Mushroom compost, what it does and where not to use it

Mushroom compost is what's left over after a mushroom crop has finished with it, so it starts life as a mix of straw and manure with some lime and gypsum worked through it, and it gets composted before it's bagged. That history is the whole story of the product, because it explains both what it's good at and the one place it causes trouble.

What it does well

It's organic matter, and organic matter is the thing most South Australian garden soil is short of. Dug through a bed it opens up heavy clay so that roots and water can actually get down through it, and in sandy soil it does the opposite job of holding onto moisture that would otherwise drain straight past. It carries some nutrition as well, gently rather than in a hit, so it's feeding the soil more than it's feeding the plant.

On a vegetable bed between crops it's about as useful as anything you can buy in a bag.

Where not to use it

Mushroom compost is usually alkaline, because of the lime and gypsum that went into growing the mushrooms in the first place, so it pushes soil pH up. For most of a garden that's either helpful or it doesn't matter. For acid loving plants it's the wrong thing entirely, so keep it well away from azaleas, camellias, rhododendrons, gardenias and blueberries, because you'd be undoing the exact soil conditions those plants need to do anything at all.

Straight out of the bag it can also be on the salty side, so it's worth forking it through a bed and watering it in rather than planting seedlings directly into a thick layer of it.

How much, and how

Spread it over the bed and fork it through the top 10 to 15 centimetres, then water it in and give it a week before you plant if you've got the time. Around established shrubs and fruit trees you can leave it sitting on the surface as a layer instead, just kept back off the trunk so the bark isn't sitting in wet compost.

It isn't a product you need to be precise about. A bag through a bed, forked in, is the sort of thing it's designed for.

It isn't a fertiliser, and the difference matters

Mushroom compost is a soil improver, so if the bed needs body and structure then this is the bag you want. If the plants themselves need feeding, you want a fertiliser as well, because the two jobs aren't interchangeable and buying one expecting the other is how people end up disappointed with a perfectly good product.

Shop related products

• Naturally Good Mushroom Compost 25L. The one this post is about.
• Brunnings Garden Compost with Seaweed 25L. A neutral compost for the acid loving beds mushroom compost has to stay out of.
• Martins Organic Sugar Cane Bale 11kg. Goes on top as mulch once the compost is in.
• Neutrog Whoflungdung Mulch 20kg. A fine composted mulch if you want the feeding and the covering in one go.

Did you know we also stock a soil pH test kit and gardening gloves in the shop, and the test kit is the cheap way to find out whether your beds want lime or want keeping away from it. They're not on the website, so give us a ring on (08) 8536 4333 to check what's on the shelf.

The rest of it's in our garden centre, and the garden range is online if you'd rather have a look first.

Read next

• Cow manure in the garden, and where not to use it
• Blood and bone, what it does and where not to use it
• How to fix a garden that stays wet after rain

I go through more of this stuff than I probably should, mostly because I'd rather fix the soil once than argue with it every season.

Cheers,
Candeece