Experimental evidence and field observations show that calcicoles are of at least two types, one more exacting in its requirement of neutral calcareous soils than the other. Both pH and Ca supply affect the success of calcicoles in a complementary manner. Calcicoles are indifferent to Mg supply except in so far as it affects soil pH. Calcifuges show definite soil preferences on experimental plots but they show much better growth on their least suitable soils than do calcicoles. Arenaria verna var. gerardi from serpentine rocks showed preference for soil enriched with MgCO3.