Testing Times for Planter Box Mixes
A question has been hanging over the construction and landscape industry. Can organic matter be used for filling planter boxes? The current thought is that it may be included in the top 300mm, but beneath that, only mineral material such as drainage sand or ash is safe to avoid causing low subsoil oxygen levels and potential root death. After careful testing, this theory has undergone a change.

The real question is, what sort of organic matter can be used and in what amounts?

The Hills BARK BLOWER ® business commissioned Sydney Environmental and Soil Laboratory to design and conduct some trials on a range of planter box “B type” layers to investigate the oxygen depletion problem in new planter box installations. This test investigated what types and quantities of organic matter were safe without risking serious anaerobia. The final results were tabulated and interpreted December 2006.


Figure 1: The Artificial Planter

Figure 1 (above) is a diagram of the trial planter box. The “A” type Planter Box Mix was the same throughout the trial, whereas various “B” type Planter Box Mixes were tested. The “B” profile tested various types and amounts of organic matter. The tubes down the side of the model were the points where soil air was extracted and measurements of oxygen and carbon dioxide taken.

The findings categorically rejected the use of fine unstable organic matter such as mushroom compost and fresh green waste deeper than 300mm in planter boxes. This type of organic matter is still decomposing and uses any available oxygen up in the process.

General Conclusions and Recommendations
The organic matter must be of a stable, particulate (high porosity) and well composted type such as pine bark, peat, coir, or well-composted sawdust. These can be used at up to 50% by volume if the mineral component is also highly porous such as sand and ash. Labile organic matter such as fresh green waste or food waste compost, fresh mushroom compost, fresh stable manures or freshly composted manures, should not be used nor should fine soil components.

The measurements showed that some mixes caused severe and sustained oxygen depletion while others did not. Generally, stable, particulate organic matter such as pine bark mixed with high porosity sand or ash gave acceptable results. See Table 1 (below).

Table 1: Percentage stable organic matter able to be added to mixes at different levels of a planter box