# Rapid Soil Tests for Assessing Soil Health
## 4 Discussion
### Choice of Soil Health Indicators  
For the soil health indicator (SHI) report, we used both Tier 1 and Tier 2 indicators, following the suggestions of the Soil Health Institute [37; 114]. Tier 1 indicators are compatible with the following criteria (i) widely considered effective to indicate soil health, (ii) defined regionally and by soil groupings, (iii) known thresholds to index outcome-based soil health status, and (iv) responsive to land use and management practices for soil function improvement. Tier 2 indicators are (i) proven relevant to soil health, (ii) impacting trends on soil health are clear, (iii) ranges and outcome-based thresholds are known for at least some regions, (iv) improvement strategies can be suggested, and (v) additional research may be needed for further validation. In Section A (physical soil health, Figure 2) of the SHI report, Tier 1 indicators are soil texture, soil pH, soil bulk density, soil electrical conductivity, cation exchange complex, base saturation, and available water-holding capacity, while Tier 2 indicators include soil sodium adsorption ratio. Section B of the SHI report (soil biology, Figure 2) includes mainly Tier 2 indicators, such as soil phospholipid fatty acid (PLFA) profiles. Section C (carbon characteristics, Figure 2) includes mainly Tier 1 indicators, i.e., soil organic carbon content. Section D (chemical soil health) mainly reports Tier 1 indicators, i.e., soil total nitrogen, extractable main nutrients (P, K, Ca, Mg), and micronutrients (Fe, Zn, Ni, Mn, Cu, B), and the beneficial nutrients Si and Co. Section E also presents Tier 1 indicators, i.e., potential contaminants, including aluminum (Al), arsenic (As), cadmium (Cd), chromium (Cr), and lead (Pb). For Section F(soil and crop husbandry management recommendations), the expected crop yield (Tier 1) has to be provided by the users of the soil tests, but the short-time mineralization rate (Tier 1) is being derived from soil test data and additional calculations. Two Tier 1 indicators, i.e., soil aggregate stability and saturated hydraulic conductivity, have not yet been included in the SHI, but efforts are made to include these indicators also in the SHI report; this requires additional calibration and validation work. The Tier 2 indicators nitrogen mineralization rate and strontium (Sr), as well as barium (Ba) contents, cannot be determined with our current soil tests, but titanium (Ti), vanadium (V), tin (Sn), and the beneficial element Se can be determined and are presented in the SHI report, although these elements are not part of Tier 1 or 2 according to the Soil Health Institute [37; 114]. Out of the long list of formal Tier 1 indicators, heavy metals will be relatively new for most soil test users. It appears that 0.01 M CaCl\({}_{2}\) is a very good extractant for evaluating the bioavailability of heavy metals in soil, especially the bioavailability for plants [115; 116]. Recently, [PERSON] et al. (2022) [117] found that Cd and Pb concentrations in soil invertebrates are best explained by 0.01 M CaCl\({}_{2}\) extractions. In general, elevated heavy metal concentrations in soil may affect biota adversely, including aquatic organisms, microorganisms, plants, animals and humans [118]. Also, the increased attention for food safety worldwide makes it necessary to monitor this aspect of soil health [119]. Soil salinity (and sodicity) indicators are also becoming more important because of their impact on crop growth and quality, and soil structure. The area of irrigated agriculture has increased greatly during recent decades, and, with the looming climate change, it is likely that the areas of land affected by salinization and/or sodification will increase further [120]. Thus, the soil salinity indicators, soil electrical conductivity and soil sodium adsorption ratio, i.e., Tier 1 and 2 indicators, respectively, are both in the SHI report (Figure 2).