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By Neville Passmore


Swiss Brown mushrooms ready to pick                          Mushroom compost ready for the garden



Mushroom compost is a popular soil conditioner and soft mulch for garden beds particularly for vegetable, fruit and herb growing in WA.  One of its greatest attributes is consistency.  I  didn't appreciate what goes into achieving this consistency until I spoke to Hilton Rich-Bowles, Growing Manager of the Costa Mushroom Exchange which harvest 70 tonnes of mushrooms per week from their Casuarina facility.

Making the growing substrate is a critical step in the growing process.  This is created by Costa Mushroom Exchange in Nambeelup where they share the site with C-Wise and Craig Mostyn Farms. 

The specifications of the ingredients are extremely tight.  Swathed wheat straw stubble is contract harvested. The requirement is that the straw should be 30cms long, anything under 20cm in length is unacceptable.   Chicken manure comes exclusively from broiler chickens grown for the meat bird industry.  Manure from caged egg layers, breeders or free range chicken is either not consistent enough or the nitrogen content doesn't comply with the necessary 4%.  Gypsum, used to flocculate the mixture so it’s not greasy, is obtained from one WA mine that has proved itself able to deliver consistent quality material year in and year out.  Water comes from an onsite bore and from runoff and wash-down water at the production facility.  All runoff water from the production process is collected in ponds where it is aerated before being pumped into the production process.

Mushroom growing substrate requires very high levels of moisture in the process.  I was amazed to hear that before loading to bring to the mushroom farm in Casuarina the mix must have 75% moisture, not 74% nor 76%. The operators are able to accurately estimate the moisture content by squeezing and listening to the mix.  In these days of technology this is a great example of the human hand being faster and more accurate than a machine. 

The ingredients are mixed to a very exact formula and then placed on a concrete pad where a windrow turning machine aerates the mix every few days adding moisture as required. Each batch of the substrate is composted for about 3 weeks at the composting facility before it’s transported to the growing facility in Casuarina.

Here the substrate undergoes controlled composting in forced air tunnels.  High temperatures are achieved to ensure pathogens are eliminated, then subsequent lower temperature bugs remove ammonia and convert the substrate into a nutrient rich material that is perfectly suited to feed the growing mushroom.  

Once conditioned, it is ready for mixing with the mushroom mycelium. These are the fast spreading root-like structures that quickly colonise the growing substrate.  The substrate is loaded into steam pasteurised wooden trays and topped with a mix of German, Irish and Baltic peat.  The acid reacting peat is mixed with limestone to bring the pH up to 7 which is neutral.  This carefully selected peat mix  becomes home for the mushrooms that emerge as pin heads around 16 days later.

Costa Mushroom Exchange sells white cap mushrooms as well as the Swiss Brown or Portobello forms.  Sizes vary from buttons to the large field style mushrooms.  Picking takes place 7 days a week and engages nearly 200 people. 

Once the mushrooms crop is finished the wooden trays are returned to the steam sterilising facility where both the wooden trays and the spent mushroom compost is sterilised to ensure that no pathogens can re-enter the process when the emptied trays are used for the next crop.  This means that the used mushroom compost is guaranteed to be clean with no weed seeds, disease organisms or insect pests.

Every step in the process to grow mushrooms reflects consistency, which means that the compost that comes out of the process is the same every week.

The spent mushroom compost may not be suitable for growing mushrooms anymore but it is a fantastic product for home gardens and professional horticulture.  The composted straw, manure, gypsum, peat and limestone make a perfect blend of rich organic materials to build soil health. 

This is a great example of recycling in action.  The mushroom growers use the by-products  from the wheat and chicken industries and then recycle their own wastes back into the soil for another beneficial re-use. What's not to like?

C-Wise have the contract to remove all of the spent mushroom compost from the Casuarina facility.  Every week this rich source of organic carbon makes its way to retail soil yards, fruit and vegetable growers and back to the C-Wise composting facility where it is blended into a number of high quality composted products. It is also registered as a certified input for organic producers.

Mushroom compost is a dual use product in the garden being a soil conditioner and a feeding mulch.   When used as a mulch it fights soil erosion. It can also be incorporated into the soil before planting.  

It has a dramatic effect on improving drainage in clay soils and holding moisture in sandy soils.  With a water holding capacity of 300% of its own weight and a pH of between 6.75 and 7 it is very plant friendly.  In commercial vineyards and orchards, mushroom compost is a long life product that is only applied every 2 to 3 years.  Use around fruit, vegetables and flowers for excellent results.   


       Buttons emerging from growing trays                               White buttons ready for dispatch