Recycling spent mushroom substrate (SMS) for fertiliser in a circular economy (MU21006)

This project has been exploring the opportunities available for the reuse of spent mushroom substrate (SMS) back into broadacre cropping systems from which wheaten straw is sourced for the original mushroom substrate. 

SMS was characterised to establish the potential value proposition of the product to the grains industry. The grains industry was also consulted to gain insight into the opportunities and barriers associated with the reuse of SMS in broadacre agriculture. 

The research team found that the chemical profile of SMS is generally similar to that of other organic materials (green waste compost, manures). Further, carbon composition analysis using solid-state 13C CPMAS NMR (Cross Polarisation Magic Angle Spinning - Nuclear Magnetic Resonance) indicates that most of the carbon in the SMS is in highly labile forms. As such, this material would be rapidly decomposed by soil microbes, with little residual carbon benefit. The nutrient profile and carbon composition analysis indicate that SMS offers no particular advantage to grain growers over other types of organic by-products that may be locally available to them. 

SMS has some challenging aspects associated with it such as transport and handling logistics. In addition, application of fertiliser products in the grains industry generally occurs at the autumn break, yet SMS is produced year-round. Some of these challenges could be addressed by “value-adding” processes (e.g., dewatering, further refinement and/or nutrient augmentation). However, consultation with the grains industry indicates significant uncertainty as to whether investment in SMS value-adding would be worthwhile. Of specific concern to growers and their advisors is the cost:benefit associated with the use of any new product. Although alternatives to high analysis fertilisers are of increasing interest, a higher value SMS product would need to compete on its own merits.

Although value-adding to SMS is technically feasible, it is potentially a very demanding and risky process given that product performance also needs to be proven with target crops in the field. 

A mapping exercise has been completed investigating the supply chain of spent mushroom substrate (SMS) in relation to mushroom producers and sources of wheaten straw. 

The Australian mushroom industry currently produces approximately 345,000 tonnes of SMS per year as a by-product of the mushroom production process. While there are established reuse options for SMS into the landscape, turf and horticultural industries, there is interest in understanding the opportunities for SMS reuse back into the broadacre cropping systems from which wheaten straw is sourced to manufacture the original mushroom substrate.

For a tight loop circular economy to be viable, where SMS is returned to wheat straw producers, several logistical and economical challenges need to be considered. Possibly the most significant of these issues is the distance between the production of SMS and wheat farms that supply straw. Economically transporting wheat straw to mushroom farms over these significant distances is feasible as the wheat straw has value to the mushroom producer. The economics of transporting SMS back to wheat farmers is not as simple, as the SMS does not have the same significant value to the wheat farmer. While empty trucks do travel to wheat farms to pick up straw, they are unfortunately not the right configuration for transporting SMS. Unless the transport issue can be resolved, the present value of SMS is diminished due to the high cost of transport over large distances. 

If the value of SMS could be proven or enhanced, it is likely that wheat farms located closest to mushroom farms would provide the best opportunity. This mapping exercise has shown that this would be most feasible in South Australia and to some extent Victoria. Alternative opportunities such as vegetable farms which are located closer to mushroom farms have been considered and are entirely feasible in terms of distance. Given that all mushroom farmers currently have sustainable disposal methods that are either cost neutral or make a small return, it needs to be questioned what the benefit of seeking alternative uses of SMS is at present, given the logistical and economical challenges that have been identified.

The next phase of the project will consider the nutritional and soil amendment properties of SMS and aims to determine if there is value for wheat or vegetable farmers and if it would be possible to enhance SMS to add value. If this is possible it may improve the value proposition of SMS.

A desktop review has investigated various means in which spent mushroom substrate (SMS) could be transformed into value-added fertiliser or soil amendment.

The Australian mushroom industry currently produces approximately 345,000 tonnes of SMS per year as a by-product of the mushroom production process. While there are established reuse options for SMS into the landscape, turf and horticultural industries, there is interest in understanding the opportunities for SMS reuse back into the broadacre cropping systems from which wheaten straw is sourced to manufacture the original mushroom substrate.

The review has identified some knowledge gaps that need to be addressed before SMS can be reused for the broadacre cropping systems:

1. Reported industry chemical analyses of SMS are outdated and may not reflect current production systems.
2. There is a geographical disconnect between the cropping farms from which the straw is sourced, the mushroom compost manufacturers, and the mushroom growers who then generate the SMS. The degree of disconnect varies according to the vertical integration of businesses across compost manufacture and mushroom production. This means that while it may be attractive for compost manufacturers to provide SMS back to cropping farmers in exchange for favourable straw agreements, the SMS may be generated by several businesses, across a range of locations.
3. The geographical spread of SMS across the industry means that any value-adding process may need to either be mobile, or multiple sites located close to the mushroom producers.
4. While value-adding of SMS through nutrient addition and palletisation may increase the attractiveness for reuse in broadacre cropping systems, there is significant cost in creating a value-added product.
5. For cropping farmers to consider receiving SMS-derived products, a range of issues around product quality and integrity, transport, logistics and soil and plant benefits need to be addressed for them to understand the value proposition of using these products either instead or, or in-addition to other farm inputs.

The overall feasibility for any change in how the mushroom industry values and redeploys SMS will need to be assessed based on opportunity cost. An evaluation to understand how recommended changes compare to current practices is required and will be addressed in future project work.