Acid Residues in Rhododendron Pot Culture
Edward S. Rothman
North Hills, Pennsylvania
Around 1913, English rhododendron experts routinely dug deep holes in their chalk ground, filled the holes with rich organic medium and grew their rhododendrons therein. In time the damp organic matter oxidized to
residues in which their plants suffered or even died. The extraordinary and quite erroneous conclusion arrived at was that the "lime" had leached upward and harmed the plants! Even now, it is commonly believed that rhododendrons need acid soil and that "lime" is toxic to them.
But today more experienced commercial growers are routinely incorporating dolomitic limestone (a natural mix of magnesium and calcium carbonates) into their potting mixes. Often they do not have full knowledge of the reason for so doing except that, empirically, they do get better results.
Incinerated rhododendron leaf ashes are rich in calcium, as the wood is in "potash," potassium. A realization that leaves contain calcium might lead to the concept of "lime" as a nutrient instead of merely an antacid to regulate pH to 5.2. But there are deeper aspects to the story than that. If I seem to wander, bear with me.
The two green chlorophylls and the red hemin pigment in blood have amazingly similar chemical structures. The rigid bonding of the centrally imprisoned metal atom is held in a process called chelation after the Greek word for claw, "chele." One can buy in the shops "chelated trace elements" for addition to finer garden plants. Some of these chelates are quite reluctant to release their bound nutrient atoms of, e.g., copper, iron, manganese, molybdenum, etc. Yet the vast excess of calcium atoms in the "lime" can knock out a few of the rarer metals simply by the sheer force of numbers.
Now to my point. The multiple carboxylic acid groups so very closely positioned on the humus - the black sludge (sewage) - function such that over-rotted potting medium also grabs, and does not easily release, nutrient metal atoms. "Water, water everywhere but not a drop to drink," as the saying goes.
Incredibly, by the same process that technically cannot be called chelation, hydrogen positively charge ions have no wish to migrate far from the negatively charged humic (COO) groups. These hydrogen positive ions are the fundamentals recordable as pH! Since they do not wish to leave their prison and swim over the glass electrode pH measuring instrument, one gets absolutely worthless readings on the expensive instruments.
In one experiment I took a water suspension of black humus from the rhododendron pot and got a pH reading of neutral 7. I added (by titration) increments of caustic lye solution strong enough to take the flesh off bones and to my amazement the pH readings remained a constant 7. This is really super buffering. A few forest plants, like rhododendrons, English ivy ( Hedera helix ), beech and oak trees can manage fairly well under blankets of fermenting leaf droppings, but these natural situations differ markedly from the over-watered, root bound, containerized rhododendron. People and plants can survive under far from optimum conditions, but thinking gardeners can easily beat the odds if they use science.
Edward Rothman, Ph.D., worked for 25 years as a USDA research scientist on steroids synthesized from wild plant sources. He is a member of the Greater Philadelphia Chapter.