The Use of Sawdust as a Fertilizer of Rhododendrons
Arthur W. Headlam, Bentleigh, Australia
The article by Porter B. Orr in the January Bulletin is one which raises possibilities for the use of sawdust as a fertilizer. One factor I have observed, both in nurseries and in my own garden at Bentleigh, is that when pots containing rhododendrons, azaleas, or for that matter any plants are sunk in damp sawdust to prevent excessive evaporation in summer months, the roots very quickly seem to find their way out through the drainage hole in the bottom of the pot and rapidly spread in the sawdust. The growth often is much more vigorous in this medium than in the pot itself. This is much more readily understandable in view of the experiments showing that nutrients are contained in the sawdust and are leached out from it with watering. This vigorous root growth has been particularly apparent when red pine (California redwood) sawdust or buzzer chips has been used as it does not rot down but stays in its natural state for some considerable time, hence the addition of ammonium sulphate is not necessary. Oregon (Douglas Fir) is another timber from which sawdust and buzzer chips are in demand for mulching and gardening purposes.
In my garden at Bentleigh, the soil is a silty loam and has a habit of compacting to a hard mass unless continually worked - as this is impossible with surface rooting rhododendrons and azaleas, many bags of red pine sawdust and copious quantities of leaf mold have been worked into the ground with quite dramatic effect in making the soil more friable, and acid.
Possibly my most successful potting mix is represented by equal parts of well rotted compost and redwood sawdust. Once azaleas and rhododendrons have been repotted into this mixture, the growth rate is quite phenomenal.
Use of sawdust by nurseries is not confined to softwoods, many use sawdust from Australian hardwoods seemingly with a considerable degree of success. An analysis as described in the January Bulletin would make an interesting study for comparison.