Assessing soil health and function
Contents
- 1
- 1.1 Ability to support vegetative growth
- 1.1.1 Vegetative growth - Soil compaction (bulk density)
- 1.1.2 Vegetative growth - Water stable aggregates
- 1.1.3 Vegetative growth - Soil structure
- 1.1.4 Vegetative growth - plant roots
- 1.1.5 Vegetative growth - Nutrient status
- 1.1.6 Vegetative growth - pH
- 1.1.7 Vegetative growth - Soil contamination
- 1.1 Ability to support vegetative growth
Site is in development
| 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 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
Each of these is discussed below.
Ability to support vegetative growth
The following indicators can be used to assess suitability of a soil to support plant/vegetative growth.
Vegetative growth - Soil compaction (bulk density)
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 |
Vegetative growth - Water stable aggregates
Vegetative growth - Soil structure
Vegetative growth - plant roots
Vegetative growth - Nutrient status
Vegetative growth - pH
Vegetative growth - 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: Most urban soils do not contain chemicals at concentrations which restrict plant growth, although concentrations of these chemicals are typically greater than natural background. 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.
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 |