The pH Requirements of Rhododendron lapponicum
L. K. Wade, M. Sc.
Dept. of Geography, Research School of Pacific Studies
The Australian National University, Canberra, A. C. T. Australia
It is a well established general rule that most species of rhododendrons, in common with most other ericaceous plants, require acidic soil conditions, the reason being that the relatively large amounts of iron required by these plants is made available to them only under acidic conditions.
Under alkaline or even neutral soil conditions iron is bound up with other soil elements and is generally unavailable to plants. R. lapponicum, a boreal to arctic species, is I believe a species that will not survive when grown under the generally required acidic conditions.
During the summer of 1963 while I was assistant ecologist under Dr. V. J. Krajina, Professor of Botany, University of British Columbia, on a Canadian arctic expedition, many observations were made of the soil pH under various species of plants, including R. lapponicum. It was found in every case that R. lapponicum grows only under neutral to quite alkaline conditions, and never in an acidic environment. The most vigorous plants were observed growing on limestone with a soil pH of at least 8.0 to 8.5. Other less robust individuals were observed on shales and sandstones, with soil pH ranging from 7.0 to 8.0.
With this information in mind it may well be suggested that R. lapponicum is not a heavy-iron-requiring species, and consequently might thrive under culture in alkaline conditions.