Measurements of the soil pH are often used to characterize the nutritional conditions for the vegetation at a particular site, for example in ecograms describing the ecological behaviour of a (tree) species. In spite of this use and interpretation of pH measurements, certain prerequisites must be fulfilled when pH-values are to be used as an expression of the nutritional conditions for plants. Our study in deciduous forests of northern Germany shows that in strongly acid soils (pH < 4) the correlations between pH-values on the one hand and S-value, base saturation and C/N-ratios on the other hand are generally low, particularly for measurements in the upper mineral horizon. By contrast, in moderately acid to neutral soils (pH 4-7), pH-values could be employed as a useful indicator of the nutritional conditions for the vegetation. The main reason for the observed discrepancy in the indicator quality of pH measurements is the presence of distinct differences in the cation exchange capacity (CEC) of the soils, which are often not taken into account when interpreting pH measurements. CEC-values close to zero may be associated with either low or comparatively high pH-values, depending on whether or not the few available cation exchange sites are occupied by protons or bases, respectively. In general, pH - S-value/base saturation/CN-ratio relationships are stronger in soil samples taken in the main (fine and coarse) root horizon than in those from the upper mineral horizon. Consequently, ecograms give a more precise description of vegetation-site relationships if the considered nutrient gradients refer to appropriate soil parameters (e.g. S-value, C/N-ratio) and to measurements in the main root horizon.