| Main authors: | Else K. Bünemann, Giulia Bongiorno, Zhanguo Bai, Rachel E. Creamer, Gerlinde De Deyn, Ron de Goede, Luuk Fleskens, Violette Geissen, Thom W. Kuyper, Paul Mäder, Mirjam Pulleman, Wijnand Sukkel, Jan Willem van Groenigen and Lijbert Brussaard |
| Editor: | Jane Brandt |
| Source document: | Bünemann, E. K. et al. (2018) Soil quality - A critical review. Soil Biology and Biochemistry, Volume 120, May 2018, pp 105-125 |
To identify the most frequently proposed (combinations of) soil quality indicators, we summarized 62 publications in which 65 minimum datasets of measured soil properties have been proposed (Supplementary Table 2).
Supplementary Table 2: Soil quality assessment publications evaluated for frequency of suggested indicators by geographical origin
| Region | References |
| USA and Canada | Larson and Pierce (1991), Arshad and Coen (1992), Doran and Parkin (1994), Acton and Gregorich (1995), Doran and Parkin (1996), Harris et al. (1996), Carter et al. (1997), Wang et al. (1997), Karlen et al. (1998), Macdonald et al. (1998), Seybold et al. (1999), Andrews and Carroll (2001), Arshad and Martin (2002), Andrews et al. (2002), Andrews et al. (2004), Wienhold et al. (2004), Shukla et al. (2006), Idowu et al. (2008), Wienhold et al. (2009), Congreves et al. (2015), de Paul Obade and Lal (2016), Moebius-Clune et al. (2016) |
| New Zealand and Australia | Schipper and Sparling (2000), Southorn and Cattle (2000), Sparling and Schipper (2002), Cotching and Kidd (2010) |
| Europe | Torstensson et al. (1998), Stenberg et al. (1998), Martin et al. (1998), Kirchmann and Andersson (2001), Huber et al. (2001), Candinas et al. (2002), Loveland and Thompson (2002), Merrington (2006), Rutgers et al. (2008), Bone et al. (2010), Wattel-Koekkoek et al. (2012), Oberholzer et al. (2012), Armenise et al. (2013), Bone et al. (2014), D’Hose et al. (2014), Volchko et al. (2014), Askari and Holden (2015), Salomé et al. (2016) |
| Asia | Wan and Zhang (1991), Chen (1996), Chen (1999), Wang et al. (2001), Hu et al. (2001), Lou (2002), Xu et al. (2008), Yao et al. (2013), Ghaemi et al. (2014), Paz-Kagan et al. (2014), Chauhan and Mittu (2015), Biswas et al. (2017) |
| South America | Velasquez et al. (2007), Lima et al. (2013), Pulido Moncada et al. (2014) |
| Africa | Tesfahunegn (2014), Swanepoel et al. (2014) |
Due to the plethora of methods and terms, a certain aggregation of measured indicators into categories was required, e.g. aggregate stability, shear strength, tilth and friability, structure, consistence and slake test were merged in a category called structural stability (Supplementary Table 3).
Supplementary Table 3: Inclusion of indicator terms from soil quality assessment publications in indicator categories for the evaluation of indicator frequencies in Figure 4
| Indicator category | Indicator terms included | |
| physical |
water storage | water-holding capacity, water content, sorptivity, water-filled pore space, water retention, field capacity, permanent wilting point, plant-available water content, Ksat |
| bulk density | bulk density | |
| texture | particle-size distribution, soil texture (sand, silt, clay) | |
| structural stability | aggregate stability, shear strength, tilth and friability, structure, consistence, slake test | |
| soil depth | soil depth, topsoil depth, maximum rooting depth, layer thickness | |
| penetration resistance | penetration resistance, previous consolidation | |
| hydraulic conductivity | hydraulic conductivity | |
| porosity | porosity, macroporosity, air capacity | |
| aggregation | aggregation, aggregate size distribution, pedality | |
| infiltration | infiltration rate | |
| stone content | stone content | |
| soil temperature | soil temperature | |
| particle density | particle density | |
| surface characteristics | surface characteristics, surface conditions, surface residues | |
| clay characteristics | clay characteristics, mineralogy, water-dispersible clay, soil color (if related to clay characteristics) | |
| biopores | biopores | |
| 137Cesium distribution | 137Cesium distribution (as an indicator of erosion) | |
| chemical | total organic matter/carbon | total organic carbon, total organic matter, soil color and odor (if related to organic matter) |
| pH | pH | |
| available P | available P, often as part of nutrient availability | |
| available K | available K, often as part of nutrient availability or extractable Ca, Mg, K | |
| total N | total N, Ntot | |
| cation exchange capacity | cation exchange capacity, exchangeable cations | |
| electrical conductivity | electrical conductivity, electromagnetic ground conductivity | |
| available N | Available N, mineral nitrogen, often as part of nutrient availability | |
| heavy metals | heavy metals (total or available) | |
| other macronutrients | total and available Mg, S, Ca | |
| micronutrients | total and extractable Al, Fe, Mn and other micronutrients | |
| labile C and N | labile C and N, active C, particulate organic matter, oxidizable C, KMnO4-extractable C, light fraction C and N, water-extractable C, water-extractable organic N | |
| sodicity, salinity | sodicity, exchangeable Na (ESP), Na adsorption ratio, salinity | |
| base saturation | base saturation, exchangeable acidity | |
| carbonate content | carbonate content | |
| total P | total P | |
| total K | total K (part of a whole list of total elements) | |
| organic pollutants | organic pollutants, PAK, PCB, xenobiotics loadings, insecticides | |
| C/N | C/N | |
| biological | soil respiration | soil respiration |
| microbial biomass | microbial biomass, microbial C, microbial N, microbial P, substrate-induced respiration, bacterial biomass, fungal biomass | |
| N mineralization | N mineralization (aerobic or anaerobic), mineralizable N | |
| earthworms | earthworms (species or biomass), incl. potworms (Enchytraeidae), biopores if counted to assess earthworms | |
| enzyme activities | enzyme activities, phosphatase, urease, dehydrogenase | |
| root health | root health, soil-borne pest pressure, root system development | |
| nematodes | nematodes (functional groups, density, diversity) | |
| microbial diversity | microbial diversity, microbial community composition, Biolog, total species number, fatty acid profiles, bacterial diversity, DNA-based methods | |
| soil faunal diversity | soil faunal diversity, total species number, microarthropod diversity and density | |
| metabolic quotient | qCO2, respiration/microbial C, metabolic quotient | |
| microbial activity | microbial activity, bacterial activity, thymidine incorporation, odour as sign of microbial activity | |
| other microbial N cycling processes | potential denitrification, nitrification, denitrification, potential ammonium oxidation | |
| Cmic/TOC | Cmic/TOC | |
| N fixation/fixing bacteria | N fixation/fixing bacteria |
We included both peer-reviewed journal articles on soil quality assessment approaches and reports on national monitoring programs, aiming at global coverage. Considering that soil quality assessment includes many steps, from the definition of objectives via the selection of indicators to the interpretation of obtained indicator values, we only included studies that address more than one of these steps and thus have a certain conceptual and generalizable nature. Consequently, studies that are entirely limited to the comparison of a set of indicators in different management systems were excluded. Even though we may have missed some publications, especially from national assessment schemes, we noted that increasing the number of evaluated datasets from 45 to 65 during the compilation hardly changed the outcome. Therefore, we are confident that our evaluation shows a valid picture of which soil quality indicators are most used.
Figure 4
Total organic matter/carbon and pH are the most frequently proposed soil quality indicators (Figure 4), followed by available phosphorus, various indicators of water storage and bulk density (all mentioned in > 50% of reviewed indicator sets). Texture, available potassium and total nitrogen are also frequently used (> 40%). The average number of proposed indicators is 11 (supplementary tables 4 and 5), which is probably more than is feasible from a practical as well as a financial viewpoint under most circumstances. Therefore, a trend towards smaller indicator sets in recent years can be seen. However, the development of novel indicators, which can be applied on a high number of samples in a fast and cheap way, could change the picture in the future.
Supplementary Table 4 
Supplementary Table 5
In most publications, at least one indicator of each category (physical, chemical and biological) is included. These categories are typically represented automatically when all soil functions or soil-based ecosystem services are addressed. However, soil biological indicators were missing from 40% of the reviewed minimum datasets.
Soil physical indicators, especially those related to water storage, were frequently proposed in the early assessment schemes and again in the last 5 years, while they were less common in between (Supplementary Table 4). Among the soil chemical indicators, soil organic carbon content, pH, available P and K, total N, electrical conductivity, cation exchange capacity, and mineral N were proposed more often than all other indicators. Likewise, soil respiration, microbial biomass, N mineralization and earthworm density were more frequent among the biological indicators than the other 10 indicators that have been proposed at least once (Supplementary Table 5).
The explicit mentioning of extrinsic factors (Supplementary Table 5) such as climate, management or site data is surprisingly rare. In particular, yield, plant nutrient status and other measures of ecosystem services are very often not included. This means that soil quality assessment is typically not explicitly linked to ecosystem services or soil threats. An example of how to establish linkages between soil properties, soil functions and ecosystem services via correlations can be found in van Eekeren et al. (2010). Recent publications advocate indicators that are applicable to several soil processes (Bone et al., 2010). In Lima et al. (2013), for example, earthworms serve as indicators for both water and nutrient cycling. However, many of the other publications lack a clear conceptual and/or mechanistic relationship between indicators and soil functions and ecosystem services.
Note: For full references to papers quoted in this article see