We examined to what extent the internal site factors (light and soil conditions) are responsible for herb layer diversity in oak-dominated forest stands growing on different substrates in central Bohemia (Czech Republic). Species richness correlated positively with soil pH and negatively with nitrogen (N) concentration in humus (P\0.05). The highest species richness was found at sites with not only low N soil concentration, but also simultaneously with high phosphorus (P) soil concentration. Despite this finding, however, herb layer diversity is evidently threatened much more in P-rich soils than in P-poor soils. It seems that the enhancement of N in an ecosystem due to litter accumulation and N deposition generally leads to only a minor increase in N availability at P-poor sites, but a considerable increase at P-rich sites. Therefore, species richness can be exceptionally high at P-rich sites, but only under conditions of strong N limitation.