Rhododendrons on limestone soils

There are many plants which can not grow in soil which is alkaline. Usually the alkalinity is due to limestone, and such plants are often referred to as calcifuges. However, there are many possible causes of poor growth. These can include toxicity due to high calcium concentrations, competition between calcium and magnesium for uptake into the roots, direct effects of pH, reduced availability of iron and of manganese at high pH, and toxicity due to bicarbonate ions has also been suggested.

limestone

Rhododendrons and related species (Ericaceae) include many plants which are important horticulturally and commercially. With a few exceptions they do not grow on soils with high pH, and if planted in such conditions they become chlorotic, and soon sicken and die. But in some parts of the world, particularly in Yunnan province in SW China, there are many Rhododendrons growing healthily in soils which can be almost pure limestone. We are studying the factors which make this possible, so that the lessons learned can be applied in cultivation.

What are the soil conditions in which the plants are actually growing? Are the roots really in contact with the limestone? Does the limestone have a high magnesium : calcium ratio? Is the limestone effectively insoluble? Are there particular climatic influences? We are looking at soil, rock and water samples, paying particular attention to the concentrations of the key metals, i.e. calcium, magnesium, iron, manganese measured using ICP-AES spectrometry on soil extracts. It appears that in many cases the plants really are growing with roots in close contact with soft, soluble, high-calcium limestone.

How do the plants respond to these conditions? Analysis of plant material, particularly leaves, shows that they can tolerate high soil concentrations of calcium and high pH, with concentrations of calcium, magnesium and iron in the plants which are within the normal ranges. But Rhododendrons and other ericaceous plants can have enormous manganese concentrations in their leaves - sometimes over 10000 ppm(mg/kg), compared with more normal levels around 50 ppm. Even in extreme limestone soils, the manganese in healthy plants is in the normal range.

Why should Rhododendrons take up such large amounts of manganese? Is there something in the wild conditions which allows sufficient manganese to be absorbed, which is lacking in cultivation? This is a major issue now being addressed, with study of plants, wild or in cultivation, which are unhealthy as a result of having alkaline soil being the centre of attention.

A list of relevant publications can be found here.

Links to other Rhododendron Information