Is the Ca:Al ratio superior to pH, Ca or Al concentrations of soils in accounting for the distribution of plants in deciduous forest?

The distributions of vascular plants in south Swedish deciduous forests were related to exchangeable (exc) and soil solution concentrations of H+ (pH), Ca, Al and the Ca:Al ratios within these fractions. Topsoils (0-5 cm) of 172 sites with a pH(KCl) of 3.2-3.9 (corresponding to 3.7-4.4 in soil solution) were used. In the soil solution both total Al, and quickly reacting Al, were determined. Exchangeable concentrations were generally well related to plant distributions, the highest correlation coefficients usually being given by pH(KCl)>Ca-exc>Al-exc.>(Ca:Al)(exc). The (Ca:AI),,, ratio was clearly inferior. Out of the soil solution variables studied, Ca concentration, followed by pH, was best correlated with plant distributions, Al-t, Al-r, and the Ca:Al ratios having similar and lower coefficients. It is concluded that the use of Ca:Al ratios as a general measure of Al toxicity in controlling plant distributions is rather problematic. It seems difficult to apply evidence for Ca-Al interactions from solution culture experiments to field conditions when measured as exchangeable or soil solution concentrations of the soil.