QBARS - v29n1 The Use of Sawdust as a Fertilizer of Rhododendrons

The Use of Sawdust as a Fertilizer of Rhododendrons
Porter B. Orr, Knoxville, Tennessee

David Leach reported in "Rhododendrons of the World" that if sawdust (age and type not specified) is extracted with water and that extract is used to water rhododendrons, the growth of the rhododendrons is increased about 50%. He further stated that if the sawdust water is boiled, it loses its growth inducing properties.
On the basis of this information, several experiments were performed in which the effect of sawdust extracts on seedlings which had been germinated in a milled sphagnum moss medium were compared with growing techniques used by the author in past years. These experiments verified that watering plants with sawdust extracts effectively promotes plant growth. After about one year of growth, seedlings watered with sawdust extract were over twice as tall as any previously grown for the same period of time, and the foliage was larger and more mature.
In efforts to determine the reason for this effect, water soluble extracts from two types of sawdust and from milled sphagnum were analyzed for plant nutrients and trace elements. Significant concentrations of nitrogen, phosphorus, and potassium and a surprisingly large number of trace elements were found in all extracts. The results of experiments to date suggest that the enhanced growth rate may be due primarily to the nutrient values extracted from sawdust, however additional experiments are presently underway which should better define the role of plant nutrients, trace elements, or possibly other water soluble constituents of sawdust.

In these experiments, seeds were sowed about the first of April on moist milled sphagnum and the flats were covered with thin polyethylene film. The first two weeks of germination was in the dark at about 60°F., and the covered flats were then placed under fluorescent lights at 65-80° F. Germination required 2-6 weeks.
When the seedlings were about ½ to ¾ inch high and had 2-3 real leaves, they were pricked out and transferred to flats filled with milled sphagnum and spaced about 1 inch apart. The flats were covered with polyethylene and put under fluorescent lights for further growth. Watering was done as needed, using the sawdust water in place of ordinary water. No fertilizer was used. Approximately 4-6 months after sowing, when the plants were 2-4 inches high, the polyethylene cover was slit, care being taken to prevent excessive opening of the cut plastic. Each day another slit was cut in the cover until the tender plants became acclimated to the dryer air of the room. After 7-8 days, the plastic was completely removed. With the flat now open to the air, frequent watering was necessary to keep the sphagnum moss damp. Each treated flat in the experiment was watered once a week with sawdust water, and if the flats became dry between treatments, ordinary tap water was used.
Since no specific data was given by Leach, a sawdust extraction procedure was selected arbitrarily, To approximately 2 liters (0.53 gal.) of sawdust, enough water was added to make a total of 3.5 liters (0.92 gal.). This mixture was agitated for 4 hours with a mechanical agitator and then allowed to settle for about 30 minutes. The supernate was decanted, allowed to settle for an additional day, and again decanted. This procedure was used to prevent fine sawdust particles from plugging pores in the growing medium. Although fine particles will continue to settle after the second decantation, these solids were re-slurried and used: because chemical analysis showed that they contain a significant fraction of the plant nutrients contained in the extract. (See below).
Two types of sawdust were leached and tested. One batch was 5-8 years old, and the other was new material not over 3-4 months old. The origin of the sawdust is not known, although the new sawdust is thought to be mainly soft woods.
To determine the optimum time that water should contact sawdust to extract the nutrients, four tests were made. Using the procedure described above, batches of old and new sawdust were extracted for four and sixteen hours each. Chemical analysis of the leach solutions are given in Table I.

Table 1. Study of Time vs. Chemicals Extracted.
Material pH Total P
Total N
Total K
Old sawdust extracted 4 hours 4.75 1 13 9
Old sawdust extracted 16 hours 4.6 1 21 11
New sawdust extracted 4 hours 5.15 4 14 20
New sawdust extracted 16 hours 5.2 3 15 25

These data indicate there is some increase in the amount of nitrogen and potassium extracted by a four-fold increase in extraction time. However, it does not appear to be enough to justify this additional time. The low phosphorus analyses for both types of sawdust results when extracts are settled for a long period of time, and demonstrates that the phosphorus content is associated with the sediment.
These extracts were used to water seedlings as previously described, and growth characteristics were compared with the growth of plants of various methods of growing seedlings in the past five years.
In additional experiments, tests were made to determine if seedlings could be effectively grown in sawdust and thereby eliminate the sawdust extraction procedure. To do this, seeds of some deciduous azaleas were germinated in sphagnum. When they were about ½ inch high, half were transplanted into flats containing the 5-8 year old sawdust and half were planted in sphagnum watered with sawdust extract.

Results and Discussions

Three months after transplanting, seedlings grown in sawdust and watered only, 
on the right, are compared with seedlings grown in sphagnum and watered with sawdust extract.
FIG. 6.  One-year old seedling, watered with
extracts of either old or new sawdust, and grown
under lights are as much as 12" high.  Growth is
double that of plants grown under similar conditions
in previous years.
Photo by Porter Orr
One-year old seedling, watered with extracts of either old or new sawdust
FIG. 5. Three months after transplanting, seedlings grown
in sawdust and watered only, on the right, are compared
with seedlings grown in sphagnum and watered with
sawdust extract.
Photo by Porter Orr

A photograph of seedlings watered with sawdust extract for about one year is shown in Figure 6. The distance from the top of the flats to the bottom of the fluorescent reflector is 9½ inches. A few of the plants are as much as 12 inches high. These plants are at least twice as high as plants grown in previous years. The foliage is also larger, more mature, and better able to withstand the shock of being moved outside to a cold frame. Comparable results were obtained with extracts from both the old and new sawdust.
In Figure 5, seedlings grown in sawdust and treated with water are compared with seedlings grown in sphagnum and treated with sawdust extract. This picture was taken about three months after transplanting. The plants in sawdust are sickly looking, while those in sphagnum watered with saw dust extract are healthy and 2-4 times as large.
In efforts to better understand this behavior, extracts of both types of sawdust and a comparable extract of milled sphagnum were analyzed for total phosphorus, nitrogen, and potassium. In addition, the extracts were dried, ashed at 1000 ° C and analyzed spectrographically for trace elements. These results are given in Tables 2 and 3.

Table 2. Chemical Analysis for Total Phosphorus, Nitrogen, and Potash
Material pH Total P ppm Total N ppm Total K ppm
Old sawdust extract 4.75 4 15 8
New sawdust extract 5.15 6 14 35
Milled sphagnum moss* 14 230 56
*"No Damp-off" brand.

Table 3.  Spectrographic Analysis of Extracts
Element New Sawdust Extract
Old Sawdust Extract
Milled Sphagnum Extract
Silver 0.02 0.005 0.001
Aluminum 1.4 0.7 5
Boron 0.01 0.005 0.01
Barium 0.04 0.005 0.003
Calcium 3.5 1.2 Major Constituent
Chromium 0.1 0.02 0.02
Copper 0.01 0.002 0.2
Iron 0.35 0.25 0.4
Lithium 0.02 0.005 0.005
Magnesium 1.4 0.25 0.2
Manganese 0.7 0.25 0.1
Sodium 1.0 0.12 3
Nickel 0.007 0.005 0.08
Lead 0.01 0.01 0.101
Rubidium 0.07 0.007 0.1
Silicon 0.7 1.2 5
Thorium 0.1 0.002 -
Titanium 0.01 0.007 0.006

All of the extracts contain significant concentrations of plant nutrients and a surprisingly large number of trace elements. The presence of some of these elements is considered to be unusual. These are silver, boron, barium, chromium, lithium, manganese, lead, rubidium, and thorium. The presence of thorium is of academic interest. This slightly radioactive element is widely distributed in small amounts in Tennessee shale. It would be interesting to determine if any particular variety of tree is more selective in the absorption of this element.
Based on these results, 2 liters of dry milled sphagnum moss contains 574 mg of nitrogen, 35 mg of phosphorus, and 140 mg of potassium. These results suggest that the nutrient values of sphagnum and sawdust extracts could be primarily responsible for enhancing plant growth. Due to the high nitrogen content of the sphagnum, it is not surprising that seedlings planted in this medium and treated with sawdust extract grew more rapidly than seedlings planted in sawdust. The high nitrogen content and sterile characteristics of sphagnum could also explain why plants will germinate and grow for awhile in this medium, then remain static until transplanted. It has been reported that seedlings can be held up to two years in this medium. It seems clear then, that the regular use of sawdust extracts, which contain small but significant concentrations of plant nutrients, could replenish the nutrients in sphagnum as they are depleted and result in enhanced plant growth.
It is possibly significant, however, that seedlings grown in sphagnum and watered with sawdust extract show more vigorous growth than plants in sphagnum which are occasionally fertilized. This could indicate that the regular use of very low concentrations of nutrients is ideally suited for plant growth or that trace elements and other possible constituents in sawdust extract play a significant role.
As previously mentioned, Dr. Leach also reported that boiling sawdust extract destroyed its growth inducing properties. Efforts have not been made as yet to verify this observation with plant growth experiments; however, boiled extracts were analyzed and compared with un-boiled extracts. These results are shown in Tables 4 and 5.

Table 4. Chemical Analysis of Boiled Sawdust Extract.
Material pH Total
Old sawdust extracted 4 hours 4.75 1 * 13 9
Old sawdust extracted 4 hours, boiled 4.6 8 12 9
New sawdust extracted 4 hours 5.15 4 14 20
New sawdust extracted 4 hours, boiled 5.05 5 15 20
* Low P result due to P being lost in sawdust sediment.

Table 5.  Spectrographic Analysis of Boiled vs. Un-boiled Sawdust Extract.
Element Un-boiled New Sawdust
Extract (ppm)
Boiled New Sawdust
Extract (ppm)
Silver 0.01 0.002
Aluminum ~5 5
Boron 0.02 0.02
Barium 0.4 0.3
Calcium Major Constituent Major Constituent
Chromium 0.1 0.04
Copper 0.02 0.05
Iron 3 2
Potassium 10 10
Lithium 0.01 0.008
Magnesium 4 5
Manganese 3 2
Sodium 3 4
Nickel 0.1 0.08
Lead 0.01 0.01
Rubidium 0.1 0.005
Silicon >5 >5
Titanium 0.3 0.08

These analyses do not show why boiled sawdust extract should not nourish rhododendron plants. However, boiling could reconstruct some of the organic compounds containing these nutrients, or trace elements, so that the plants could not assimilate them.
Because of the very pronounced beneficial effect of sawdust extract on plant growth, additional experiments are presently in progress in efforts to understand the observed behavior. In these experiments, seedlings of a like variety, are being treated with boiled and un-boiled sawdust extract, solutions which contain only nitrogen, phosphorus, and potassium at the concentrations found in sawdust extract, and solutions which contain only the trace elements. The results of these findings will possibly be reported in a subsequent article.


1. David G. Leach, Rhododendrons of the World , pg. 79, Chas. Scribner's Sons, 1961.

2. David G. Leach, Rhododendrons of the World , p. 354, Chas. Scribner's Sons, 1961.