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	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
Plant roots	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.

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.

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, 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. 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.

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.

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.

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.

Available water capacity

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.

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

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

Soil health - additional reading

• 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