The probability that plant–available Mg concentration in soil is below RB209 Index 2, i.e. <51 mg L‐1, is presented here using a combination of colour, numerical (percentage) and verbal scales to communicate the uncertainty of this outcome.

These probabilities are obtained from the combined statistical analysis of two national-scale data sets on soil chemical properties.

The scales to present the uncertainties follow the IPCC guidelines for communicating uncertain information with "calibrated phrases", Mastrandrea et al., 2010 ( https://archive.ipcc.ch/pdf/supporting-material/uncertainty-guidance-note.pdf), modified, as described by Lark et al (2019) in the references below, to reflect the findings of psychological studies on how such scales are interpreted.

This map will allow users better to understand the uncertainty in the predicted soil concentrations of plant-available Mg, understood in comparison with the Index 2 threshold for which there are soil fertility management recommendations in the Nutrient Management Guide (RB209) published by AHDB.