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Minnesota Stormwater Manual

Assessing soil health and function
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Indicators for determining soil health
Indicator Vegetative growth Soil biotic function Soil hydrologic function Soil erosion potential
Compaction/bulk density Yes Yes Yes If surface compacted
Water stable aggregates Yes
Soil nutrient status Yes
Soil structure Yes
Plant roots Yes

Soil health is an assessment of how well soil performs all of its functions now and how those functions are being preserved for future use. The assessment of soil health depends on the desired functions of the soil. In agricultural applications, for example, soil health is determined by assessing properties that affect plant crop growth, such nutrient status, pH, and bulk density.

For stormwater applications, soil health can be assessed for the following functions.

  • Ability to support vegetative growth
  • Fully functioning soil ecology
  • Supporting hydraulic/hydrologic function
  • Ability to minimize erosion

Assessments of soil health are typically done by using indicators. Indicators are measurable properties of soil or plants that provide clues about how well the soil can function. Indicators can be physical, chemical, and biological properties or processes. The adjacent table illustrates which indicators are useful in evaluating the four functions identified above.

Soil compaction (bulk density)

curve showing relationship of root penetration and penetration resistance
Curve showing relationship of root penetration and penetration resistance. Source: Penn State University Extension.

Importance: Soil compaction results from repeated traffic, generally from machinery, or repeated tillage at the same depth, which results in a compacted layer at the tillage depth. Compaction inhibits infiltration, gas and water movement, may impede root growth, disrupts habitat for soil biota, and affects nutrient cycling. See Soil physical properties and processes for a discussion of bulk density.

Assessment There are multiple methods for measuring bulk density and compaction (resistance). See methods for measuring and methods for measuring compaction. Recommended methods of assessment include the following.

  • Penetrometer - a penetrometer is a portable, easy to use tool. The penetrometer is pushed into a soil and a gauge shows the resistance. Readings greater than 300 psi indicate conditions restrictive to root growth. If more than 50 percent of readings froma field exceed 300 psi, compaction is considered moderate; severe if more than 75 percent of readings exceed 300 psi. Typical cost of a penetrometer is about $200.
  • Bulk density - bulk density is a relatively easy to perform measurement but requires determining the water (moisture) content of the soil. Relationships of bulk density to root growth are shown in the adjacent table.

General relationship of soil bulk density to root growth based on soil texture
Link to this table

Soil texture Ideal bulk densities (g/cm3) Bulk densities that may affect plantgrowth (g/cm3) Bulk densities that restrict root growth (g/cm3)
sands, loamy sands <1.60 1.69 >1.80
sandy loams, loams <1.40 1.63 >1.80
sandy clay loams, loams, clay loams <1.40 1.60 >1.75
silts, silt loams <1.30 1.60 >1.75
silt loams, silty clay loams <1.40 1.55 >1.65
sandy clays, silty clays, clay loams with 35-45% clay <1.10 1.49 >1.58
clays (>45% clay) <1.10 1.39 >1.47


Water stable aggregates

Importance: Stable soil aggregates, in the presence of water, is important for water and air transport, root growth, habitat for soil biota, minimizing soil erodibility, protecting soil organic matter, and nutrient cycling.

Assessment: Methods for assessing aggregate stability are somewhat qualitative and different methods do not correlate well. The method selected should simulate field processes likely to affect aggregate stability (e.g. rainfall impact, ponded (flooded) conditions, tillage). For more information about aggregate stability tests, link here.

  • Sieve or strainer method: Aggregate stability is assessed by moving a sieve with aggregates up and down in a bucket of water. This method may represent stability under rainfall.
  • Slake test: Dried aggregates are placed into a container filled with water and aggregates are assessed after specified times. This method may represent stability under flooded (water immersed) conditions.
  • Vibration methods: An ultrasonic probe immersed in water containing soil aggregates vibrates at different vibration amplitudes. This method may represent stability under tillage conditions.

Soil structure crusting, and macroporosity

Available water capacity

Infiltration

Organic matter and organic carbon

Soil electrical conductivity

Biotic assessment (diversity)

Soil Enzymes

Soil Respiration

Plant roots

Nutrient status (fertility)

Information: An excellent resource applicable to a wide variety of vegetated stormwater BMPs, including bioretention BMPs, is Plants for stormwater design by Shaw and Schmidt (2003).

Evaluating the nutrient status of a soil focuses on determining if a soil is deficient in one or more macronutrients (nitrogen (N), potassium (K), sulfur (S), calcium (Ca), and magnesium (Mg)) or micronutrients (boron (B), zinc (Zn), manganese (Mn), iron (Fe), copper (Cu), molybdenum (Mo), and chloride (Cl)). Additional parameters may include organic matter, pH, soluble salts, and cation exchange capacity.

Importance: Soil nutrients are essential for plant growth and soil biotic processes essential to plant growth. Soil pH of 5-8 is typically acceptable for plant growth and biotic processes, but outside this range metals may be mobilized and other biologic processes adversely affected. Cation exchange capacity is a measure of a soils ability to retain nutrients that can be used by soil biota, including plants. Soluble salts may build up in soils after excess fertilizer applications, leading to drought stress in plants. Soil organic matter serves many functions in soil, including supplying nutrients, improving water storage and transport, improving soil structure and aggregation, and providing habitat for soil biota.

Assessment: Most Minnesota soils with organic matter are not deficient in soil micronutrients, Ca, Mg, or S. Thus, testing for organic matter, pH, N, P, and K is generally sufficient. Organic matter is analyzed in a laboratory, while the other parameters can be tested in the field. For purposes of assessing soil nutrient status or fertility, field tests are generally adequate. Lab tests provide more accurate results and some labs offer standard soil tests that assess soil fertility. Links to videos discussing and demonstrating field testing are provided below.

  • Organic matter content: Generally determined in the lab. See here for methods. A field method is described here. Organic matter concentrations between 4 and 6 percent, by weight, are ideal, with 2 to 8 percent being good. Soils with less than 2 percent organic matter may require fertilizer additions or organic amendments. Excess organic matter can lead to leaching of nutrients and in some cases, soil acidification.
  • pH: test strips or field meters are adequate. For lab analysis, link here. Soil pH should be 5-8, with a pH of about 6.5 optimum for most plants.

Video links for field testing

Further reading

pH

Soil contamination

Importance: Soils may contain concentrations of certain chemicals that are toxic to plants. Pollutants of greatest concern include metals (copper, lead, cadmium, nickel, zinc), sodium and chloride from road salt application, pesticides, and some hydrocarbons (e.g. oil, PAHs). Sites with known contamination may contain other pollutants, such as arsenic, but these soils are generally not suitable for stormwater applications without remediation.

Assessment: Risk assessments for metals concentrations in soil are generally based on human exposure, and there is limited information on toxic concentrations for different plants. Nevertheless, most urban soils do not contain chemicals at concentrations which restrict plant growth, although concentrations of these chemicals are typically greater than natural background ([1], [file:///C:/Users/franc/Downloads/environments-07-00098-v2.pdf], [2], [3], [4], [5], [6]). Chemical sampling is expensive, particularly for organic contaminants. An assessment of soil contamination should therefore begin with a site investigation to identify the presence of contaminant sources or historical activities that may have resulted in soil contamination.

  • Site visit: Conduct a site visit and determine if any of the following exist at the site - soil stockpiles, tanks or drums, odor(s), visual staining of soils, dead or dying vegetation, and debris that may be a source of contaminants.
  • Site review: Conduct a site review consisting of a search for nearby contaminated sites, site historical search to identify past uses, and review of historical aerial photos. Link here for more information and sources.
  • Assessment: If a site visit or assessment indicate potential contamination at a site, sampling may be warranted.

Regardless of the results for a site visit and site review, soil sampling is warranted for certain land use settings. The adjacent table provides a summary of potential pollutant concerns for specific land uses. If sampling is warranted, use appropriate sampling and test methods, described on this page.

Pollutants of Concern from Operations (adapted from CWP, 2005).
Link to this table.

Pollutant of concern Vehicle operations Waste management Site maintenance practices Outdoor materials Landscaping
Nutrients X X X
Pesticides X X
Solvents X X
Fuels X
Oil and grease X X
Toxic chemicals X X
Sediment X X X X
Road salt X X
Bacteria X X
Trace metals X X
Hydrocarbons X X


Relating indicators to function

Soil health - additional reading

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