The Use of Recyclable Products for Horticultural Purposes

While in the past, emphasis has been placed on the collection of recyclable materials, the challenge of the future is to find appropriate uses for the products of recycling. We spoke to Oliver Pohls who is conducting research at Burnley College, Institute of Land & Food Resources, Melbourne University into possible materials that can be used to create ‘sustainable soils’.

1. Can you explain briefly what your research has entailed?
My research has involved investigating a variety of recyclable products, both from domestic and industrial sources. The materials examined are basalt dust, clay tailings, food waste, municipal waste, local council green waste and sewerage sludge.
The individual components were blended together using various proportions of each and the final blends were tested under the guidelines within the Interim Australian Standard - Soils for landscaping and garden use (AS4419 (Int) - 1996).
The aims of this research into ‘sustainable soils’ are to identify possible soil media components, establish their properties and assess the performance of particular blends and combinations. This study is limited to a certain degree, as any one of these objectives are in themselves extensive research projects. The term ‘sustainable’ alone suggests many areas of research, especially the estimations of cash values placed on environmental factors that are deemed environmentally progressive.
Despite these concerns, the investigation of using by-products, waste products and recyclable materials as soil substitutes contributes to the overall integrated approach to resource recovery, higher quality soil blends and environmentally beneficial practices. Some of the benefits include: reducing landfill by waste avoidance, waste reduction and waste reuse and also minimising the removal of diminishing topsoil resources for relocation and its accompanying environmental impact.
2. From your research so far, which area of recycling appears to hold the most promise for horticultural applications?
While all the materials tested have potentially desirable properties, they all seem to have limitations also. For instance while the clay slurry was very beneficial in improving the soil structure and nutrient levels, its high pH meant that it limited the growth of some plants ie acid-loving plants.
While sewage sludge was excellent for reducing dispersibility (which is a useful anti-erosion feature of soils and also prevents soils from drying rock-hard or cracking when dry) the high zinc and copper levels are problematic. If the pH falls too low, these elements may become toxic to plants.
In my trials, composted municipal wastes returned the highest growth rates in the plants (silverbeet), but this material (like all the organic components) requires constant testing and monitoring because it is so diverse. The growth rates from the composted food wastes were not as vigorous as the other organic treatments and the material had a strong odour. Incidentally the sewage sludge did not smell at all.
3. Many councils are now separately collecting green waste and recycling it as garden mulch. How much of a threat does this pose in terms of spreading plant diseases and weeds?
The threat is significant if the material is not properly composted and matured. The composting process takes 2-3 months. To kill off known pathogens and weeds the compost needs to be above 55 degrees Celsius for 3 days. Ongoing research is still seeking to establish whether all pathogens and weeds are killed at this temperature. (Weeds came up in the all the composted materials I was supplied for the trials.)
It is a cause for concern that where the composting is placed in the hands of private contractors, the appropriate procedures may not be followed. All workers involved in the process need to be educated about how the composting process works.
4. How should green waste be treated?
Handreck & Black (1994) suggest the following:
Green waste must be mulched
The moisture content should be between 50 and 65%
The heap must be turned 1-3 times a week initially and then turned whenever the temperature of the heap falls below 45 degrees C or rises above 60 degrees C.
All material must spend 3 days in the interior of the heap at 55 degrees C
Active composting takes about 8 weeks. After this the compost should be cured for about 1 month so as to enable the formation of more humus and the conversion of ammonium to nitrate.
The compost should be screened.
In regards to the length of composting time some companies suggest three weeks is enough.
5. What are the major issues involved in the recycling of sewage for horticultural purposes?
The high metal content mentioned before is a problem as is the level of soluble salts. It is possible to stabilise the heavy metals by keeping the pH at high levels i.e. pH 7 and above. However, soil conditions may change over time and we do not know what will happen with the metals, they may come back into solution and be taken up by plants or leached. Sewage sludge is not used in urban landscape applications in Victoria at the moment, though it is used in NSW. The EPA may review its use in Victoria next year.
Other issues include the public perception of using sewage sludge in public areas. I’m not sure whether the public would find it acceptable. Certainly, correct composting procedures are essential if pathogens are to be reduced to safe levels. If the compost is not adequately aged, then smell will be a limiting factor on its use. The high phosphorus content of sewage sludge may pose a problem for Australian plant species (e.g. Proteacae species) as they have evolved in soils with low phosphorus content.