Assessing soil health and function

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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
Soil respiration	B	B
Soil enzymes	B	B
Biotic assessment (diversity)	B	B
1 B=biologic function; N=nutrient cycling function; H=hydrologic function; E=erosion control function 2 P=physical; C=chemical; B=biologic 3 F=field test; L=lab test; FL=field and lab test; O=office evaluation; () indicates test can be done but is not recommended (e.g. F(L) means test can be done in lab but field is recommended 4 management strategies focus on the primary function but are generally applicable to all functions

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.

How do I assess soil health

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

• Ability to support biologic function, including plant growth, soil biota, and species diversity (B)
• Ability to support nutrient cycling, pollutant attenuation (N)
• Ability to support hydraulic/hydrologic function (H)
• Ability to minimize erosion (E)

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.

Which indicator should I use?

Stormwater management goal1
Indicator	Infiltration	Plant selection	Runoff/erosion potential	Pollutant retention	BMP selection	GSI suitability2
Compaction/bulk density	A		O
Water stable aggregates		O	R
Infiltration	R		R	X	X	X
Soil structure, crusting, and macroporosity	O		R
Available water capacity		O
Nutrient status		R			X	X
pH		O
Soil contamination		O		X	X
Soil electrical conductivity		O
Organic matter and organic carbon		R		X	X	X
Soil respiration				X
Soil enzymes				X
Biotic assessment (diversity)				X
1 R=recommended indicator; A=alternative or optional to recommended indicator 2 GSI=green stormwater infrastructure

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.

• Infiltration - recommended. Direct measurement of infiltration (e.g. permeameter, double ring infiltrometer) rates in a soil or at a site.
• Soil compaction/bulk density - alternative or optional. Not a direct measurement of infiltration, but compacted soils will likely restrict infiltration.
• Soil structure, crusting, and macroporosity - alternative or optional. Not a direct measurement of infiltration, but soils with poor structure, surface crusted soils, or soils with low macroporosity will likely restrict infiltration.

Plant selection: Use one or more of the following indicators to determine which vegetation is suitable for a site or soil.

• Organic matter and organic carbon - recommended. Soils with adequate organic matter (2% or more; 4-8% preferred) will generally provide soil structure, water availability, and nutrients suitable for plant growth.
• Nutrient status - recommended. Soils with suitable concentrations of macronutrients will generally be suitable for plant growth. Micronutrient analysis might be appropriate for some soils.

• pH - alternative or optional. pH of most Minnesota soils is suitable for plant growth. Certain species may require pH adjustments to a soil. Some natural soils may be acidic, while some soils may be influenced by human activity (e.g. soils that have received organic fertilizers, lime, or amendments that affect soil pH).
• Available water capacity - alternative or optional. Sandy or clayey soils or soils with less than 2% organic matter may exhibit reduced available water capacity and should be tested.
• Soil contamination - alternative or optional. Sites with known or likely contamination should be evaluated to determine if toxic conditions exist for plant growth,
• Soil electrical conductivity - alternative or optional. Soils that have been heavily fertilized, particularly with organic material, soils that have been irrigated, and clay soils in dry climates should be tested.

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.

• Infiltration - recommended. Use direct measurements of infiltration (e.g. permeameter, double ring infiltrometer) to indicate the rate at which water will infiltrate and therefore not be available for runoff.
• Water stable aggregates - recommended. This measurement should be taken near the soil surface, as unstable soil aggregates will likely lead to soil crusting unless surface cover exists.
• Soil structure, crusting, and macroporosity - recommended. Surface crusting is an important indicator of runoff potential.
• Soil compaction (bulk density) - alternative or optional. While soil density and compaction are good indicators of infiltration, they are not direct measurements.

Pollutant retention: Use one or more of the following indicators to determine a site or soil's ability to retain pollutants.

• Infiltration - alternative or optional. On sandy and gravelly soils, use direct measurements of infiltration (e.g. permeameter, double ring infiltrometer). These soils may have very high infiltration rates that restrict retention of pollutants.

Links to individual indicator sheets

• Soil compaction (bulk density)

This page provides a discussion for several soil health indicators and links to summary sheets for each indicator.

Discussion of soil indicators

Soil compaction (bulk density)

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 from a 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.

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

Water stable aggregates

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.

• 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. Soil stability is evaluated by measuring the mass of soil remaining, as a percent of initial soil mass, on the sieve after a specified number of dipping cycles (e.g. 5 cycles). Soils with high aggregate stability will typically retain 50 percent or more of the initial soil mass.
• Slake test: Dried aggregates are placed into a container filled with water and aggregates are assessed after specified times (e.g. 20 minute intervals). This method may represent stability under flooded (water immersed) conditions. This is a qualitative test.
• Vibration methods: An ultrasonic probe immersed in water containing soil aggregates vibrates at different vibration amplitudes. This method may represent stability under tillage conditions. Vibration methods relate stability to the energy needed to break down aggregates.

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.

Recommended reading

• Soil Quality Indicators: Aggregate Stability
• Aggregation and Aggregate Stability
• Soil aggregate stability

Infiltration

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)

• Permeameter: An Initial soil moisture content measurement is determined, then the permeameter cylinder is filled with water. As the permeameter drains, measurements of stage and time are taken. Then, a final measurement of soil moisture content is aggregated into a post-processing spreadsheet, where saturated hydraulic conductivity is calculated.
• Double ring infiltrometer: Two rings (e.g. 20 and 30 cm diameter) are driven into the soil. A constant water level is maintained in the outer ring. The water level in the inner ring may be maintained at a constant level or a falling level. Saturated conductivity is achieved when the rate of infiltration in the inner ring reaches a constant.
• Textural analysis: Soil borings or pits are used to determine soil texture.

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.

For more information, see Determining soil infiltration rates. Links to videos demonstrating direct measurements can be found here.

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.

Caution: The table for design infiltration rates has been modified. Field testing is recommended for gravelly soils (HSG A; GW and GP soils; gravel and sandy gravel soils). If field-measured soil infiltration rates exceed 8.3 inches per hour, the Construction Stormwater permit requires the soils be amended. Guidance on amending these soils can be found here.

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
A	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.		gravel sandy gravel	GW - Well-graded gravels, fine to coarse gravel GP - Poorly graded gravel
	1.63a	4.14	silty gravels gravelly sands sand	GM - Silty gravel SW - Well-graded sand, fine to coarse sand
	0.8	2.03	sand loamy sand sandy loam	SP - Poorly graded sand
B	0.45	1.14	silty sands	SM - Silty sand
	0.3	0.76	loam, silt loam	MH - Elastic silt
C	0.2	0.51	Sandy clay loam, silts	ML - Silt
D	0.06	0.15	clay loam silty clay loam sandy clay silty clay clay	GC - Clayey gravel SC - Clayey sand CL - Lean clay OL - Organic silt CH - Fat clay OH - Organic clay, organic silt

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

• NOTE that this table has been updated from Version 2.X of the Minnesota Stormwater Manual. The higher infiltration rate for B soils was decreased from 0.6 inches per hour to 0.45 inches per hour and a value of 0.06 is used for D soils (instead of < 0.2 in/hr).
  

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	1	1	5
1000 to 5000	2	2	10
5000 to 10000	3	3	15
>10000	41	41	202

¹an additional soil boring or pit should be completed for each additional 2,500 ft² above 12,500 ft^2
2an additional five permeameter tests should be completed for each additional 5,000 ft² above 15,000 ft²

Soil structure, crusting, and macroporosity

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.

Assessment:

• Soil structure is a field qualitative study to assess the dominant type of soil ped. The assessment should be performed by a professional soil scientist or someone experienced in evaluating soil structure. The USDA-NRCS publication Field Book for Describing and Sampling Soils, version 3.0 (pages 2-52 to 2-61), or the USDA-NRCS Soil Field Test Guide (pages 24-26 and 76-77) provide descriptions, tables, and schematics that can be used to assess soil structure. Non-granular soils exhibiting weak platy or columnar structure indicate poor soil structure, versus strong blocky structures.
• Surface soil crusts are qualitatively evaluated in the field. The assessment should be performed by a professional soil scientist or someone experienced in evaluating soils. The USDA-NRCS publication Field Book for Describing and Sampling Soils, version 3.0 (pages 2-77 to 2-79) provide descriptions, tables, and schematics that can be used to assess surface crusting. Surface crusts are often tested for their stability, similar to tests of aggregate stability. Crusts that are stable indicate conditions likely to restrict infiltration and water and air exchange.
• Macroporosity: Soil macroporosity can be quantified in the field and expressed as a percent of surface area or soil volume. Typical methods include measuring the number and area of soil macropores (pores > 75 microns in diameter) in different soil layers, which can then be converted to a soil volume or a percent of soil volume. Examples of field techniques are described in several papers ([1], [2], [3], [4], [5]). Desirable values for macroporosity are not well defined in the literature, though a value of 10 percent or greater is cited in the literature ([6], [7]). Higher values are more desirable in clay soils or soils with poor structure, where water and air transport can be restricted.

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.

Available water capacity

General relationship between soil moisture and texture. Ohio Agronomy Guide, 14th edition, Bulletin 472-05.

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.

Recommended reading

• Soil Quality Indicators - Available Water Capacity - USDA-NRCS
• Available Water Capacity - Soil Quality for Environmental Health
• Available Water Capacity - Cornell University
• Fact sheets - water availability - Healthy Soils for Sustainable Farms
• Available Water Capacity - What is Available Water Capacity? - Soil Health Nexus
• Soil and water - FAO

Organic matter and organic carbon

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.

• Organic carbon combustion method
• Organic matter Walkley-Black method
• Organic matter loss on ignition method - recommended due to ease of use and since organic matter is more commonly used compared to organic carbon

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.

• Compost: Aged compost is recommended. Compost has moderate hydrologic, physical, and nutrient benefit.
• Manure: Manure must be composted prior to use to reduce pathogen and ammonia concerns. Composted manure has good nutrient benefit and low to moderate hydrologic and physical benefit.
• Biosolids: Class A biosolids have been treated to reduce concerns with pathogens. These biosolids provide moderate hydrologic and nutrient benefits. Heavy metals are a concern.
• Wood products: These provide hydrologic and structural benefits to soil but can tie up nitrogen. To avoid nutrient deficiency compost these before using and consider adding a nitrogen fertilizer.
• Peat: Peat has very good hydrologic benefits. It has limited nutrient benefit, can acidify soils. Most importantly, it is considered a nonrenewable resource.

For more information, see [8], [9], [10], and [11].

Further reading

• Measuring and managing soil organic matter
• Soil organic matter - USDA-NRCS
• Soil organic matter - Cornell University
• Soil organic matter in cropping systems - University of Minnesota
• What Does Organic Matter Do In Soil? - Noble Research Institute
• Why Soil Organic Matter Is So Important - Sustainable Agriculture Research and Education

Soil electrical conductivity

Schematic illustrating salt accumulation in soil. Image source: Oxford Research Encyclopedia of Environmental Science

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).

• Field methods include the use of sensors which can be inserted directly into the soil (contact sensors) or placed on the soil surface (non-contact sensors)(see descriptions here). Contact sensors are more accurate but involve more effort to employ. EC is affected by soil water content, temperature, texture, and porosity, in addition to salt concentration, so these must be considered in the analysis.
• EC measurements of soil water are often preferred as they better reflect conditions affecting plant growth. Soil water is extracted from a soil (e.g. with a lysimeter) and EC is measured with a calibrated probe.
• The most common lab method consists of collecting a soil sample and creating a saturated paste or slurry in the lab. This is considered an accurate method but involves sample collection and lab analysis. For more information, see here or here.

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.

Recommended reading

• Soil electrical conductivity - USDA-NRCS
• The Ultimate Guide to Testing Electrical Conductivity of Soil
• Using Soil Electrical Conductivity (EC) to Delineate Field Variation
• Water and Soil Characterization - pH and Electrical Conductivity
• Soil electrical conductivity: a beginner’s guide to measurements

Biotic assessment (diversity)

Example of a soil food web. Image source: 2012 Nature Education Adapted from Gupta et al. 1997.

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.

• Taxonomic tests analyze and classify parts of the microbial material itself. Examples of taxonomic tests include DNA fingerprinting, pitfall trapping of surface-active invertebrates, and monoliths.
• Functional tests look at the metabolic products of microbes (e.g. respiration).

For taxonomic tests, the following biodiversity parameters are often used to assess soil biotic health.

• Taxonomic richness at species and strain (rhizobia) level
• Abundance and biomass of taxa
• Abundance and biomass of functional groups (FGs)
• Relative proportions of FGs
• Shannon-Wiener Index, Simpsons and associated evenness

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.

Recommended reading

• The Soil Biota
• Use of soil biota in the assessment of the ecological potential of urban soils
• Standard methods for assessment of soil biodiversity and land use practice
• Rangeland Soil Quality—Soil Biota - USDA-NRCS
• How can we measure soil biodiversity?

Soil Enzymes

Soil enzymes. Image source: Environmental Brigade

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).

Recommended reading

• Soil Enzymes : Their Importance And Classes
• Importance of Soil Enzymes
• Soil enzymes
• Soil Enzymes: Importance, State and Kinds
• History and Interpretation of Soil Enzyme Activity

Soil Respiration

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.

• Gas chromatography
• Infrared gas analyzer -
• Chemical titration - potassium hydroxide or sodium hydroxide
• Colorimetric techniques

Management: Management activities that increase soil respiration include the following.

• Reduced tillage or conservation tillage (no-till, strip-till, mulch till, etc.)
• Application of organic matter
• Rotations with high residue and deep-rooted plants
• Cover and green manure plants
• Irrigation or drainage
• Controlled traffic

Recommended reading

• Interpreting, measuring, and modeling soil respiration
• Talking SMAAC: A New Tool to Measure Soil Respiration and Microbial Activity
• Examination of four methods for measuring soil respiration
• Solvita field test kit
• Measuring Soil Respiration with an infrared gas analyzer

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

• Soil Test - pH and NPK Nitrogen Phosphorus and Potassium
• Best Home Soil Test Kit for NPK - 3 Test Results Compared
• Home Soil Test vs Lab Soil Test. Demonstration and review
• Soil Test Kit Review - Which is the best soil test?
• In Hand Soil Test Method - Nitrogen, Phosphorus, pH

Further reading

• Micronutrients
• Soluble salts in soil and plant health
• Soil Test Interpretation Guide
• Assessing soil fertility; the importance of soil analysis and its interpretation
• Soil Quality Indicator Sheets - see Chemical Indicators

pH

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.

Recommended reading

• Soil pH - USDA-NRCS
• Soil pH
• Managing Soil pH in Horticultural Crops
• Soil pH Management
• Changing the pH of your soil
• MEASUREMENT AND MANAGEMENT OF SOIL PH FOR CROP PRODUCTION IN DELAWARE
• Measuring and managing soil 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 ([12], [file:///C:/Users/franc/Downloads/environments-07-00098-v2.pdf], [13], [14], [15], [16], [17]). 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

Soil health - additional reading

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.

• Soil Health Assessment
• Soil Health Concepts
• Soil Health Management
• Soil health fact sheets
• Example soil health report
• Main page, including links to the full document
• Introduction to the Soil Health Assessment Framework

Additional reading is summarized below.

• Soil Health Assessment - USDA-NRCS
• Soil Quality Indicator Sheets - USDA-NRCS
• Using Healthy Soils to Manage Stormwater - American Society of Landscape Architects
• Soil Health Fact Sheets – Ocean County, Stormwater Basins
• Soil Health - USDA - NRCS
• Top Healthy Soil Resources - Ocean County, New Jersey, Soil Conservation District
• Soil Health Indicators - University of Minnesota
• Minnesota Office for Soil Health
• Soil Health Project Update - Minnesota Office for Soil Health
• Soil health - Resources and publications - USDA-NRCS