by David Rankin
This article appeared in the Journal of the Scottish Rhododendron Society:
Growing rhododendrons on limestone is impossible. More or less. Of course,
there are one or two lime-tolerant species - the European R. hirsutum is
a familiar example - but the vast majority of species become chlorotic
and sick if there is lime in the soil. So if you live in a limestone area,
don't waste your time on rhododendrons - grow something else instead.
That is what we have been led to believe - but nature has different ideas.
In Western China and the Himalaya there are many species of rhododendron
which are commonly found growing in limestone regions, sometimes quite
close to the rock outcrops. This apparent contravention of the rules has
been observed and commented on by the plant explorers from Forrest and
Kingdon Ward to the present day. So in keeping with the inclination to
argue away the evidence that doesn't agree with our prejudices four theories
have been proposed and have gained credence.
Theory Number 1. Although the rhododendrons are growing over or near limestone,
they are actually growing in decayed vegetation and the nature of the underlying
rock is irrelevant.
Theory Number2. The limestone is dolomitic, with a magnesium content comparable
to that of calcium. Plants need magnesium for chlorophyll production, so
although there may be competition between calcium and magnesium for uptake
by the plant roots, enough magnesium is absorbed. (However, alkalinity
can still cause problems by making iron unavailable.)
Theory Number 3. The limestone is hard and effectively insoluble so as
with theory number 1, it has no effect on the metals available to the plants.
Theory Number 4. As most of these regions have very high rainfall, the
water permeating the soil effectively washes away calcium as it dissolves
from the rock, so again it is not available to the plants.
Whenever I discuss this topic, one or more of these theories usually is
given as an explanation. Yet so far as I can see, there is very little
in the way of hard evidence for (or against) any of them. Such a situation
is intolerable to a Rhodophile who happens to earn his bread and butter
as a research chemist. So a visit to North-West Yunnan in July 1995 provided
the opportunity to investigate what is really going on.
Our aim was to test theories (really hypotheses) to see which one was true,
although, of course, there could be different explanations for different
species of rhododendrons or different situations. The strategy was to take
soil pH readings as a quick and easy way of identifying rhododendrons growing
in alkaline or roughly neutral conditions. Samples of the growing medium,
subsoil and rock were then taken, and in a few cases it was possible to
collect water directly from the soil. This was filtered on the spot - a
tediously slow and difficult process, particularly when the monsoon is
dripping down your neck. Chemical analyses and inspection by geologists
revealed the nature of the rock formations, and we were pleased to find
that, without any geological knowledge we had correctly identified limestone
locations in the large majority of cases. Soil samples were analysed for
organic content, and for calcium, magnesium and iron under several different
conditions. these included simply adding water (to represent natural rainfall),
extractions with high and low pH buffered solutions, and analysis of the
total content of these metals in the soils. Finally, water samples revealed
the amounts of the metals present in solution in natural conditions.
Detailed results will be reported elsewhere; here we merely see what happened
to those theories.
Theory number 1 is wrong. In most cases the roots were growing in soil
which contained large amounts of limestone. At one extreme, lumps could
be picked out; at the other, analyses showed the soils to contain large
percentages of finely divided calcium carbonate. Sometimes there was so
much shattered stone that it was hard to find soil at all, and the plants
were effectively growing in stabilised scree. And in the most extreme situation,
there were dwarf rhododendrons growing on a flood plain with the ground
submerged in a sea of milky white glacier melt carrying a vast amount of
finely divided limestone silt and saturated with calcium salts.
Theory number 2 is wrong. In every case, the limestone was effectively
pure calcium carbonate with less than 1% magnesium. In natural water samples
and in water extracts from soils, the ratio of magnesium to calcium was
higher, but at about 1 : 7 still not sufficient to compete with and counter
the effects of calcium.
Theory number 3 is wrong. All the limestone samples were described by geologists
as being soft. the calcium was readily available under the conditions of
all our analyses, and was present in the water samples.
Theory number 4 is wrong. A study of the rate at which calcium dissolved
showed that even under the high rainfall conditions found in those mountains,
the dissolved calcium concentration will build up in the soil. In any case,
the calcium was abundant in water samples.
So we have a dilemma!
The lime is there in the soil and it is available to the plants. So we
need some more theories or, at least one. My suspicion is that there are
in fact many more lime-tolerant species than we had been led to believe,
or that at least there are tolerant strains. What we want to do next is
to see whether these species growing on limestone are able to avoid absorption
of calcium by their roots. If not, we must see where it goes in the plant.
Material for these further studies is to be collected on an expedition
this summer. We will then have to compare these data with similar observations
for supposedly less tolerant species, when fed a lime-rich diet. In the
longer term, we would like to see stocks of the species/strains from the
limestone mountains in cultivation and tested in limy conditions. If that
succeeds, the horticultural implications are considerable. Not least, the
potential membership of the Scottish Rhododendron Society could double!
Acknowledgments Field work was carried out with Sun Hang and Andrew
Rankin; laboratory analyses were performed by Alison Corteen. I thank The
Royal Society, the Royal Society of Edinburgh and the University of Edinburgh
for financial support, and Kunming Institute of Botany for help organising
Professor David Rankin is in the Department of Chemistry at the University
of Edinburgh. He can be reached by e-mail at: firstname.lastname@example.org
or at his web page: http://www.ed.ac.uk/chemistry/dwhrankin