More About Rhododendron Seedlings in Sphagnum Moss
Peter B. Orr, Knoxville, Tennessee
In an earlier report, the growth stimulation of 1973 rhododendron seedlings in milled sphagnum, by the use of a water solution of sawdust extracts was described. It was also shown by chemical and spectrographic analyses that not only did the sawdust extracts contain nutrients available to the seedlings, but the sphagnum itself contained most of these same elements.
Starting with these data, more chemical and physical analyses, as well as growth observations, have been made on 1974 seedlings using three types of sawdust extracts, a synthetic sawdust extract minus the trace elements and an artificial trace element solution without phosphorus and potassium. These were done to determine if the growth inducement was due to something special in sawdust extract, or if the same end could be achieved using ordinary fertilizer chemicals.
The initial large amount of chemicals in some batches of sphagnum and the frequent application of the above solutions, particularly the synthetic N, P, and K solution, caused the plants to transpire at least 12 elements from both sides of some leaves and even some of the stems.
Seedlings of R. oreodoxa, from ARS Seed Exchange seed, were germinated in a flat of "horticultural grade" sphagnum. When they were about ½ to ¾ inch high, they were transferred to six flats also containing milled sphagnum. One flat was watered only with extract from old sawdust, another received extract from new sawdust, a third was watered with boiled extract from new sawdust, and the fourth was watered with a synthetic nitrogen, phosphorus, and potassium solution calculated to be about the same amounts as that found to be in the new sawdust, (but actually analyzed somewhat higher.) The fifth flat of seedlings was watered with an artificial trace element solution in the same proportion as found in new sawdust extract. The sixth flat was a control, watered with tap water.
For about the first six months, the six flats were enclosed in plastic, under fluorescent lights, and were only watered with their particular solution as needed. No record was kept of the number of watering during this period, but it was no more often than once a month, if that frequently.
For the first few flats that were opened (not all were opened exactly at the same time), and as the flats became dry, each was still watered with its own particular solution. When many of the leaves on these (and seedlings other than R. oreodoxa ) became spotted with a white powder, the treatment of all flats with these solutions was suspended and tap water substituted in all cases.
FIG. 29. After removal of plastic covers,
a white powder gradually began to cover
the leaves of rhododendron seedlings,
indicating that fertilizing solutions were too
frequent and/or strong.
Photo by Porter Orr
Results and Discussion
The growth of the plants in three of the six flats of R. oreodoxa were different from the remaining three. The one treated with the synthetic solution containing nitrogen, phosphorus, and potassium (made to resemble new sawdust extract) contained plants about an inch taller than all the other flats. The control flat treated only with tap water grew well for a few months, but after reaching about 1 to 11/z inch in height, remained static without the addition of nutrients. The flat containing the plants watered with the trace element solution were somewhat smaller than the three watered with sawdust extracts. Practically no difference could be seen in the plants watered with the three types of sawdust extracts.
It is possible that the treatment of the plants with the synthetic trace elements was not a valid experiment. Since the actual chemical composition of the trace elements extracted from sawdust is probably organic in nature, and is virtually impossible to duplicate in a synthetic solution, nitrates and chlorides were the only water soluble compounds available for most of these elements. Since the chloride ions were thought to be possibly harmful to the plants, the nitrates were used. Although some authorities indicate that nitrogen in the nitrate ion is not acceptable to rhododendrons, it is quite possible that the nitrate solution of these trace elements affected this flat of seedlings, masking any effect of the trace elements.
Three main things were shown by these tests:
- The extracts of sawdust are good for fertilizing the rhododendron seedlings, but are no better than a synthetic preparation, which is easier to procure.
- The addition of trace elements is not necessary for the period of time (about 12-13 months) the seedlings are in the flats; the sphagnum has a sufficient amount.
- Boiled sawdust extracts were not altered in chemical composition to the point that they would not induce growth in seedlings.
A few weeks after the R. oreodoxa test flats (and flats of other seedlings treated with the synthetic N, P, and K solution) had their plastic covers opened to room conditions, patches of white dust began to appear on the tops of some leaves. Later, the same material appeared on the underside of some of the leaves and in a few cases it appeared on the plant stems. On some plants this white powder increased to completely cover the leaf. When it be came widespread, the only conclusion acceptable was that the fertilizing solutions had been used too frequently, were too strong, or both. Photographs show affected leaves and plants.
FIG. 30. Analysis of white powder from
the surface of a leaf is depicted in graph.
Manganese is the predominant peak.
The addition of all types of chemical solutions was stopped, and only tap water used. In spite of the fact that the plants transpired the chemicals through the leaf tissues, much of the powder was not soluble in water and after being washed off the leaves with de-mineralized water, it was put into solution with hydrochloric acid for analysis. Wet chemical analysis showed the presence of nitrogen, phosphorus, and potassium. Spectrographic analysis of the same solution showed cobalt, copper, iron, potassium, magnesium, sodium, and silicon to be present also. Phosphorus and nitrogen are not detected by this method. It should be noted here that cobalt was not detected by Spectrographic analysis of either the sawdust or sphagnum water extracts in a previous report. Subsequent samples of sphagnum analyzed by this method have shown the presence of cobalt and other elements not detected previously. Some of these are: cadmium, cesium, gallium, mercury, tin, strontium, yttrium, vanadium, zinc, and zirconium. Few analyses are reproducible, indicating a very poor homogeneity of the samples. To date a total of 30 elements have been detected in various samples of sphagnum by spectrographic analysis.
A thick coating of the white powder was removed from the leaves of the plants with the sticky side of "scotch tape" and analyzed while on the tape by the x-ray fluorescence method. A photograph of the graph shows the presence of chlorine (possibly from chlorinated tap water), potassium, manganese, iron, and zinc. Manganese is the predominant peak. Elements of lower atomic numbers than sodium are not detected by this method. Again as was the case of the cobalt above, zinc was detected by this method but not previously seen in the sphagnum or sawdust extracts. As in the case of cobalt, some sphagnum samples show the presence of zinc and some do not. It is likely that many of these trace elements are concentrated by the plants to the point of easy detection. The non-homogeneity of sphagnum samples from a particular 10 pound bag was not only true of the trace elements, but there was considerable variation of N. P. and K analyses as well. This experiment points up the following:
- Although the fertilizing solution used contained no more than nitrogen 23 ppm, phosphorus 25 ppm, and potassium 60 ppm (and usually less than half these amounts), fertilizing more than once every 3-4 weeks with these concentrations is inviting trouble.
- Since sphagnum contains ample N, P, K, and trace elements for a while, additional fertilization is unnecessary and possibly harmful during the first 3-4 month period while the plants are still enclosed in plastic.
- Although the evidence is not conclusive, there are indications that flats of seedlings in areas of higher temperatures transpired more chemicals than those in cooler zones.
- Despite terrible looking foliage and early dropping of some leaves, few plants were lost. The plants were kept in active growth as much as possible by bud pinching and appear to be transpiring the stuff out of their systems. The sphagnum in the worst flats was washed with water hopefully to remove excess chemicals which were held by simple absorption. However, the washing of the sphagnum with water will not remove those cations held by ion exchange.
- After the worst of the white powder was transpired from the plants, the usual symptom of too much chemicals appeared. The tips of some of the leaves began to brown and curl. Many leaves eventually completely died.
- From the considerable variation in analyses of the chemicals found in sphagnum, it is thought that when a flat of this material is prepared, it should be washed well before use to remove excess chemicals. Then a solution containing about 6-8 ppm nitrogen, 3-4 ppm phosphorus, and 3-4 ppm potassium applied once a week, after the plastic covers are removed, will keep the small plants in active growth safely, if lighting and temperature conditions are optimum.
1. Porter B. Orr, The Use of Sawdust as a Fertilizer of Rhododendrons, ARS Bulletin, Vol. No. 1, 1975