Prediction of soil pH patterns in nature areas on a national scale

Question: To assess the acidification process, nationwide information about soil pH on a site level is called for. Measurements of soil pH may be used, however there are not sufficient measurements available to map soil pH nationwide on site level. Instead we developed a soil pH map based on vegetation data. Location: Natural terrestrial areas in The Netherlands. Methods: 271,693 vegetation plots were used to estimate average soil pH per plot with indicator values, based on field measurements, of plant species. By spatial interpolation average pH values between the plots, with the soil type, groundwater table and vegetation management type as ancillary explanatory variables we created a soil pH map. The map covers all terrestrial nature areas (all areas that are not built up areas, agricultural areas and infrastructural areas) in the Netherlands with a map resolution of 25 × 25 m 2 raster cells. Results: The predicted pH of the map varied between 3.0 and 8.6 with standard errors between 0.13 and 0.93. Most of the standard errors range from 0.4 to 0.55, with an average just below 0.5 pH unit. Cross-validation shows that for 33% the difference between observed and predicted is between −0.1 and 0.1 pH-unit and for 83% the difference is between −0.5 and 0.5 pH-unit. Validation shows that the pH map is unbiased (mean error is almost zero), accurate (root mean squared error is 0.64) and nicely captures spatial patterns (r = 0.77). We applied the pH map to assess the impact of acidification on the abiotic quality of nature areas in the Netherlands. Conclusions: The model fit in the predicted soil pH is in good resulting in a low standard error and a high correlation. The measures taken to prevent acidic deposition causing further acidifying of nature areas can be considered as successful.