|Indicators for determining soil health|
|Indicator||Function||Type of indicator||Test||Management strategies|
|Compaction/bulk density||H/E||P||FL||Amend with organic matter; tillage|
|Water stable aggregates||P|
|Infiltration||H||P||F||Amend soil with organic matter to increase (clayey soils) or decrease (sandy soils) infiltration rates|
|Soil structure, crusting, and macroporosity||H||P||F||Tillage; amend with organic matter|
|Available water capacity||H||P||Amend soil|
|Nutrient status||N||C||Amend with organic matter or fertilize|
|pH||N||C||Add lime for acidic soils, sulfur compound for basic soils|
|Soil contamination||C||Remediate or avoid contaminated areas if feasible|
|Soil electrical conductivity||C|
|Organic matter and organic carbon||N||C||Add organic matter|
|Biotic assessment (diversity)||B||B|
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.
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.
|Stormwater management goal1|
|Indicator||Infiltration||Plant selection||Runoff/erosion potential||Pollutant retention||BMP selection||GSI suitability2|
|Water stable aggregates||O||R|
|Soil structure, crusting, and macroporosity||O||R|
|Available water capacity||O|
|Soil electrical conductivity||O|
|Organic matter and organic carbon||R||X||X||X|
|Biotic assessment (diversity)||X|
We generally assess site and soil suitability on the ability of the site or soil to support a specific stormwater function. For example, we may want to know if a site or soil is suitable for infiltration or which green infrastructure practice is most suitable. The adjacent table provides a list of recommended soil indicators that can be used to assess site or soil suitability. Additional discussion is provided below.
Infiltration: Use one or more of the following indicators to determine if a site is suitable for stormwater infiltration.
Plant selection: Use one or more of the following indicators to determine which vegetation is suitable for a site or soil.
Runoff/erosion potential: Use one or more of the following indicators to determine if a site or soil has high erosion potential. Note that slope is not included as an indicator, but slope should be considered in addition to the indicators discussed below.
Pollutant retention: Use one or more of the following indicators to determine a site or soil's ability to retain pollutants.
This page provides a discussion for several soil health indicators and links to summary sheets for each indicator.
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.
Management If a soil is compacted based on penetrometer readings, or if bulk density for a particular soil type exceed the value in the table, the soil should be amended. Addition of organic matter is recommended for reducing compaction and bulk density, with tillage as another option. For information on soil compaction and alleviating compaction, link here.
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|
Importance: Stable soil aggregates, in the presence of water, are 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.
Management Aggregate stability can be improved through a combination of reduced tillage, addition of organic matter (e.g. compost), increasing the amount of crop residues in the soil, and providing soil cover.
Importance: affects water storage and transport of solutes and pollutants; adequate infiltration is required for certain stormwater practices.
Assessment: Direct measurement is recommended (e.g. permeameter, double ring infiltrometer)
Multiple measurements are highly recommended since the infiltration rate can vary by orders of magnitude over very short distances, even within a single soil series. The adjacent table can be used to assess suitability of a soil for stormwater infiltration bmps, with A and B soils suitable for infiltration and C soils suitable for partial infiltration.
Management If infiltration is limited due to compaction, then amending the soil with organic matter or tillage are recommended. For soils with naturally low (e.g. clay) or high (sand) infiltration rates, amend with organic matter (e.g. compost). Long-term infiltration can be enhanced with deep-rooted vegetation and enhancing biologic activity, both which promote macroporosity.
Design infiltration rates, in inches per hour, for A, B, C, and D soil groups. Corresponding USDA soil classification and Unified soil Classifications are included. Note that A and B soils have two infiltration rates that are a function of soil texture.*
The values shown in this table are for uncompacted soils. This table can be used as a guide to determine if a soil is compacted. For information on alleviating compacted soils, link here. If a soil is compacted, reduce the soil infiltration rate by one level (e.g. for a compacted B(SM) use the infiltration rate for a B(MH) soil).
Link to this table
|Hydrologic soil group||Infiltration rate (inches/hour)||Infiltration rate (centimeters/hour)||Soil textures||Corresponding Unified Soil ClassificationSuperscript text|
|Although a value of 1.63 inches per hour (4.14 centimeters per hour) may be used, it is Highly recommended that you conduct field infiltration tests or amend soils.b See Guidance for amending soils with rapid or high infiltration rates and Determining soil infiltration rates.||
|GW - Well-graded gravels, fine to coarse gravel
GP - Poorly graded gravel
GM - Silty gravel
SP - Poorly graded sand
|0.45||1.14||silty sands||SM - Silty sand|
|0.3||0.76||loam, silt loam||MH - Elastic silt|
|0.2||0.51||Sandy clay loam, silts||ML - Silt|
GC - Clayey gravel
1For Unified Soil Classification, we show the basic text for each soil type. For more detailed descriptions, see the following links: The Unified Soil Classification System, CALIFORNIA DEPARTMENT OF TRANSPORTATION (CALTRANS) UNIFIED SOIL CLASSIFICATION SYSTEM
Source: Thirty guidance manuals and many other stormwater references were reviewed to compile recommended infiltration rates. All of these sources use the following studies as the basis for their recommended infiltration rates: (1) Rawls, Brakensiek and Saxton (1982); (2) Rawls, Gimenez and Grossman (1998); (3) Bouwer and Rice (1984); and (4) Urban Hydrology for Small Watersheds (NRCS). SWWD, 2005, provides field documented data that supports the proposed infiltration rates. (view reference list)
aThis rate is consistent with the infiltration rate provided for the lower end of the Hydrologic Soil Group A soils in the Stormwater post-construction technical standards, Wisconsin Department of Natural Resources Conservation Practice Standards.
bThe infiltration rates in this table are recommended values for sizing stormwater practices based on information collected from soil borings or pits. A group of technical experts developed the table for the original Minnesota Stormwater Manual in 2005. Additional technical review resulted in an update to the table in 2011. Over the past 5 to 7 years, several government agencies revised or developed guidance for designing infiltration practices. Several states now require or strongly recommend field infiltration tests. Examples include North Carolina, New York, Georgia, and the City of Philadelphia. The states of Washington and Maine strongly recommend field testing for infiltration rates, but both states allow grain size analyses in the determination of infiltration rates. The Minnesota Stormwater Manual strongly recommends field testing for infiltration rate, but allows information from soil borings or pits to be used in determining infiltration rate. A literature review suggests the values in the design infiltration rate table are not appropriate for soils with very high infiltration rates. This includes gravels, sandy gravels, and uniformly graded sands. Infiltration rates for these geologic materials are higher than indicated in the table.
References: Clapp, R. B., and George M. Hornberger. 1978. Empirical equations for some soil hydraulic properties. Water Resources Research. 14:4:601–604; Moynihan, K., and Vasconcelos, J. 2014. SWMM Modeling of a Rural Watershed in the Lower Coastal Plains of the United States. Journal of Water Management Modeling. C372; Rawls, W.J., D. Gimenez, and R. Grossman. 1998. Use of soil texture, bulk density and slope of the water retention curve to predict saturated hydraulic conductivity Transactions of the ASAE. VOL. 41(4): 983-988; Saxton, K.E., and W. J. Rawls. 2005. Soil Water Characteristic Estimates by Texture and Organic Matter for Hydrologic Solutions. Soil Science Society of America Journal. 70:5:1569-1578.
Recommended number of soil borings, pits or permeameter tests for bioretention design. Designers select one of these methods.
Link to this table
|Surface area of stormwater control measure (BMP)(ft2)||Borings||Pits||Permeameter tests|
|1000 to 5000||2||2||10|
|5000 to 10000||3||3||15|
1an additional soil boring or pit should be completed for each additional 2,500 ft2 above 12,500 ft2
2an additional five permeameter tests should be completed for each additional 5,000 ft2 above 15,000 ft2
Importance: Soil functions related to soil structure include sustaining biological productivity, regulating and partitioning water and solute flow, cycling and storing nutrients, water and air exchange, plant root development, and habitat for soil organisms. Soil crusts can impede infiltration and water and air exchange and transport. Macropores can enhance air and water transport, but may result in short-circuiting (bypass) of water and solutes, including pollutants, to deeper depths within the soil profile.
Management: Amending soil with organic matter (e.g. compost) is recommended for soils with poor structure. Organic matter may also stimulate biologic activity that results in increased macroporosity. Tillage can alleviate surface crusting and improve soil structure in the tillage layer, but will disrupt macropores. Utilizing deep rooted vegetation increases macroporosity over time.
Importance: Available water capacity is the amount of water available between a soil's field capacity and wilting point, or the maximum plant available water that a soil can hold. For areas subject to periodic dry spells or regions where seasonal evapotranspiration can exceed rainfall, a soil's ability to store water is critical to plant growth. Loam soils have the highest water holding capacity.
Assessment To calculate water holding capacity, a soil's water content at field capacity and wilting point must be determined. These are typically determined in the lab, though field measurement is possible using methods such as near-infrared reflectance. Field collection of undisturbed cores is preferred, but soils are sometimes repacked in the lab. Measurement at field capacity involves saturating a soil column and then allowing it to drain for 48 hours. A pressure membrane is required to determine the soil wilting point. Water contents are measured after drainage (field capacity) and at -15 bars pressure (pressure plate), with the difference being water holding capacity.
Management Organic matter (e.g. compost) can be added to a soil to increase water holding capacity.
Importance: Soil organic matter (SOM) is the organic matter component of soil, consisting of plant and animal material, including cells, tissues, and substances produced or synthesized by soil organisms. Organic matter exists at varying levels of decomposition. Organic carbon is a component of soil organic matter, making up about 60 percent of soil organic matter. Organic matter provides numerous benefits to soil function, including improvement of soil structure, aggregation, water retention, soil biodiversity, absorption and retention of pollutants, buffering capacity, and the cycling and storage of plant nutrients. Soil organic carbon is the primary food (energy) source for soil microbes.
Assessment: Soil organic matter and carbon are generally determined in the laboratory. Because organic matter varies with depth, it may be important to determine soil organic matter at discrete depths. Preferred ranges for soil organic matter are 4-8 percent by weight, with 2 percent being a recommended lower limit. Common methods of analysis are provided below.
Management: Although soil organic matter can be increased by simply adding organic material to a soil, the choice of material is important since materials vary in their rate of decomposition, nutritional value, effects on pH, and other factors. Below are some commonly used organic amendments.
Importance: Soil electrical conductivity (EC) is a measure of the amount of salts in soil (salinity of soil). It is an indicator of nutrient availability and loss, soil texture, and available water capacity, which in turn affect plant growth and biotic processes. Excessive salts hinder plant growth by affecting the soil and water balance. Excess soil sodium leads to poor structure and surface crusting.
Assessment: EC does not provide a direct measurement of specific ions or salt compounds, but it has been correlated to concentrations of nitrates, potassium, sodium, chloride, sulfate, and ammonia. EC is expressed in milliSiemens per meter (mS/m) or deciSiemens per meter (dS/m).
An optimum EC value is approximately 0.5-1.0 dS/m, though plants vary in their tolerance to soil salinity. EC values above 1.0 dS/m are considered to represent conditions that may begin to adversely affect many plant species, while values above 2 represent saline soils. These levels also increase production of nitrous oxide (N2O) gas from denitrification under anaerobic conditions.
Management: Soil EC is affected by cropping, irrigation, land use, and application of fertilizer, manure, and compost. Addition of organic matter or inorganic fertilizers increases EC. Repeated irrigation that does not adequately leach soluble salts increases EC. Monitor soils that are amended and irrigated to ensure EC values are appropriate. Salt buildup is more likely in fine-textured soils (e.g. clays) where drainage is often poor.
Importance: Soil biota consist of the micro-organisms (bacteria, fungi, archaea and algae), soil animals (protozoa, nematodes, mites, springtails, spiders, insects, and earthworms) and plants living all or part of their lives in or on the soil. Soil biota mediate several functions, including the formation, turnover and mineralization of soil organic matter; nutrient cycling; disease transmission and prevention; pollutant degradation; and improvement of soil structure.
Assessment: There are two basic types of tests for soil biota.
For taxonomic tests, the following biodiversity parameters are often used to assess soil biotic health.
Biodiversity is typically determined by comparison of the test site or soil with a reference site or soil, usually a healthy, undisturbed site or soil or soils from similar land uses.
Management Many activities decrease soil biotic diversity and function, including erosion, soil compaction, tillage, use of pesticides, nutrient depletion, and poor plant management. Practices that minimize these will maintain or improve soil biotic diversity.
Importance: Soil enzymes are protein structured molecules that increase chemical reaction rates by catalyzing reactions without any permanent transformation. Soil enzymes play an important role in organic matter decomposition and nutrient cycling, facilitate the breakdown of organic matter, and are involved in nutrient mineralization. Absence or suppression of soil enzymes prevents or reduces processes that can affect plant nutrition. Poor enzyme activity can lead to accumulation of chemicals harmful to the environment.
Assessment: Enzymes are measured indirectly by determining their activity in the laboratory using biochemical assays. Several methods are available depending on the objectives of the analysis. Assessment is typically for one of four enzymes - amidase (carbon and nitrogen), urease (nitrogen), phosphatase (phosphorus), and sulfatase (sulfur). Results of enzyme assays are typically expressed as mass of product produced from the assay reaction, per unit time (e.g. ug-glucose released per g-soil in 24 hours). Higher values indicate greater enzyme activity. A sampled soil is often compared to an undisturbed soil as an indicator of the sampled soil's health.
Management: Organic amendment applications, crop rotation, cover crops, and reduced tillage have been shown to enhance enzyme activity in agricultural settings. Enzyme activity is affected by amendments to adjust soil pH, such as addition of lime (Bandick and Dick, 1999).
Importance: Carbon dioxide (CO2) release from the soil surface is referred to as soil respiration. Respiration is a measure of microbial activity, soil organic matter content, and decomposition. Unlike concentrations of a chemical such as nitrogen or phosphorus, respiration is an indicator of soil microbial biomass, nutrient cycling, the soil's ability to sustain plant growth. It can also be used as an indicator of a factor or factors that are limiting biologic activity, such as low temperatures, saturated or droughty soil, or insufficient available nitrogen.
Assessment: Soil respiration is often assessed by measuring changes in carbon dioxide (CO2) concentration within a controlled volume over some period of time. There are several methods described in the literature, Below are commonly discussed methods.
Management: Management activities that increase soil respiration include the following.
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.
Video links for field testing
Importance: Soil pH generally refers to the degree of soil acidity or alkalinity. Soil pH affects many soil processes, such as nutrient cycling, mobility of metals, and biological activity. In acid soils, calcium, magnesium, nitrate-nitrogen, phosphorus, boron, and molybdenum are deficient, while aluminum and manganese may occur at levels toxic to some plants. Phosphorus, iron, copper, zinc, and boron may be deficient in alkaline soils.
Assessment: While pH may be measured in a laboratory, field tests are simple and reasonably accurate. Field tests are therefore recommended. Field tests include the use of test strips or portable meters. If measured with a meter, ensure the meter is properly calibrated. Optimal pH for most plants and soil biota is 5.5-8.0, with 6.0-7.5 generally preferred. Soil plants (e.g. roses, azaleas) prefer acidic soils. If selecting vegetation, ensure the soil pH is appropriate.
Management: Liming is typically recommended for increasing soil pH. Ash or some organic residues rich in basic cations may also be used to raise soil pH. Soil pH can be lowered by adding organic matter (e.g. compost, peat, acid moss, pine needles, sawdust) or applying ammonium based fertilizers, urea, or sulfur/ferrous sulfate. Increasing organic matter increases buffering capacity, which helps prevent rapid fluctuations in soil pH.
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 (, [file:///C:/Users/franc/Downloads/environments-07-00098-v2.pdf], , , , , ). 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.
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|
|Oil and grease||X||X|
Cornell University has developed a manual for soil health and soil health assessment. The manual provides detailed discussion of several soil indicators, methods of analysis including sample collection, interpretation of results, and management strategies. Some useful links to this information include the following.
Additional reading is summarized below.