# Rapid Soil Tests for Assessing Soil Health
## 1 Introduction  
Soil testing is a tool for crop growers and land managers to help optimize fertilization of soil-crop systems, and thereby to optimize the soil fertility level and crop production [1]. The focus in soil testing is often on the main essential nutrient elements phosphorus (P), potassium (K), and nitrogen (N), soil pH, and soil organic matter, although it is well-known that there are up to 17 essential nutrients and possibly more elements potentially limiting plant growth and development, and that soil water holding capacity, soil infiltration and drainage, soil depth, soil structure, soil bulk density, heavy metal contents and soil faunal and microbial diversity may affect crop growth and development [2, 3]. Next to the crop production function, soils have a range of other functions, which are generally defined as'soil ecosystem services' [4, 5], and which receive increasing attention from governments, land management, agrifood industries, and nature conservation agencies because of their role in sustainable land management. That is why the attention in soil management has shifted over time from soil fertility to soil quality to soil health, e.g., [6, 7], and why there is increasing interest in soil health assessments. Soil health is commonly defined as thecontinued capacity of soils to contribute to ecosystem services [8]. Soil health reflects the multifunctionality of soils. As a consequence, a range of soil characteristics has to be quantified for an adequate soil health assessment. In addition to land users and governments, soil health also gained a lot of interest from the agrifood-industry, including the fertilizer industry (for example Yara [9] (Olso, Norway), LCL [10] (Tel Aviv, Israel), Anglo America [11] (London, UK)), the potato industry [12], dairy industry [13; 14], the seed industry [15; 16], and crop protection industry [17; 18]. These companies all accentuate their commitment to maintain or increase soil health. Agrifood companies like Nestle [19] (Vevey, Switzerland), Cargill [20] (Wayzata, MN, USA), Kraft Heinz [21] (Chicago, IL, USA), Carlsberg [22] (Copenhagen, Denmark), PepsiCo [23] (Purchase, NY, USA), and supermarkets including Aholddelhaize [24] (Zaandam, The Netherlands), Lidl [25] (Neckarsulm, Germany), and Waltmart [26] (Bentonville, AR, USA) do the same. Some of these companies have their own advisors to jointly work with land users on soil health, sometimes in collaboration with extension services. Consultancy firms like Arvesta [27] (Leuven, Belgium) Agrifirm [28] (Apeldoorn, The Netherlands), ProAgria [29] (Helsinki, Finland), Procam [30] (Lavenham, UK), and Seges [31] (Aarhus, Denmark) all significantly increased their signature on soil health. [PERSON] et al. [32] called these agricultural extension services the 'foundation of sustainable agricultural development'  
The importance of soil health assessment is also recognized by governments. Governmental agencies in some countries recently launched soil health monitoring projects (e.g., Canada [33], and the Netherlands [34]). Further, some 193 counties recently agreed on the seventeen Sustainable Development Goals (UN-SDGs), and soil health is integral to achieving many of these UN-SDGs, including zero hunger (SDG2), good health wellbeing (SDG3), clean water and sanitation (SDG6), sustainable consumption and production (SDG12), climate action (SDG13) and life on land (SDG15) [35; 36]. The increased attention on soil health raises the need for uniform definitions [37], and clear ambitions and goals [38].