The PEDON – A Soiled Subject

by Norman Todd – December 2001


The unit of classification for soils is called the PEDON[1].  Since soils do not occur as discrete entities, this unit of measurement is not an obvious one and I have only the vaguest appreciation of its significance.  I do, however, know what a Pedon means in terms of impact.  Recently, my disrobed two-year-old grandson rushed out, anticipating his bath, and aimed perfectly at my left leg, sock and slipper.  I felt the warm gush of affection.  A soil Pedon usually has a vertical dimension of one meter and the affected extent of this human Pedon was approximately the same.


The practice of returning organic matter to the soil has been recognized as being beneficial since the beginnings of agriculture.  This recent experience suggests that some distribution methods are more desirable and effective than others.  Still, it is hugely significant that almost all of the energy in the world’s terrestrial ecosystems is supported, contained and dissipated in the tiny layer we know as soil.


There are certain platitudes that I find myself repeating, almost to the point of self-disgust.  One such adage is ‘a $50 hole for a $5 plant’.  I hope it is obvious that it is what goes in the hole that is important.  Recently, I found myself in trouble with a landscaper.  I had been emphasizing to some gardeners (including it turned out, the landscaper’s client) the importance of creating an enticing rooting environment for new rhododendrons.  I also stressed the need to expose the roots of the new plants to the garden soil by teasing out the roots of the plants – especially if they had been container grown.  I have been told that over half of potted rhododendrons planted in North America die in the first year – mainly because the roots do not leave the original root mass and never do extend into the surrounding soil.  The landscaper had recently planted some ‘balled and burlapped’ rhododendrons, leaving the loosened burlap around the root ball.  This is a standard practice.  However, the client wanted the roots to be roughened and exposed and insisted that the landscaper return and do just that.  The landscaper agreed to go back and tease out the roots.


I recommend digging in a lot of bark mulch to garden soil – 50%  -  to give a depth of 30 cm.  This gives the air the roots need (20% of the soil’s volume) and gives a fairly safe assurance that the acidity will be about right.  A pH of 5.5 is generally considered ideal for most rhododendrons.


Rhododendron growers know that if the soil pH is high, this generally means a calcareous soil and the plants will not thrive.  There are acres of paper warning gardeners that rhodos and lime are like chalk and cheese.  In our part of the world most soils are acidic and so getting a suitable pHis not of great concern or difficulty.  It is, therefore, something of a diversion to discuss rhododendrons growing on limestone.  However, as it occurs quite frequently, it is an interesting and revealing diversion.  Especially when we realize that no one can yet explain how rhododendrons grow so happily in these heretical conditions.


There were, or are, a number of commonly touted theories as to how rhododendrons grow on limestone.  The main ones are: (1) They grow in pockets of organic material out of contact with the limestone; (2) they grow on dolomitic limestone; (3) The limestone is hard and more or less insoluble; (4) The heavy rain of monsoons washes any dissolved limestone out of the soil.  Professor  Rankine of the University of Edinburgh has written about these theories in the ARS Journal and elsewhere.  He studied rhododendrons growing on limestone in Yunnan, China, measuring the chemistry of the soils (examining many Pedons) in which the plants were growing, and measuring the chemistry of the plant tissue.  He showed that none of the foregoing hypotheses held water.  He is not very definite about the physiological processes that do allow the rhododendrons to thrive in such limey soils.  These interactions are very complex but he suggests that manganese is of great consequence.  We gardeners usually think only of a plant’s need for nitrogen, phosphorous and potassium.  In occasional lapses of inspiration, we may also think of iron, perhaps sulphur and magnesium (we all know to give our roses Epsom Salts) but only scientists think of manganese.  I recall consulting a doctor about a medical problem.  “What is the cause of my problem?” I asked.  With studied professionalism and ponderous severity, he replied, “It’s multifactorial.” – which was underwhelmingly reassuring.  And that, I’m afraid, is where we have to leave the case of the lime-loving rhododendrons – at least for the time being – no one as yet understands all the processes.


A more common problem with local soils is clay.  Clay soils are generally fertile soils but are not good for rhododendrons and other fine rooted plants that prefer an open, airy soil.  To correct a clay soil one needs to add enough coarse mineral material to give the desirable, open consistency.  In practice, this means adding a third to a half of sand to the depth of soil being amended.  This depth need not be over 30cm for rhododendrons, provided there is good drainage below the new soil.  Ted Irving faced the clay problem at the Horticultural Centre of the Pacific.  He lifted all the rhododendrons, dug in 15cm of coarse sand and then 15 cm of leaf mould, and replanted the rhododendrons, which are now resplendently thriving.  Ted emphasizes that applying gypsum to clays to improve texture is, at best, a very short-lived palliative.


For an amateur to say much about soils is a very stupid and risky business – the subject is complex in the extreme.  One wonders how plants grow so well when they know so little of all the machinations, animate and inanimate, going on around their roots.  That’s why doctors resort to gross generalities – they are the safest – when asked to explain the physiology of our bodies.  It is also probably why a lot of us rely on folk remedies.


There is a great deal of concern in our society these days about toxic and contaminated soils.  Large chunks of public and private money are being spent on testing, removing and treating soils that are shown, or deemed, to be contaminated.  There is a thriving business proving that such soils cannot support happy and thrifty human or vegetative populations.  The future is never really predictable but what is predictable is that there will always be a demand for soil experts.  This non-growing business is growing.  Another field where demand for expertise is sure to grow is constitutional law.  I am torn between these as the preferred career choice for my grandson.  Currently, I am favouring the soil business because he already knows what a Pedon is.


[1] A Pedon is defined as a 3-dimensional sampling unit of soil, with depth to the parent material and lateral dimensions great enough to allow the study of all horizontal shapes and intergrades below the surface.