Charles M. King
Reprinted from the from the October 2003 Siuslaw Chapter newsletter
In discussions of how we should fertilize rhododendrons we sometimes refer to this process as "feeding" our rhodies. Lest you think that this process is analogous to visiting your favorite restaurant, it may be of interest to consider how rhododendrons actually "feed." When we fertilize rhododendrons we are providing them with those inorganic chemicals that they cannot obtain in sufficient quantities on their own. This means that we treat rhodies with mixtures that will give them nitrogen (N), phosphorus (P) in the form of phosphate, potassium (K) and smaller quantities of several metals and sulfur.
Nitrogen is required by all living things, be it animal or plant, for synthesis of most of their chemical components, protein, nucleic acids (i.e., DNA and RNA) and lipids to name a few. Although air is 79 percent nitrogen, animals cannot convert it to chemical forms that can be used by them. A few plants, in conjunction with associated microbes, are capable of converting atmospheric nitrogen into chemical forms that can be used by both plants and animals, a process called nitrogen fixation. Unfortunately, rhododendrons are dependent on absorbing water-soluble nitrogen compounds from their roots since they are not capable of utilizing gaseous nitrogen.
Phosphate is also required for living things to synthesize DNA and RNA as well as lipids and proteins. Made up of phosphorus and oxygen in a ratio that is easily utilized by cells, phosphate is readily incorporated into these vital cellular components.
Soluble salts of potassium, potash, are utilized by plants and animals to provide appropriate ionic (i.e., salt) conditions for their cells. The extent to which potassium accumulates in plant tissues can be seen in the need for renal patients to limit their consumption of vegetable products rich in potassium; individuals on diuretics may require potassium replacement through increased intake of such things as orange juice. Perhaps the most dramatic effect of the potassium content of plant tissues is the use of wood ash to produce soft soap by boiling with animal fat. In fact, potash was produced until the late 19th century from wood ash. Soft soap consists of potassium salts of fatty acids that form when the high pH produced by the ashes splits the fat into fatty acids and glycerin. The alkaline (i.e., high pH) effects of the potassium residue in wood ash may well be something that would prompt you to limit the exposure of your rhodies to this potential.
Metallic salts and sulfur are generally required by rhododendrons in smaller quantities than nitrogen, phosphate and potassium. Calcium, magnesium and sulfur, required in somewhat larger quantities than the other metallic salts, usually have adequate levels in agricultural soils, but may need to be added to our highly acidic coastal soils. Most of the other metallic salts are involved at low concentrations in helping catalyze many of the chemical reactions of the plant rather than in the production of large quantities of structural components. These so-called "trace" elements include iron, zinc, manganese, molybdenum, boron and copper.
There are two important factors missing from the above components that distinguish plants from animals. First, fertilizers donít provide plants with a carbon source since atmospheric carbon dioxide is used for the synthesis of virtually all of their carbon containing constituents. Second, in contrast to animals that utilize food for energy, fertilizers do not give plants the energy needed for growth and survival. Plants, but not animals, use the process of photosynthesis to harness the sunís energy to convert carbon dioxide into carbohydrates that can be utilized for both energy and growth. Since photosynthesis utilizes one of our waste products, carbon dioxide, it seems only fitting that it produces something that we can use, oxygen, in the process. Carbon containing compounds derived from photosynthesis constitute the bulk of the plantís mass, which is why they burn so well. Obviously, the small amount of inorganic fertilizer spread around the roots of our rhodies contributes only a minor fraction to the mass of the plant.
Humans survive only by consumption of animal and plant products that give us carbohydrates, fats and protein for the production of energy, as well as certain essential amino acids, fatty acids, vitamins and minerals that we cannot synthesize. In contrast, rhodies thrive on simple inorganic elements provided by our fertilizers, but they actually grow through the combined effect of energy from sunlight and carbon from carbon dioxide. This relative nutritional independence of plants has often been exploited through hydroponic techniques that permit growth in soil-less systems. Knowledge of what plants actually utilize for optimal growth should enlighten us as to what role "organic" fertilizers might play in the cultures of our plants. It is likely that they serve best to alter the texture of the soil, retain moisture and modulate the release of their own nutrients.
In human terms, it would seem logical to regard plant fertilizers as nutritional supplements akin to vitamins and nutritional supplements rather than as calorie-laden delicacies that by themselves alone can add pounds. To carry the analogy further, you can conclude that our rhodies cannot exist on the empty calories provided by the combination of sunlight and carbon dioxide alone - they also need fertilizer to produce their fantastic foliage and eyepopping trusses. As stated more explicitly by Dr. Jim Gerdemann, "We do not feed plants. They produce their own food. We fertilize them."
Charles King, a biochemist, is a member of the Siuslaw Chapter.