Scientists use soil quality indicators 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.
Source: 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. The categories do not neatly align with the various soil functions, so integration is necessary. The table below shows the relationship between indicator type and soil function.
| Indicator category | Related soil function |
| 1. Chemical | Nutrient Cycling, Water Relations, Buffering |
| 2. Physical | Physical Stability and Support, Water Relations, Habitat |
| 3. Biological | Biodiversity, Nutrient Cycling, Filtering |
Organic matter, or more specifically soil carbon, transcends all three indicator categories and has the most widely recognised influence on soil quality. Organic matter is tied to all soil functions. It affects other indicators, such as aggregate stability (physical), nutrient retention and availability (chemical), and nutrient cycling (biological); and is itself an indicator of soil quality.
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 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; and levels of soil contaminants and their availability for uptake by animals and plants. Indicators include measures of:
- Electrical Conductivity
- Soil Nitrate
- Soil Reaction (pH)
- more…
Physical indicators provide information about soil hydrologic characteristics, such as water entry and retention, that 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:
- Aggregate Stability
- Available Water Capacity
- Bulk Density
- Infiltration
- Slaking
- Soil Crusts
- Soil Structure and Macropores
Biological indicators can tell us about the organisms that form the soil food web that 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). Indicators include measures of:
- Earthworms
- Particulate Organic Matter
- Potentially Mineralisable Nitrogen
- Respiration
- Soil Enzymes
- Total Organic Carbon
Source: http://soilquality.org
