# Rapid Soil Tests for Assessing Soil Health
## 2 Materials and Methods
### Step 1a: Near Infrared Spectroscopy  
Near-Infrared Spectroscopy (NIRS) allows for fast, quantitative, non-destructive and cost-effective estimation of multiple physical, chemical, and biological soil characteristics. Since the introduction of NIRS for soil testing into practice in 2004, many soil characteristics have gradually been introduced in routine soil tests. The current calibration models are based on a minimum of 1000 to more than 100,000 reference samples depending on the year of introduction, the specific soil characteristic, the efforts made to create high-quality calibration models, and the number of 'outliers' (which are re-analyzed by the reference methods and after that included in the calibration system). To evaluate the calibration system, validation tests were conducted in several European countries, China, New Zealand, and Vietnam, with initial efforts initiated in California (USA) [(43; 51)] (Table 1). Since the 2022 publication, the calibration results have been updated, and the results from California, which were not previously published, are now included. \begin{table}
\begin{tabular}{l l l l l l l l l} \hline
**Type** & **Region** & **n** & **P5** & **Mean** & **SD** & **P95** & **R2** & **RPD** & **RMSEP** \\ \hline Calibration & - & 23,322 & 0.7 & 1.8 & 1.7 & 5.2 & 0.99 & 13.2 & 0.48 \\ Validation & California, USA & 40 & 0.4 & 1.3 & 0.9 & 3.0 & 0.93 & - & 0.13 \\ Validation & China & 223 & 0.4 & 1.0 & 1.8 & 3.1 & 0.96 & 4.7 & 0.29 \\ Validation & Europe & 4037 & 1.0 & 3.3 & 2.7 & 9.3 & 0.98 & 8.7 & 0.32 \\ Validation & New Zealand & 234 & 2.0 & 6.1 & 5.5 & 14 & 0.99 & 14.1 & 0.33 \\ Validation & Vietnam & 213 & 0.3 & 1.4 & 0.6 & 4.2 & 0.96 & 4.7 & 0.12 \\ \hline \end{tabular}
\end{table} 
Table 1: Results of the calibration and validation of the determination of SOC percentages by NIRS. Samples have been taken in different countries but were analyzed following the same standard procedures [(50)]. Results are presented for number of samples, the 5 th (P5), and 95 th (P95) percentiles, mean, standard deviation (SD), determination coefficient (R2), relative percentage difference (RPD for \(n\geq 100\)), and root mean squared error of prediction (RMSEP). SOC, soil organic carbon; NIRS, near-infrared spectroscopy. Figure 1: The conceptual framework of the three-step approach to assess and monitor soil health. Step 1b: Soil Intensity Characteristics Measured with Multi-Nutrient Extractions with 0.01 M CaCl\({}_{2}\)  
Multi-nutrient extractions with 0.01 M CaCl\({}_{2}\) were first proposed for assessing the readily available nutrients in soil more than half a century ago. Currently, this extraction method is used worldwide and is embedded in many scientific soil testing programs ([PERSON] et al., 2022) [42]. The method has been promoted since the unbuffered solution of a 0.01 M CaCl\({}_{2}\) extract has a comparable ionic strength to soil solutions of most soils; thus, the measured nutrients in the extract reflect the availability of the nutrients at the pH and ionic strength of the soil solution. Moreover, various nutrients as well as metals can be measured in a single extract simultaneously, which allows us to consider the relationships between available nutrients. Prior to extraction, soil samples were dried at 40 \({}^{\circ}\)C, gently milled, and sieved (2 mm) to remove gravel, stubble, and roots. After extraction at a 1:10 extraction ratio (\(w/v\)) for two hours at 20 \({}^{\circ}\)C, nitrogen and phosphorus concentrations in the filtered extract were determined by DA and all other elements by ICP-MS under controlled conditions. Results were verified analytically through reference samples, duplicate samples, and ring tests. Below, we present a summary of the results for bio-available heavy metals, including nickel--an element also recognized as an essential micronutrient. These data, which were not included in our previous 2022 publication [42], are based on 1886 routine soil tests conducted across the Netherlands between 2024 and 2025.