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What to do with compost: making your own potting soil mix

If you find yourself wondering what to do with compost that you’ve made, chances are you’ve realised that home made compost just isn’t suitable for potting up plants and seedlings. Being made entirely of rotted plant material, it is too high in nutrients and too easily waterlogged to support tender young roots. To make a potting compost (also known as potting soil), it needs to be blended with other ingredients to make it less rich and drain better.

a compost sieve helps turn home made compost into potting soil

a sturdy potting riddle

Sieve your compost

To break your compost up into a nice friable texture and get rid of any uncomposted woody chunks, it needs to be seived. You can do this with a potting riddle, raking the compost back and forth with a trowel until only the bigger chunks are left. These then go back into your active compost heap, leaving you with lovely crumbly compost that’s easy to mix with your other ingredients.

If you want to make your potting soil on a much larger scale, make yourself a box frame with a galvanised mesh bottom (6mm works best) and really strong post corners. If you can, make it to just fit your wheelbarrow so that you don’t need to make a bottom section to catch the sieved compost.

home made compost: powerful stuff

Other potting compost ingredients

home made potting soil is perfect for containers

Loam

Loam gives the compost mix structure and stops it from slumping (reducing in volume) too much. Loam is a type of soil that has sand, silt and clay in fairly equal amounts.  If you lifted any lawn when you were landscaping or making paths, you may have made some by stacking the turves to rot down under a tarpaulin for at least a year; just sieve it before use. If you don’t have any loam but your topsoil is reasonable, you can sieve some of that instead. If your own topsoil is very sandy or has a lot of clay in it, you may need to buy a bag of loam or balanced topsoil.

Leaf mould

Leaf mould is the best ingredient for adding humus to your potting soil, giving bulk without additional nutrients. It’s easy to make your own leaf mould, but if you don’t have any you can use coir or peat-free soil conditioner from the garden centre. Warning: avoid anything that says it has added nutrients.

Making a potting compost mix

Once your three ingredients have been sieved, mix equal volumes of each in a bucket or wheelbarrow. If you want higher nutrient levels (for example, in square foot gardening) add a tablespoonful of fish, blood and bone meal per bucketful. If the mix is very dry, water it lightly from a can with a fine rose but don’t overdo it. Leave it to stand for half an hour before mixing again.

Your blended potting compost mix is best used immediately, but it can be stored in a plastic bag in cool conditions. We recommend not keeping it for longer than three months, because the nutrient levels will gradually fall.

What to do with compost

Once your compost has been sieved and blended you can use it for everything you’d normally use commercial compost for: container growing, potting plants up, topping up raised beds and square foot gardening. Don’t forget that unless your home-made compost was created using the hot method, there are likely to be viable weed seeds, slug eggs and other such undesirables in it. This isn’t the end of the world: it just means that it behaves exactly like regular garden soil. Keep an eye out for trouble, and you’ll be fine.

home made potting compost is suitable for raised beds such as this quare foot garden

Potatoes image courtesy of Jo-H; square foot garden image courtesy of Robert Goodwin

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4 Responses to “What to do with compost: making your own potting soil mix”

  1. dave mcdee says:

    Yet more useful information. I `d wondered why all the peas I planted in neat compost just rotted away. With your advice hopefully next time will be more successful.

  2. rita gascoyne says:

    The best compost info I’ve come across. Explained simply & clearly exactly what I wanted to know. Grateful thanks from a keen but groping-in-the-dark gardener!

  3. John Daly says:

    Is it possible to mix a portion of homemade çompost with water to improve watering quality

    • Andy McKee says:

      I’ve never found it to be a problem, but if things have really dried out too much you can add a drop or two of a plant-based dishwashing liquid (such as Ecover) to each litre of water and it should soak in just fine.

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