Sawdust as a Mulch and Soil Amendment for
Rhododendrons and Azaleas
Dr. A. N. Roberts
A lecture presented to the Society February 15, 1951 at Portland, Oregon
There is an increasing interest on the part of both the home gardener and the commercial nurseryman in the use of sawdust as a mulch or soil conditioner for ericaceous plants. This is not surprising because in recent years sawdust as a soil amendment has received considerable study and much of the misinformation regarding its effects on the soil and plant have been corrected. We now know that if properly used, it can have a decidedly beneficial effect on the soil even though it does have little fertilizer value.
Function Of Soil:
In order to fully evaluate the role of sawdust in the media for growing rhododendrons and azaleas, we must consider what function the soil has in the growth of this type of plant and how sawdust can improve this function. Generally the role we associate with the soil is that of supplying mineral nutrients, water and oxygen to the plant. Too often the latter of these is overlooked in the soil management program. If ample oxygen is not present, it is impossible for the roots to function properly and the water and mineral nutrients will not be absorbed
Before discussing further some of these soil factors, it is well to consider the type of root system the rhododendron possesses and the conditions which favor its extensive development in the soil. These plants have fine, fibrous roots which have a tendency if undisturbed to develop extensively in the surface of the soil. The extent of this root system depends on the physical properties of the soil to a greater extent than its chemical nature. Experience has shown that they develop most profusely in an open, well-aerated media that is cool, moist and of the correct acidity. These factors that determine the growth of the root system therefore determine to a great extent the amount of water and nutrients that can be absorbed for use by the plant.
Research and experience are showing that in the culture of rhododendrons and other plants of this type that the physical properties of the media is probably far more important than a high level of nutrient salts. It would seem necessary therefore that conditions unfavorable to the development of an extensive root system be avoided.
Conditions which are sure to result in poor root development and growl h of the rhododendron are many and varied. The most common cause in Oregon is planting too deep in heavy, compact, poorly drained soils. This results in poor aeration of the roots. Over-fertilization is another sure way to discourage good root development, just as can a lack of adequate soil nutrients. Excessively high soil temperatures in the summer or excessively low soil temperatures in the winter can result in poor root development which will restrict top growth. Of course, we must not fail to mention the factor of soil reaction, since some elements become unavailable to the rhododendron root if the soil is not sufficiently acid. Under western Oregon conditions this factor should seldom be a problem and is often overemphasized without giving sufficient attention to the physical properties of the soil.
A poorly developed root system will often manifest itself in the form of (1) nutrient deficiency symptoms, (2) summer scorch of the leaves and (3) freezing injury of the top in severe winters.
Nitrogen: Nitrogen is the element most likely to be deficient in rhododendron soils. However, these plants are sensitive to excessive amounts of nitrogen fertilizers and care must be exercised in its use. The presence of large amounts of organic matter in the media, which is reduced to humus by soil organisms, has an important bearing on the nitrogen status of the soil. This humus acts as a reserve supply of readily available nitrogen that is non-toxic to roots. Mineral fertilizers can be safely added to a media, high in organic matter. These can supplement the nitrogen supply, when the soil organism need this element in decomposing the freshly added organic matter.
The form of supplement nitrogen used is of considerable importance. While the organic fertilizers such as tankage, cottonseed, soybean and blood meal are ideal rhododendron fertilizers for supplying nitrogen, they are often too expensive for extensive use. Their chief advantage like the naturally occurring soil humus is the gradual availability of their nitrogen. This makes them safe to use, even in quite large amounts. However, the inorganic mineral fertilizers have the advantage of being rapidly available and where large amounts of sawdust are to be decomposed, this is a decided advantage. The best of these for rhododendrons is ammonium sulfate because it leaves an acid residue. Some forms such as calcium nitrate or sodium nitrate will tend to neutralize the soil to a point where iron chlorosis may develop. Iron becomes unavailable to rhododendrons if the soil is not sufficiently acid.
Phosphorus and Potassium: A rhododendron soil well supplied with organic matter will ordinarily not require supplement feeding with phosphorus and potassium in the form of commercial fertilizers. However, it is interesting that under some conditions, iron-chlorosis symptoms develop that have as their underlying cause a deficiency of these major elements. More needs to be known as the importance of these elements in rhododendron nutrition. What is their effects on flower quality, winter-hardiness etc.?
Calcium and Magnesium: Under some conditions calcium may be deficient in rhododendron soils. If the soil is extremely acid, responses may sometimes be obtained from the use of dolomite limestone. this supplies both calcium and magnesium. The latter element may also be deficient under certain conditions and like potassium may first appear as iron chlorosis. However, iron will only temporarily correct the deficiency and magnesium is needed to permanently correct the disorder.
Rarely do any of these mineral deficiencies appear in plants grown on soils liberally supplied with decomposing organic matter and mulched with the same. If any deficiency appears under these conditions it is usually that of nitrogen which can be corrected in a matter of days by the addition of quickly available forms of nitrogen fertilizers. At the same time moisture and oxygen requirements are in good balance. This brings us to the consideration of the role of sawdust in supplying this organic matter.
Role Of Sawdust:
Sawdust has been used as a mulch and soil amendment by professional nurserymen, small fruit growers and orchardists for a number of years, despite the dire predictions of some and the lack of enthusiasm on the part of the professional horticulturist and soil scientist. Some of these users have been enthusiastic over their results from trial and error methods. Others have been disappointed because of a lack of information on the problems involved in using sawdust.
Agricultural scientists have been reluctant to accept sawdust as a desirable form of organic matter to use as a mulch or soil amendment because of some of the problems it presents when added to the soil. Its slow rate of decomposition, which may or may not be a disadvantage, and its temporary depression of nitrates have been the principal objections to its use. The very low value of sawdust as a source of readily available plant nutrients is a recognized disadvantage. Fresh sawdust contains only approximately 0.048 per cent nitrogen; 0.007 per cent phosphorus; 0.0117 per cent potassium, and 0.106 per cent calcium. However, in spite of these disadvantages, the benefits that are being derived from the use of these wood wastes in improving the physical properties of the soil for certain plants can not be overlooked.
Moisture, Temperature And Aeration:
The fact that sawdust increases the water holding capacity of the soil and prevents rapid changes in moisture and temperature about the roots has been demonstrated many times. When sawdust is incorporated into the soil and is used as a mulch the improvement in these factors as well as that of aeration is strikingly shown by the extensive root system which results. To know that acid-loving types of plants respond to this type of media, one needs only to examine the fibrous mat of roots that develop in straight sawdust where it is only used as a mulch with a fairly heavy soil below. This will occur in fresh sawdust and as it ages and decomposes, the growth of roots in it is even more striking.
Sawdust Decomposition And Nitrogen Relationships:
The value of sawdust as a soil amendment increases as this woody organic matter is transformed into humus. Humus is generally considered to be a complex aggregate of dark colored amorphous substances, which profoundly modifies the fertility of the soil. It is largely a ligno-protein complex, which forms when soil organisms bring about the decay of organic matter. As this organic matter changes to humus, most of the nitrogen and a considerable portion of the soil phosphate, sulphate and other inorganic substances become a part of the humus fraction of the soil. Evidence to date indicates that a large part of humus formation is derived from the lignin in organic matter which decompose slowly in the soil. This is the reason for the slow decomposition of sawdust in the soil. Sawdust is composed of about 40 per cent lignin and about 60 per cent cellulose along with various waxes, resins and oils. This high lignin content makes sawdust potentially a good source of humus. The problem presented is in bringing about this decomposition.
The decomposition of sawdust to all important humus is brought about by soil organisms. Like plants these organisms require food other than the lignin and cellulose in the wood. The principal nutrient they draw heavily on is nitrogen, and in decomposing fresh sawdust they use lots of it. For this reason during this process there is a competition between the soil organisms and the plant for available nitrogen and the plant usually comes out second best. As a result plants grown in soil with large amounts of sawdust incorporated will turn yellow and show nitrogen starvation. This can easily be overcome by adding liberal amounts of nitrogen during this period. Later a balance is reached where little supplemental nitrogen needs to be added and in fact nitrogen and other nutrients will be released to the plant as the organisms break down.
While it is not necessary to use old sawdust that is partially decomposed, one can see the advantage in its use, since less supplemental nitrogen will be needed. After a planting is once established and a balance reached in this decomposition process, the annual addition of more mulch presents very little if any nitrogen problem. In any case the balance can be maintained by the addition of some form of readily available nitrogen.
The amount of supplemental nitrogen that will have to be added to soils where large amounts of sawdust have been added will vary with the amount added, the fertility of the soil and the type of plant to be grown in it. Generally speaking 2 lbs. of ammonium sulfate per inch of sawdust incorporated into 100 sq. ft. of soil will take care of the nitrogen problem the first year. This can be cut in half the second year and each year thereafter until the proper balance has been restored in the soil. This rate will vary somewhat with the type of sawdust whether it be fir, alder, pine, redwood, etc. but the problem for all is essentially the same. In reality it is a very simple problem. If the plants begin to show signs of nitrogen deficiency, more nitrogen is added to the surface of the mulch and watered in. Fairly large amounts of commercial fertilizers can be added to a rhododendron soil that has large amounts of this type of organic matter without danger of being toxic to the plants. This is another of the advantages of this type of soil amendment.
When sawdust is used as a surface mulch, it does not present the nitrogen problem that it does when it is worked into the soil. Normal applications of nitrogen to the surface of the mulch will usually be sufficient in this case. The rain will leach soluble nitrogen fertilizers down to the roots. However, the physical condition of the soil will be much better if a part of the sawdust is incorporated in the soil prior to planting and taken care of by additional nitrogen feeding.
The principal advantage of our favored peat moss as an amendment to rhododendron soils over that of sawdust is that it has previously gone through the decay process. This advantage can be overcome to some extent by compositing the sawdust prior to the time it is added to the soil. This decomposition takes place slowly in old sawdust piles. It can be done much more rapidly by compositing the sawdust with manure or commercial fertilizers similar to the way we handle leaves and garden residues. The English gardeners have elaborate methods of compositing sawdust and have excellent results with the product.
In all of the reports we have seen on the use of sawdust as a mulch or soil amendment, none have been other than enthusiastic when they knew the problems involved and allowed for them in their soil management. Some of the most recent enthusiastic reports have been those where rhododendrons and azaleas were involved.