the current site before, during, and after construction.
5. The bulk density of all unpaved pervious surfaces intended for seeding and planting shall have the following maximum bulk densities:
  1. Clays and Silts: 1.25 g/cm³
  2. Loams: 1.40 g/cm³
  3. Sands: 1.60 g/cm³
6. A 50-ft. minimum of perennially rooted vegetated buffer for delineated wetland boundaries shall be maintained, established, or enhanced.
7. Topsoil from the project site shall not be sold or exported until all landscaped areas (tree, shrub, perennial, annual, or lawn plantings) have received an average 12-in. deep respread using soil from the project site. Existing site topsoil shall be stockpiled and protected, or topsoil should be imported for an average respread depth of 12 in. in all proposed planting and seeding areas. No topsoil should be screened with less than a 3-in. screen. Minimum respread depth is not required for green roofs.
8. At least 3.5% organic material by soil weight should be achieved in planting and seeding areas by adding sufficient organic matter to soil below this threshold.
9. If urban soils are present, in-site landscaped areas soil should be amended to mimic the physical and biological capabilities of natural and agricultural soils to achieve the following metrics:
  1. Soil texture: Determine which of the 12 soil classes are present on the site.
  2. A pH between 4.5 and 8.5.
  3. Nitrogen-Phosphorous-Potassium (NPK) fertility greater than medium, as tested by University of Minnesota Soil Testing Laboratory “Lawn, Garden, and Landscape” Soil Analysis Request Sheet.
  4. Meeting the following bulk density requirements for the listed soil types:
    1. Organics: less than 1.0 g/cm³
    2. Clays: less than 1.25 g/cm³
    3. Loams: less than 1.4 g/cm³
    4. Silts: less than 1.25 g/cm³
    5. Sands: less than 1.6 g/cm³
  5. Organic matter content should achieve a minimum of 3.5% by weight through the incorporation of Class A Biosolids, US Compost Council Certified Compost, and activated biochar (as defined in this B3 Guideline) in the following depths for the following soils:
    1. Predevelopment: incorporated (e.g., V-ripper or Paraplow) into site soils to a minimum depth of 24 in.
    2. Postdevelopment: incorporated (e.g., V-ripper or Paraplow) into site soils to a minimum depth of 12 in.
    3. Minor Modification Amendments: applied as topdressing or incorporated into site soils to a minimum 6-in. depth.
  6. Soil should be modified to achieve the following NRCS infiltration rate for the following conditions:
    1. Natural stage soils improved to have a higher infiltration rate than Group A.
    2. Agricultural stage soils improved to have a higher infiltration rate than Group B.
    3. Urban stage soils improved to have a higher infiltration rate than Group C.
    4. Stormwater infiltration stage soils improved to have a higher infiltration rate than Group A.
  7. A cation exchange capacity (CEC) of at least 15 should be achieved.
  8. A base saturation percentage of at least 30% should be achieved.
  9. A mycorrhizae count of at least two Glomus species per ounce of soil should be achieved.
10. Atypical soils: If the project has atypical soils or substrates for a specialized NPC, these shall be preserved in the landscaped areas of the site according to the following:
  1. All naturally occurring atypical soils of an area greater than 5,000 sq. ft. shall be preserved as required to support NPCs in seeps, fens, bogs, bedrock outcrops, sand blow-outs and sand dunes (as defined by DNR Natural Heritage and Nongame Research Program), Spodosols, Histosols, Psamment, Entisols, and Sodic soils.
  2. The boundaries of these atypical soils and substrates should be field mapped, marked, and delineated with visible flagging on project site. This NPC delineation prohibits entry of any vehicles with tires before, during, or post construction. These soils or substrates should not be disturbed, buried, blasted, or removed from their original location onsite.
  3. Minnesota Biological Survey staff of the DNR should be consulted to create and execute the following:
    1. A specialized, NPC planting plan with a conservation status rank (S-ranks) of S1 or S2 that most appropriately matches the site’s atypical soils and substrates.
    2. Guidance on site preparation (weed and erosion control), site drainage, and revegetation (seeding, planting, etc.), and long-term maintenance (fire management, weed control, etc.) for that specialized NPC.
    3. Operations and maintenance plan to ensure that this restored vegetation and naturally occurring atypical soil is rigorously protected and maintained.
  4. Upon complete installation of this specialized NPC, a perimeter exclusion fence should be installed with one permanent, outdoor sign (dimensions at least 24 in. x 36 in.) interpreting the specialized NPC using Tilden’s 5 Principles of Interpretation.

Note: Specialized planting areas may count against local open space ordinances at a 3:1 ratio (i.e., 1 sq. ft. of specialized, NPC will equal 3 sq. ft. of local open space). Also note that other vegetation requirements are listed under S.1 and S.4.

Recommended Performance Criteria

If the soils are NRCS-defined udorthents or udipsamments, then the results of a Modified Philip-Dunn Infiltrometer measurements of saturated hydraulic conductivity with at least four tests taken per acre should be included in the descriptions of the site soils.
A CEC of more than 15 should be maintained.
A pH factor of the soil between 5.5 to 8.5 pH should be maintained.

Meeting the Guidelines

The SSIM should be developed using an engineering scale typical to a development of site size (e.g., 1 in. = 30 ft., but no coarser than 1 in. = 100 ft.), and labeled with a bar scale and north arrow. An interpretive legend of symbols, colors, shades, hatched markings, etc. and current NRCS soil terms should be used. The SSIM as a layer on the site’s legal land survey should be mapped with topographical controls, benchmarks, etc. The NRCS WSS: https://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm should be used to determine the following:

The eight soil orders, 1,000 soil series, and seven slope classes present on the project site.
The soil limiting constraints for organic, wetland, or expansive clay soils for the proposed development uses (buildings, roads, parking, trails, landscapes).
The three stages that currently apply to the state of the site’s soils: natural (never plowed, often containing O or E horizons), agricultural (standard cultivation practices with A, B, C horizons), or urban (buried horizons, missing major horizons, such as A or B or C; C or R horizons at the surface plane).

Additional information on using the NRCS WSS is listed under Appendix S-3.

The SSIM should identify where one or more of the state’s soil orders are encountered intact, as these areas will need to be actively protected from filling or compaction per guideline S.3B. Existing natural soil horizons (A, B, C, R or O, A, E, B, C, R or A, E, B, C, R) should be preserved. The site’s damaged soils should be mapped, classified, protected, and/or mitigated.

Exclusion barriers for any identified SSPZ should be installed prior to site mobilization to ensure soil protection during the construction process. Access by vehicles with tires should be prohibited and a perimeter exclusion fence a minimum of 42 in. in height implemented

Atypical, naturally occurring soils, and substrates greater than 5,000 sq. ft. in area discovered on subject site should be identified, mapped, delineated, and preserved. These soils and substrates are required to support specialized (S1 and S2 rank) NPCs found in Minnesota’s seeps, fens, bogs, bedrock outcrops, sand blow-outs, and dunes.

The specialized NPC and atypical soils may necessitate the use of other strategies to increase site biomass to match the ecosystem province in which the subject site occurs. In those cases, the use of green walls (vines on trellis), green roofs (extensive), and tree canopies over impervious surfaces that do not increase building footprint but do increase overall site biomass are encouraged.

Soil management and erosion control plans must list activities used to protect the soil profile of the current site before, during, and after construction. The following definitions should be used:

Natural: never plowed, often containing O or E horizons with A, B, C horizons also.
Agricultural: standard cultivation practices with A, B, C horizons present.
Urban: buried horizons; missing major horizons, such as A or B or C or E or R horizons within 30 in. of the existing ground surface plane.

Raising or maintaining the percentage of organic material content in the existing or imported site soil will help build the site’s natural mycorrhizae and microbial population and enhance the health of the soil. The soil in planting and seeding areas should be tested and amended with organic material as needed to achieve at least 3.5% organic material by soil weight.

Soil testing should be done using the University of Minnesota’s Soil Laboratory sampling protocol at the following rates for the following human soil stage: natural: 1 soil test per acre; agricultural: 3 soil tests per acre; urban: six soil tests per acre. Testing during occupancy should be submitted at the following rates for the following human soil stages: natural: two soil tests per acre; agricultural: three soil tests per acre; urban: four soil tests per acre.

Minnesota Soil Background

Of the 12 soil orders in the United States, Minnesota contains eight: 32% Mollisols (prairie); 27% Alfisols (deciduous forest); 9% Inceptisols (mixed forest); 18% Entisols (boreal forest or river floodplains); 5% Histosols (peat marshes or blanket bogs); 1.0% Vertisols (Glacial Lake Agassiz/Red River Valley); 0.2% Spodosols, (sandy saturated coniferous boreal forest). These soil orders are inextricably linked to the state’s parent material climate and the vegetation of its dominant ecosystem provinces.

Surface transported glacial parent materials of the Wisconsin Ice Age has been the most influential factor in forming Minnesota’s soils. The dominant, surface glacial parent materials are till, outwash, and moraines from the Des Moines and Superior Lobes. The shale-rich (Canadian sedimentary bedrock) Des Moines Lobe entered Minnesota from the northwest and terminated in Des Moines, Iowa. The Superior Lobe that scraped the Canadian Shield was iron-rich, igneous rock (basalt, granite). The Superior Lobe entered Minnesota from the northeast and ended its journey in north central Minnesota.

Minnesota’s most recent glaciation, the Wisconsin Ice Age (about 10,000 BP), was the Laurentian Ice Sheet that covered about 90% of the state for thousands of years, with ice anywhere from hundreds to thousands of feet thick. The state’s soils are geologically new and extremely fertile. Much of the state’s Entisols and Inceptisols (27% of the state) are less than a thousand years old. Mollisols, the state’s dominant soil order, covers about one-third (32%) of Minnesota. Minnesota’s Mollisols are among the richest soils on earth and typically greater than 5,000 years old. One acre of Minnesota Mollisols yield on a per-acre basis three to five times the agricultural lands of the southern and southeastern United Stated, and ten times more than ancient tropical soils. It is strongly discouraged to develop or disturb current Minnesota farmland.

Ancient or Glacial Lake Agassiz was located in the northwestern portion of Minnesota, now called the Red River Valley. At its maximum, Glacial Lake Agassiz covered most of Ontario and Manitoba, dwarfing the Great Lakes in size. The Minnesota River Valley (the Glacial River Warren) was the outlet channel for Glacial Lake Agassiz. The fluvial force of emptying Lake Agassiz carved a mile wide canyon, hundreds of feet deep. After this glacial meltwater formed Lake Agassiz, lacustrine parent material precipitated in the lake. At this location, fertile clays, with abundant illite, smectite, and vermiculite, were deposited. Because of high moisture content, these Vertisols are much harder to cultivate than Mollisols. However, these Vertisols, are as exceptionally fertile as Mollisols. As these are poorly drained shrink-and-swell-prone clays on a planar flat landscape they are generally not well-suited for construction activities and are recommended to be used only for row crop agriculture.

Submittal Requirements

Predesign:

3A: Submit soil testing results as required by guidelines and a listing of the human soil stage(s) of site.

Design:

3A: Submit description of site soils, including the following: a SSIM, test results, and sampling locations; which of the eight soil orders, 1,000 soil series, and seven slope classes are present onsite; the soil development limiting constraints for organic, wetland, or expansive clay soil; which of the three stages apply to the current state of the subject site’s soils; whether the site is considered a greenfield; and a listing of NPCs identified onsite.
3C: Submit identified SSPZ and delineated exclusion barriers/zones.
3I: Submit urban soil restoration amendment plan of future landscaped area in 1,000 sq. ft. units.
3J: Include results of preliminary coordination with Minnesota Biological Survey staff for NPC restoration, including preliminary planting list, details of site preparation, drainage, and revegetation.
3K: If soils are udorthents or udipsamments, submit saturated hydraulic conductivity test results.

Final Design:

3B: If project is developing on a greenfield site, submit rationale for developing on site and a site plan documenting minimization of the disruption of existing, native, non-invasive vegetation.
3C: Submit site plan delineating limits of soil disturbance during development, including SSPZ.
3D: Submit a soil management and erosion control plan.
3E: Submit Contract Documents section with specifications for bulk density requirements for all unpaved pervious surfaces.
3F: If wetlands are present on site, submit site plan noting 50-ft. vegetated buffer for delineated wetland boundaries.
3G: Submit Contract Documents, which prohibits selling or exporting any topsoil from project site, and delineate onsite plan storage areas for site’s topsoil that will be reused.
3H: Submit Contract Documents specifying that the soil must have a minimum of 3.5% organic material by soil weight.
3I: Submit Contract Documents with specifications for urban soil restoration amendment: Soil texture, pH, NPK fertility, bulk density, organic matter, infiltration rate, CEC, base saturation, and mycorrhiza count requirements for each 1,000 sq. ft. of site landscape area.
3J: Results of coordination with Minnesota Biological Survey (MBS) staff for NPC restoration, including planting list, details of site preparation, drainage, revegetation; design of permanent outdoor interpretive sign meeting guideline requirements in site operations and management (O&M) manual to vigorously protect and maintain restored vegetation on these atypical soils with MBS and DNR staff guidance.
3L: Submit Contract Documents requiring CEC of at least 15.
3M: Submit Contract Documents requiring pH factor between 5.5 and 8.0.

Closeout:

3A: Submit soil reports as required.
3C: Note any incursions into the SSPZ and remedies employed to mitigate soil damage.
3J: Site O&M manual for restored NPCs on atypical soils, as created in coordination with MBS staff.

Occupancy – Submitted annually for ten years:

3A: Submit ongoing soil sampling as required at least every three years.

Training Resources

‘Understanding Site and Water in B3 Version 3.1’ Training Session Video (YouTube)

‘Understanding Site and Water in B3 Version 3.1’ Training Session Slides (PDF)

Additional Resources

DeepRoot tree and stormwater products: http://www.deeproot.com/.

MN DNR Minnesota Biological Survey. Conservation Status Ranks for Native Plant Community Types and Subtypes (S-ranks): http://www.dnr.state.mn.us/npc/classification.html.

MN Board of Water & Soil Resources Wetland Delineation Guidance: https://bwsr.state.mn.us/delineation-guidance-resources.

MN DNR County Biological Survey, County Maps: http://www.dnr.state.mn.us/eco/mcbs/maps.html.

MN PCA – Brownfields: https://www.pca.state.mn.us/waste/brownfields.

MN PCA – Dominant soil orders: https://stormwater.pca.state.mn.us/images/e/e6/Minnesota_dominant_soil_orders.jpg.

Dominant Soil Material Mapping Statewide: https://www.mngeo.state.mn.us/chouse/soil.html.

Urban, James. Up By the Roots. International Society for Arboriculture, 2008: http://www.jamesurban.net/up-by-roots/

USDA Web Soil Survey (WSS): https://stormwater.pca.state.mn.us/images/thumb/b/bd/Minnesota_dominant_soil_suborders.jpg/300px-Minnesota_dominant_soil_suborders.jpg.

UMN Soil Testing: http://soiltest.cfans.umn.edu/testing-services.

United States of America Department of Agriculture (USDA) Web Soil Survey: http://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm.

US Compost Council Certified Compost: https://www.compostingcouncil.org/.

Glossary

Activated Biochar:

Activated Biochar is biomass (e.g., woodchips) produced via pyrolysis (400–500 degrees Celsius), and activated to allow immediate application.

Landscaped Areas:

Nonbuilding or paved areas on which plants will be grown in soil (trees, shrubs, perennials, annuals, turf) or prairie, wetland, woodland, buffers.

NRCS:

Natural Resource Conservation Service, within the USDA.

Soil Orders:

Coarsest resolution of soil taxonomic nomenclature, defined by NRCS. Twelve found worldwide; eight found in Minnesota.

Soil Series:

Finest resolution of soil taxonomic nomenclature. >19,000 found in the USA; >1,000 found in Minnesota, defined by NRCS.

Slope Classes:

Classification of the slopes present on a project site based on percentage grade (slope A B C D E F G, where A is shallowest <2%. G is steepest >35%).[1]

Human Soil Stages:

Natural: never plowed, often containing O or E horizons with A, B, C horizons also.
Agricultural: standard cultivation practices with A B C horizons present.
Urban: buried horizons; missing major horizons, such as A or B or C or E or R horizons within 30 in. of existing surface plane.

Soil Texture Classes & Triangle:

Twelve classes representing percentage ratios of the fine earths (<2 mm) or mineral portions of soil: sand, silt, clay. For example, in the case of sandy loam, the first word is a modifier (sandy) of the dominant mineral portion (loam).[2]

Infiltration rate scale uses NRCS Infiltration Rate Scale, which is based on the speed, measured in inches per hour, at which gravity water passes through soil. Listed as A, B, C or D. A is the fastest (>1.1 in./1 hr.); D is the slowest (<0.05 in./1 hr.).[3]

Bulk Density:

Soil compaction rate is measured in grams of density within a fixed volume of cubic centimeters.[4]

Cation Exchange Capacity (CEC):

Capacity of a soil to retain macro and micro nutrients during standard weathering. 0 (zero) is an exhausted deeply weathered soil; 15 is adequate; >25 is a fertile soil.[5]

Base Saturation Percent:

Anion elements (calcium, sodium, magnesium) as a percentage of nutrient fertility: >35% is a young fertile soil; <20% is a weathered acidic nutrient-poor soil.[6]

Topsoil:

Defined as an A horizon with a Munsell soil color darker than 10YR 4/3.

Atypical Soils:

Naturally occurring atypical soils are soils with any of these characteristics: