Calcium - A Needed Element
R. L. Ticknor, Aurora, Oregon
Reprinted from the Portland Chapter Newsletter
Many people have the impression that calcium is not needed by rhododendrons since it is often suggested that lime not be applied around rhododendrons. Actually in the leaves, calcium is the mineral element present in the second largest quantities on a dry weight basis. Usually 0.8-1.5% of the dry weight is calcium, exceeded only by nitrogen which is usually in the 1.5-3.0% range.
The reason that lime (limestone or calcium carbonate) is not applied in many cases is to avoid the bicarbonate ion which is sometimes formed during the breakdown of the carbonate ion. It has been stated that bicarbonate is toxic to rhododendrons. It is possible to avoid the use of the carbonate ion and the chance of bicarbonate injury by using the sulfate form of calcium - gypsum or land plaster to supply the needed calcium. Gypsum has another advantage over limestone in that it supplies sulfur which is needed by plants as well as calcium.
A word of caution when you buy gypsum be sure the bag does not contain borated gypsum. Boron is an element needed by plants in very small amounts and is often deficient in western Oregon soils. Most plants need only one pound per acre of boron while the three pounds suggested for the best growth of broccoli or cabbage is enough to kill beans or strawberries when planted the following year. To make it easier to apply a small amount of boron, 1½ lbs. is mixed per 100 lbs. of gypsum to form borated gypsum. If borated gypsum was substituted for normal gypsum and is applied at 400-2000 lbs/acre or 1-5 lbs/ 100 sq. ft. to supply calcium to the soil, it would kill practically all plants.