Disease and Nutritional Imbalance From the Use of Sawdust
David G. Leach
The October 15, 1960 issue of The Quarterly Bulletin contained several comments commending the use of sawdust both as a mulch and as a soil amendment in the cultivation of rhododendrons.
The experience of growers in the Pacific Northwest stands as its own testimonial to the benefits of sawdust but it is no indication of similar usefulness in the East. Both nurserymen and amateur growers here have sustained such heavy losses after using sawdust, particularly as a source of humus in the soil, that it has almost been abandoned. Those newly interested in rhododendrons in the East should be forewarned that the favorable experience elsewhere may not apply to this part of the country.
In the first place, the application of sawdust as a mulch corresponds with the appearance of rhododendron wilt (Phytophthora cinnamomi) with suspicious frequency. It is my theory - and it is only a theory - that the rather hard crust which usually forms atop the sawdust mulch reduces the aeration of the soil beneath it, a condition well known to favor the development of the Wilt fungus. If the soil is the least bit heavy, or if there is an exceptionally wet growing season, this smothering blanket is all the more likely to induce an epidemic of this destructive disease.
Of equal or greater importance is the nutritional imbalance which results from the use of sawdust as a source of humus in the soil. I live in a region where unlimited amounts of sawdust are available free for the hauling from mills in the surrounding forests. A few years ago I used sawdust liberally as a soil amendment for a field block which was being prepared to receive several thousand seedlings from crosses which had been made three years earlier. Ammonium nitrogen was applied at the recommended rate to compensate for the calculated withdrawal which would occur with the decay of the sawdust.
A few weeks after the rhododendrons were planted severe chlorosis began to appear. Assuming that the sawdust had decayed more rapidly than had been anticipated, more nitrogen was applied but the yellowing of the foliage became progressively worse and the plants started to die in sizable numbers. A small portion of the field was selected for further trial and additional nitrogen was applied periodically until the plants were actually killed by its excess without improving the chlorosis in the slightest.
Obviously, something not in the books had occurred. I had previously worked with Michigan State University in a very small scale program intended as a start toward establishing the nutritional requirements of rhododendrons so there was some data available from plants in optimum growth for comparison, and I therefore sent leaf samples from the severely chlorotic plants to their laboratory for spectrographic analysis. When the report of the analysis came back it was immediately apparent that the sawdust had caused a vast elevation in potassium to 1.33% (dry weight basis) and the concurrent depression of the magnesium level which is to be expected in such circumstances, to 0.11%. The "normal" levels which would be expected on the basis of my previous leaf analyses of rhododendrons in optimum growth are 0.40% for potassium and 0.31% magnesium.
Thus the sawdust was responsible for an increase of potassium to more than three times its normal level and a decrease in magnesium to one-third its normal level. Within three weeks after this imbalance was corrected the chlorosis disappeared and the plants were flourishing once again.
Nurserymen who have run into trouble following the use of sawdust have consulted me at least half a dozen times in the last several years. In two cases their plants were the victims of rhododendron wilt but in the remainder the yellowing foliage promptly disappeared following the recommended treatment designed to reduce the potassium and increase the magnesium level.
The most recent consultation was with a nurseryman from New Jersey this spring. In this case the grower forwarded two representative plants to illustrate the difficulty he had encountered, but did not offer any information about any treatments that had been given to the soil. The leaves on the plants looked suspiciously familiar and so I arranged for a spectrographic analysis. The report showed 2.40% potassium and 0.2 % magnesium. I suggested to the grower that such an imbalance indicated the use of sawdust and the nurseryman immediately confirmed that this was the case. His stock is once again in good condition following the correction of the surplus potassium and the deficient magnesium.
A further difficulty encountered occasionally is that sawdust piles are sometimes havens for grubs which are ruinous to young rhododendrons, girdling them at ground level.
So it is apparent that the use of sawdust in the East is not the cheap and carefree mulch and humus source that it appears to be in the Pacific Northwest. Nor is the nitrogen balance so easy to maintain here as the experiment station bulletins lead us to believe. Sawdust may range from very old and thoroughly rotted to fresh, and the intermediate stages are often hard to estimate in their degree of decay. Yet its age determines how much nitrogen to use. Further, while some sorts of sawdust de-cap rapidly, the next delivery may come from another site and be predominantly from a wood which rots slowly. There is no formula for applying nitrogen which allows for such variation. It is a matter of judgment.
Even the best calculation may be upset by the weather. If it should turn unseasonably warm for an extended period the decay accelerates, and then protracted rains may leach the nitrogen from the soil. Nitrogen chlorosis appears and the grower makes a second application of ammonium sulphate to correct it. Should the weather then turn cool and dry there is an excellent chance that there will be a surplus of nitrogen in the soil which will stimulate unseasonably late growth when the fall rains arrive. The plants, not ripened for the onset of cold weather, split their bark and may be killed by the first blast of winter.
If sawdust is used as a mulch after it has also served to modify the texture of the soil there is a further problem in management. When the mulch is renewed in the spring, nitrogen must be added to compensate. At the same time, a supplement should be provided to balance the withdrawal which takes place during the second year of decay for the sawdust previously mixed into the soil. The amount required to restore the nitrogen balance in the soil depends on how rapidly decay has progressed the first year. Then the calculations truly deteriorate into the realm of guesses.
In the East, rhododendrons under ordinary field culture appear to be a good deal more critical than most other nursery stock in the latitude of the nitrogen level which will allow good growth, but they are usually much better off with too little of the element than with an excess. The nitrogen balance problems I have described may not be commonplace but the conditions under which they can arise occur just often enough to be cause for concern. At least, I have observed with considerable interest that the occasional grower who uses sawdust with apparent success for several years seems to encounter problems, sooner or later which lead to its abandonment.
Several growers have told me that the sawdust from the cutting of White Pine (Pines strobes is toxic to rhododendrons.
In any case, the sum total of experience with sawdust in the East suggests its use for the first time by any grower, amateur or. professional, on an extremely cautious, experimental basis, on a small scale and for an extended period, before making any decision. for large scale use on rhododendrons.