Mushroom Compost - From Farm to Garden

What is Mushroom Compost? - Mushroom compost (also referred to as Spent Mushroom Compost) is compost which has been used on mushroom farms to grow and harvest mushroom crops.

To get the best yields, fresh new compost is used for each harvest, and the spent mushroom compost is left over as a by-product – a by-product which is much appreciated by gardeners! It is high in organic matter and nutrients and can greatly help struggling gardens.

Life on a Mushroom Farm

Here at Online Soil, we are lucky that located just 3.5 miles from our site is one of the UK’s largest mushroom farms. Twice a week, our tractors make the short trip backwards and forward carting mushroom compost to our yard. Here it is composted down further ready for the finished product to be dispatched to our customers. The compost that we collect has been busy playing a vital role on the farm, so what has it been up to before we go and collect it? I popped down to the farm to find out…

Mushroom Farming Stages

Step 1: Compost Mixing

The first stage on a mushroom farm is to mix the mushroom compost – Phase I composting. This is a mixture of horse manure, chicken manure, straw and water. Our local mushroom farm mixes around 280 tonnes at a time and the compost spends two weeks in a mixing tunnel.

Step 2: Pasteurisation Tunnel

Following mixing, the compost goes into a pasteurisation tunnel – this is Phase II composting. This kills any insects and pests and conditions the compost. Conditioning is essential as this removes ammonia formed during mixing which can have negative impacts on mushroom growth.

Step 3: Mushroom Spawn Added

The compost is now ready, and it is time to add the mushroom spawn. The mushroom spawn is mixed with the compost and put into wooden growing trays.

Step 4: Peat Layer Added

Water is very important to mushroom growth, so a layer of peat is added to the surface to help water retention. This is referred to as casing, and the peat is kept moist acting as a water reservoir. This layer is where the mushrooms eventually form.

Step 5: Time To Grow

The trays are now moved to a growing tunnel where they grow in the dark. Here, temperature, moisture, humidity and carbon dioxide levels are closely monitored and controlled to achieve the best yield.

Step 6: Harvesting/ Flushing

After around 18-21 days, the mushrooms will be ready to be picked. Once the first ‘flush’ is picked, two more subsequent flushes will grow and be picked off the same crop. There is normally 7-10 days between the flushes so each full harvest takes around 35-42 days.

Step 7: Compost Sterilisation

After the mushrooms have been harvested, the trays and compost are sterilised with steam. This kills off any insects and pathogens along with any remaining spawn so they don’t take over in your garden!

Step 8: Arrival of Online Soil Tractors

Online Soil arrive onsite with tractors and trailers to collect the spent compost. We transport it the short trip back to our yard where we tip it into a big pile. It is left here to compost down further, regularly getting mixed, until it is ready to pack into bulk bags and dispatched to our customers.

This process is then repeated, so that all year round you can buy fresh mushrooms from the supermarket, and we continue to collect the mushroom compost by-product. From start to finish, from mixing the compost at the start to Online Soil collecting the spent compost at the end, the process takes approximately 12 weeks.

Using Mushroom Compost in Your Garden

Spent mushroom compost is very high in organic matter and is a great addition if your garden is in need of a bit of a boost. It can either be incorporated into you soil before you begin planting, or it can be used as a topdressing - both ways will help improve the quality of your soil.

Where to Use Mushroom Compost?

  • Mushroom compost can be used as a soil improver in poor soils, and for mulching to help with water retention.
  • Mushroom compost is alkaline, so is a good addition to very acidic soils which are low in organic matter, helping to neutralise the soil.
  • It is good for vegetable gardens, and can even help to prevent clubroot disease in commonly affected vegetables such as cabbage, broccoli and cauliflower.
  • It is good for improving soil structure, particularly in clay and sandy soils.
  • It is important to note that if you are growing ericaceous plants (acid-loving plants) such as rhododendrons, then mushroom compost should be avoided.
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