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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 are required for New Construction and Major Renovation projects that include at least 5,000 square feet of land disturbance, or at least 50 cubic yards or more of cut and fill, excluding the land disturbance and cut and fill within or below the building footprint. Exterior scope related only to ADA improvements or utility connections may be excluded from contribution toward these amounts.

The soil conditions of the nonbuilding area of the project site shall reference data gathered from the National Resources Conservation Service (NRCS) Web Soil Survey (WSS) 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-based limiting constraints for organic, wetland, or expansive clay soils (shrink/swell) for the project site’s proposed
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 NPK soil fertility testing for topsoil, using the following rates, at a minimum, for the following human soil stages:
  1. Natural: No test shall represent more than five acres of a site with a different soil type and/or different usage history. A minimum of two soil tests are required for any natural site.
  2. Agricultural: No test shall represent more than four acres of a site with a different soil type and/or different usage history. A minimum of three soil tests are required for any agricultural site.
  3. Urban: No test shall represent more than one acre of a site with a different soil type and/or different usage history. A minimum of four soil tests are required for any urban site.
  4. 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.
These soil tests do not require boring or heavy equipment and should be carried out when soil is not frozen. These tests can be met by using the University of Minnesota Soil Testing Laboratory Soil Analysis or equivalent testing may also be accepted if done by a different laboratory. Test results must include Nitrogen-Phosphorous-Potassium (NPK) fertility composition, soil texture classification, percentage of organic matter in the sample, soil pH, and recommended strategies for fertilization and/or cropping, depending on the state of the project soil.
For projects developing on a greenfield site, the following shall 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.
Soil disturbance is defined as grading, compacting, piling, tilling, scraping, or storing. Disturbances of this kind shall be limited and the removal of soil within natural and agricultural human soil development spectrum areas shall 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); these areas shall be delineated using exclusion barriers to ensure soil protection during construction.
2. Soil in the following areas shall not be disturbed:
  1. 40 feet beyond the building perimeter.
  2. 15 feet beyond the primary roadway curbs, parking lots, main utility branch trenches, or impervious areas.
  3. Five feet beyond walkways.
  4. Any area under or closer than one foot of tree driplines per one inch of diameter at breast height (DBH) trunk diameter (e.g., 12-inch DBH will require tree protection fence at least 12 feet 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 ten feet of each other, with tree fence protection zone distances as defined above in C.2.iv. Please note other tree requirements are listed in S.4B.
Soil management and erosion control plans shall be created and implemented to protect the soil profile of the current site before, during, and after construction.
The bulk density of all unpaved pervious surfaces intended for seeding and planting shall have the following maximum bulk densities, obtained via testing:
1. Clays and silts: 1.25 g/cm³
2. Loams: 1.40 g/cm³
3. Sands: 1.60 g/cm³
A 50-foot minimum of perennially rooted vegetated buffer for delineated wetland boundaries shall be maintained, established, or enhanced.
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-inch deep respread using soil from the project site. Existing site topsoil shall be stockpiled and protected, or topsoil shall be imported for an average respread depth of 12 inches in all proposed planting and seeding areas. No topsoil shall be screened with less than a 3-inch Minimum respread depth is not required for green roofs.
At least 3.5% organic material by soil weight shall be achieved in planting and seeding areas by adding sufficient organic matter to soil below this threshold.
If urban soils (defined as soils missing major horizons within 30 inches of existing surface plane) are present, in-site landscaped areas soil shall 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. pH between 5.0 and 8.5.
3. NPK fertility greater than medium, as tested by using the University of Minnesota Soil Testing Laboratory Soil Analysis. Equivalent testing of NPK fertility may also be accepted if performed by a different laboratory.
4. 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 at 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. Post-development treatment for all soils with all soil disturbance types: incorporated (e.g., V-ripper or Paraplow) into site soils to a minimum depth of 24 inches.
  2. Pre-development treatment for soils with continuous pasture cover: incorporated (e.g., V-ripper or Paraplow) into site soils to a minimum depth of 12 inches.
  3. Pre-development treatment for soils with continuous forest cover: Minor modification amendments: applied as topdressing or incorporated into site soils to a minimum depth of six inches.
6. Soil modification to achieve the following NRCS infiltration rate for the following soil state conditions (calculated infiltration rates for each soil group):
  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.
8. A base saturation percentage of at least 30%.
9. A mycorrhizae count of at least two Glomus species per ounce of soil.
Naturally occurring atypical soils: If the project has atypical soils or substrates for a specialized Native Plant Community (NPC), these shall be preserved in the landscaped areas of the site according to the following:
1. All naturally occurring atypical soils (eg. Spodosols, Histosols, Psamment, Entisols, and Sodic soils) of an area greater than 5,000 square feet shall be preserved as required to support NPCs in seeps, fens, bogs, bedrock outcrops, sand blow-outs, and sand dunes (as defined by MN DNR).
2. The boundaries of these atypical soils and substrates shall be field mapped, marked, and delineated the same way as other Site Soil Protection Zones (SSPZ) on the project site. This NPC delineation prohibits entry of any vehicles with tires before, during, or post construction. These soils or substrates shall not be disturbed, buried, blasted, or removed from their original location onsite.
3. The MBS Sites of Biodiversity Significance from the DNR and the DNR Native Plant communities layers (viewable at Minnesota Conservation Explorer) shall 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 shall be installed.
Note that atypical soil definition is included in the glossary. Specialized planting areas may count against local open space ordinances at a 3:1 ratio (i.e., one square foot of specialized, NPC will equal three square feet 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 shall be included in the descriptions of the site soils.
Maintenance of a CEC of more than 15.
Maintenance of a pH factor between 5.5 to 8.5 of the soil.

Meeting the Guidelines

The SSIM should be developed using an engineering scale typical to a development of site size (e.g., 1 inch = 30 feet, but no coarser than 1 inch = 100 feet), 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 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, and 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, and protected from further damage, 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, with a minimum of 42 inches in height, implemented.

Atypical, naturally occurring soils, as well as substrates greater than 5,000 square feet in area 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 the building footprint but do increase overall site biomass are encouraged.

Three stages of soil impacts

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, and/or A, B, C horizons.
Agricultural: standard cultivation practices containing A, B, C horizons.
Urban: buried horizons; missing major horizons, such as A or B or C or E or R horizons within 30 inches 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.

Determining the presence of pre- or post-development soils

Post-development and pre-development soils refers to the presence or absence of significant human activities or disturbance via machine of/on site soil. Determining whether soils are in a pre-development or post-development condition necessitates the digging of exploratory test pits prior to any disturbance of the site because exposure of the soil horizons is needed for the evaluation. Exploratory test pits are not needed for soil areas that will be built on—e.g., under buildings, roads, parking lots, walkways/trails, and curbs. However, exploratory test pits are needed for all future lawn, landscaping, and garden areas; and for all future stormwater management areas, e.g. rain gardens, swales, bio-infiltration, etc., at a rate of one test pit per 1,000 square feet for areas less than one acre in size, or one test pit per 2,000 square feet for areas larger than one acre in size.

Post-development soils consist of human machine soil disturbances comprising anything larger than ten cubic yards in volume or 500 square feet in area. Post-development soil disturbance applies when the natural horizons of a site’s soil have been removed, displaced, or mixed prior to any disturbance of the project’s site. Post-development disturbance also refers to soil compaction in which large machinery (e.g., semi-tractor trailers, dump trucks, one-ton pick-ups, pan-scrapers, bulldozers, tractors, skid-steers, backhoes, rollers, and others) have traveled over unprotected soils for haulage or vehicle access more than three times. If exploratory soil test pits reveal that none of the four natural soil horizons, O, A, E, B, are visible as distinct layers, one on top of the other and in order, then the area is considered post-development.

Pre-development soils are generally found in locations where the following vegetation cover exists: continuous grazing land or old meadow that has been in that condition for at least 25 years; continuous forest cover in which 70% of tree stems are greater than 10 inches in diameter at breast height (DBH). If exploratory test pits reveal any one or all of the following intact soil horizons (O, A, E, B) are present as distinct layers, one on top of the other and in order, then this area is considered pre-development.

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 have 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 and has an alkaline pH (7.0 to ~14.0). The Superior Lobe that scraped the Canadian Shield bedrock was iron-rich, igneous rock (basalt, granite) with an acidic pH (~3.0 to 6.9). The Superior Lobe entered Minnesota from the northeast and ended its journey in north central Minnesota.

During Minnesota’s most recent glaciation, the Wisconsin Ice Age (about 10,000 BP), the Laurentian Ice Sheet covered about 90% of Minnesota for thousands of years, with ice anywhere from hundreds to thousands of feet thick. The state’s soils are geologically young 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, formed under prairie, are among the richest soils on earth and approximately 5,000 to 10,000 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, fine lacustrine parent material precipitated in the lake, depositing rich soils. Thus, the Red River Valley has fertile clays, with abundant illite, smectite, and vermiculite. Because of high moisture content in these very fine clays, these Vertisols are much harder to cultivate than Mollisols. However, these Vertisols of the Red River Valley are as exceptionally fertile as Minnesota’s Mollisols. As these Vertisols are both poorly drained with high shrink/swell prone clays on a planar flat landscape, they are generally not well-suited for construction activities and are only recommended for row crop agriculture.

Submittal Requirements

Predesign:

3A: Submit soil testing results as required by guidelines and a listing of the three stages of human soil impacts of the 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 of human soil impacts 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 square-foot 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, noninvasive 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-foot vegetated buffer for delineated wetland boundaries.
3G: Submit contract documents, which prohibit 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 square feet of site landscape area.
3J: Submit NPC restoration plan for atypical soils, including planting list, details of site preparation, drainage, revegetation; and management (O&M) manual to vigorously protect and maintain restored vegetation on these atypical soils.
3L: Submit contract documents requiring CEC of at least 15.
3M: Submit contract documents requiring pH factor between 5.0 and 8.5.

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.

Occupancy – Submitted annually for ten years:

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

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/wetland-delineation

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.

MN Geospatial Commons (MBS Sites of Biodiversity Significance and the DNR Native Plant Communities are available here): https://gisdata.mn.gov

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. (select the Lawn and Garden Soil Analysis Request Sheet): 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.

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]

Three Stages of Human Soil Impacts:

Natural: never plowed, often containing O or E horizons and/or A, B, or C horizons.
Agricultural: standard cultivation practices with one or more A, B, and/or C horizons present.
Urban: buried horizons; missing major horizons, such as A, B, C, or E or R horizons within 30 inches 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]

Soil Infiltration Rate:

The rate of infiltration of water into soil. The 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 micronutrients during standard weathering. Zero is an exhausted, deeply weathered soil; 15 is an adequate soil; >25 is a fertile soil.[5]

Pre-Development and Post-Development Soils:

Refers to the presence or absence of significant human activities or disturbance via machine of/on the soil of sites.

Post-development soil disturbance describes the removal, displacement, or mixing of that site’s soil’s natural horizons prior to any disturbance of that project’s site; Post-development disturbance also refers to soil compaction in which large machinery have traveled over unprotected soils, e.g. for haulage or vehicle access.

Pre-development soils are locations where the following vegetation cover exists: continuous grazing land or old meadow in that condition for at least 25 years, or continuous forest cover in which 70% of tree stems are greater than ten inches in diameter at breast height (DBH).

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: