Soil Quality Indicators: Measures of Soil Functional State
Scientists use soil quality indicatorsA soil quality indicator is a chemical, physical or biological property of soil that is sensitive to disturbance and represents performance of ecosystem function in the soil of interest. Indicators are dynamic soil properties. to evaluate how well soil functions since soil function often cannot be directly measured. Measuring soil quality is an exercise in identifying soil properties that are responsive to management, affect or correlate with environmental outcomes, and are capable of being precisely measured within certain technical and economic constraints. Soil quality indicators may be qualitative (e.g. drainage is fast) or quantitative (infiltration= 2.5 in/hr).
Ideal indicators should:
- correlate well with ecosystem processes
- integrate soil physical, chemical, and biological properties & processes
- be accessible to many users
- be sensitive to management & climate
- be components of existing databases
- be interpretable
Doran and Parkin, 1996
There are three main categories of soil indicators: chemical, physical and biological. Typical soil tests only look at chemical indicators. Soil quality attempts to integrate all three types of indicators. These categories do not neatly align with the various soil functions. That is why integration is necessary. The table below shows the relationship between indicator type and soil function.
| Indicator category | Related soil function |
|---|---|
| Chemical | Nutrient cycling, Water relations, Buffering |
| Physical | Physical stability and support, Water relations, Habitat |
| Biological | Biodiversity, Nutrient cycling, Filtering |
Some indicators are descriptive and can be used in the field as part of a health card. Others must be measured using laboratory analyses. Some examples of indicators (and indicator groupings) that fall into the three broad categories of chemical, physical and biological, are provided below.
Indicator Categories
Chemical indicators can give you information about the equilibrium between soil solution (soil water and nutrients) and exchange sites (clay particles, organic matter);plant health; the nutritional requirements of plant and soil animal communities, levels of soil contaminants and their availability for uptake by animals and plants. Indicators include:
- Organic matter measures, such as total organic carbon and total nitrogen
- Soil fertility measures, such as soil test phosphorus, mineral nitrogen, exchangeable potassium, and micronutrients
- Soil reaction (pH)
- Measures of toxic contaminants, such as arsenic or copper.
- Measures of salinity or sodicity, such as electrical conductivity, sodium adsorption ratio, and exchangeable sodium percentage
Physical indicators provide information about soil hydrologic characteristics such as water entry and retention which influences availability to plants. Some indicators are related to nutrient availability by their influence on rooting volume and aeration status. Other measures tells us about erosional status. Indicators include measures of:
- Water flow into and through soil, such as porosity, water infiltration rate and water retention
- Soil structure, such as porosity, bulk density, and soil depth
- Soil aggregate size
- Physical stability, such as aggregate stability, soil depth, or soil loss
Biological indicators can tell us about the organisms that form the soil food web, which are responsible for decomposition of organic matter and nutrient cycling. Information about the numbers of organisms, both individuals and species, that perform similar jobs or niches, can indicate a soil's ability to function or bounce back after disturbance (resistance and resilience). The full consequences of extinction of organisms are unknown. Indicators include measures of:
- Active organic matter pools, such as microbial biomass carbon or respiration
- Diversity indices for various soil organism populations, such as earthworms, nematodes, and microarthropods
- Biological activity, such as enzyme activity, potentially mineralizable nitrogen or respiration (CO2 production)
Doran, J.W. and T.B. Parkin. 1996. Quantitative indicators of soil quality: a minimum data set. In J.W. Doran and A.J. Jones, eds. Methods for Assessing Soil Quality. SSSA, Inc., Madison, Wisconsin, USA.