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.