QBARS - v30n3 Some Chemical Properties of Sphagnum Moss

SOME CHEMICAL PROPERTIES OF SPHAGNUM MOSS
Porter B. Orr, Knoxville, Tennessee

Introduction
During some recent fertilization tests on rhododendron seedlings,1,2 a closer examination of sphagnum was made in an effort to more clearly define its somewhat unique characteristics. Its chemical retention properties appear to be at least threefold: it has a remarkably good cation exchange capacity, both anions and cations can be held for plant use by simple absorption, and there are elements which are incorporated into the cellular structure of sphagnum which cannot be removed short of its complete destruction. The differentiation of these properties opens up some very interesting possibilities for more rigidly controlled plant uptake studies.

Experimental
A Pyrex glass tube about 1 5/8 in. diam x 39 in. long was heated at one end and drawn down to about 6 mm diameter. A small wad of glass wool was inserted in the large end and pushed to the small end to form a retaining bed.

In test number 1, this column was filled with a weighed quantity of dry milled ''horticultural grade" sphagnum. One liter of de-mineralized water was added and the sphagnum permitted to soak for at least an hour to make sure it was completely wet, and then the water was drained out and sampled for analysis. A second one liter water wash was made and sampled. In order to strip as much of the ionic contents from the sphagnum as possible, 2 one liter washes of about 4.5 M hydrochloric acid were passed through the column at a flow rate of about 25-35 ml per minute and saved for analysis. These were followed with 2 one-liter de-mineralized water washes, both sampled for analysis. The sphagnum was now in the hydrogen form.

The second experiment was to determine whether or not sphagnum was an anion exchanger. The column was again filled with a weighed amount of dry sphagnum, and treated with HCl as was done previously.

The next solution put through the column was 4 M nitric acid to displace any chloride ions (from the HCl) which might have been held as anions. The effluent from the column drained into a vessel containing a solution of silver nitrate. Any chloride ions from the column would precipitate out as silver chloride, to be filtered out and weighed. No silver chloride precipitate was formed.

In another attempt to sorb chloride ions, the column was again filled with hydrogen form sphagnum and treated with a solution of high molarity lithium chloride. The column was then washed with de-mineralized water to remove excess LiC1.

The sphagnum was again eluted with 4 M nitric acid into silver chloride. Again no AgCl precipitate was formed.

Test number 3 was done to measure the water retaining properties of sphagnum. A weighed quantity of new milled sphagnum was put into a glass column similar to the one used in the previous experiments. The column exit was closed off and a measured amount of de-mineralized water added. After the air bubbles stopped rising, the column was filled completely and allowed to stand for l-2 hours to completely wet the sphagnum. After this period, the drain was opened and a total of 1 liter of water put through and permitted to drain for about 3 hours until essentially all the excess water had drained out. The total water which went through was measured and the absorption calculated.

Experiment 4 measured the simple absorption of both cations and anions by sphagnum, and also its cation capacity. Some hydrogen form sphagnum was prepared as described, and air dried. A weighed quantity of this material was placed in a column and soaked with a 0.5 M solution of dibasic potassium phosphate until the sphagnum was thoroughly wet. This solution was drained out and the remainder of a liter of this solution also put through. The column was drained and a small stream of compressed air blown through the column until the solution ceased dripping. The column was next washed with 2 1-liter water washes and each retained for analysis. These were to determine the amount of potassium and phosphorus held by absorption only. A 1-liter strip of both potassium and phosphorus was made with 4 M HCl and analyzed to determine the amount of potassium held by cation exchange. Finally the sphagnum was washed with 1 liter of de-mineralized water and analyzed.

To determine any difference in absorption using stronger solutions, the above procedure was repeated using a l.4 M solution (2.8 times as strong) and a 4 M solution (8 times as strong) of the K ₂ HP0 ₄ . Analyses were made of the eight solutions as was previously done with the 0.5 M solution.

Results and Conclusions

Analytical results from test No. 1 are given in Table l. These data show that in the first 2 one-liter water washes, a total of 34.3% N, 21.3% P , and 59.5% K was removed from the sphagnum. This indicates that these elements were held by absorption, regardle ionic species. A water wash of over-fertilized sphagnum will remove the absorbed elements, but will not remove those held by true ion exchange. It is interesting to note that almost 3 times as much K (always a cation) was washed from the sphagnum as was P (always an anion). Since the second experiment later showed that sphagnum is not an anion exchanger, these data in the 1st test indicate the P must be bound tightly in the cell structure. The N removed by water was either held as the nitrate (anion) or if it was as the ammonia ion, the amount exceeded the cation capacity of the sphagnum.

The water retaining capacity of sphagnum was measured to be 6.8 times its own weight, but since there is so much variation in sphagnum batches, it is possible water retention capacity will also vary somewhat from sample to sample. The 4th experiment, to determine the simple absorption of different ions by sphagnum, showed that the largest amount of chemicals are held in this manner. Tables 2, 3, and 4 show the P and K held by absorption as well as the K held by ion exchange, which was removed with the HCl washes.

In Table 2, a total of 0.178 gm of potassium per dry gm of sphagnum, and 0.07525 gm of phosphorus per dry gm of sphagnum were removed by water from sphagnum treated with 0.5 M K ₂ HPO ₄ . When the K ₂ HP0 ₄ molarity was increased 2.8 times to l.4 M, the amount of K absorbed.

(Table 3) was increased 2.5 times and the P increased 2.6 times. Since the absorption of the l.4 M K ₂ HPO ₄ was almost linear with the increase in concentration, a third column of hydrogen form sphagnum was preconditioned with 4 M K ₂ HPO ₄ and eluted in the same manner as was previously done (shown in Table 4). This indicated that an increase of concentration by a factor of 8 was only absorbed by a factor of 4.74 for potassium and 4.89 for phosphorus.

This is good evidence that sphagnum has a very high chemical absorption capacity, which is possibly related to its water retention capacity. A very important method of absorption of cations was shown in this experiment. The water washes removed almost all of the anionic P, but in all three portions of the tests, almost exactly the same amount of potassium remained and was removed by the HCl wash. This is the amount of this element which was held by true cation exchange. As measured by this method, sphagnum has a remarkably high cation capacity of between 0.51 and 0.665 meq K/gm of dry sphagnum.

Several possible studies to observe the effect of various anions and cations on rhododendron plants are opened up by these studies of sphagnum moss, with seedlings or rooted cuttings used for the experiments. These are:

1. Some plants could be tried in hydrogen form sphagnum. Figure 1 is a curve showing some of the trace elements found in new sphagnum by the x-ray fluorescence method of detection. Figure 2 is hydrogen form sphagnum with many of the previously detected elements removed and the most of the remainder greatly reduced. However chlorine is increased due to the HC 1 used in the preparation of the hydrogen form. These analyses could indicate that the residual elements in hydrogen form sphagnum will not be available for rhododendron utilization.
2. Plants grown in hydrogen form sphagnum treated with nitrogen as the ammonia ion could be compared with those grown in sphagnum containing the nitrate ion, with and without potassium and phosphorus.
3. Plants in hydrogen form material treated with K and P in various ratios could be treated.
4. Hydrogen form adjusted with N, K, and P, with or without trace elements could also be explored.
5. Since some of the literature on calcium uptake by rhododendrons has been based on erroneous characteristics of sphagnum, the old question of calcium with its faults and /or advantages might be finally laid to rest.

After the hydrogen form sphagnum is prepared, care should be taken to make sure all the HCl has been removed. Copious quantities of either de-mineralized, distilled, or clear rain water should be used. Tap water will add unwanted ions.

1. Porter B. Orr, The Use of Sawdust as a Fertilizer of Rhododendrons, ARS Bulletin, Vol. No 1, 1975
2. Porter B. Orr, More About Rhododendron Seedlings in Sphagnum Moss, ARS Bulletin, Vol. No. 3, 1975