It is best to add compost underneath mulch rather than on top as the compost can then naturally work its way into the soil by worms and soil microbes. Cover the compost on with a 2 inch layer of mulch to keep the moisture in and help to prevent weeds.
This article will explore all you need to know about composting on top of mulch and how to use both ingredients together for the best result.
Why you should mulch on top of compost
When aiming to improve the soil in your flower, vegetable or raised garden beds mixing compost through is a fantastic idea. You can do this at the end of the vegetable growing season in Fall or Spring or any time you are preparing a bed for new plants.
Compost should always be put directly onto the soil and can be mixed through with your garden fork or left to mix through naturally. Adding layers of compost is a great way to improve your no-dig garden. Layer 3 inches of compost straight onto the top soil if it is weed free.
If there are weeds or if you are starting a new garden bed, first layer thick cardboard covering the whole area. You can fill any gaps with newspaper and then layer 5 inches of compost on top of this. Layer 2-3 inches of straw or bark mulch on top and allow it to sit for around 4 weeks.
The cardboard will start to break down and you can plant into the compost. Plants will reach their roots down into the soil gradually and the weeds will be stopped by the cardboard.
How to remove mulch before adding compost
Before adding new compost to your garden remove the mulch with a rake. You can use a rake to drag the mulch back into the corner of your garden bed. Then layer compost on top of the soil around 3 inches thick. Rake it flat and then cover the compost with mulch.
You do not need to dig the compost through the soil and it is best not to if you are placing it over established plant roots. This is called top dressing and is perfect for feeding the soil of citrus, mangoes, established trees like camelia and palms.
How to add compost to a mulched garden bed
To add compost to your mulched garden bed you will need a rake and a bucket or wheelbarrow to move the mulch. Use a rake to move the mulch back from the surface of the soil and layer the compost on top. If there are no plants in the garden you can gently mix the compost through to help it to incorporate quicker.
If you already have plants growing in the garden bed leave the compost on top of the soil. This is also a great method if you are creating a no-dig garden. Smother any weeds with newspaper or cardboard first to stop them growing through the compost and mulch.
The next step is to cover the compost with a 2-3 inch layer of mulch.
You can use the old mulch to cover the compost. Old mulch is great for garden beds as it will have already started to break down so will incorporate and improve the soil well. Over time you can add more mulch to keep the mulch layer up to 2 inches thick.
You could use straw, pea straw, bark mulch or leaf mold. For more on making compost or mulch out of leaves, check out this article, How to use compost bags | Easy guide
Is it OK to put top soil over mulch?
It is best to avoid putting top soil over mulch. As mulch breaks down under the top soil, it will draw nitrogen from the soil locking it up and taking it away from the plants temporarily. It will also dry out the soil as the mulch has low water holding capacity and lots of air pockets.
What can be put on top of mulch
Layering new mulch on top of old is a great idea. Once the mulch has started to break down and is less than 2 inches thick, top it with fresh mulch to keep the layer thick enough to prevent weed seeds from growing.
Mulch should always be the top layer of your garden and always remember to dig down to the soil before planting new plants.
Does mulch turn into soil?
Mulch will eventually turn into organic matter and will mix through soil over time. Soil is made up of many parts including organic matter, clay, silt, sand and minerals. Bark mulch will take over 6 months to break down on top of your soil and turn into compost which will mix through soil.
Letting the mulch naturally break down and be mixed through by worms and soil bacteria is the easiest way to improve your soil.
What to use with compost to mix through your soil
Alongside compost you can also mix through aged animal manures to improve your soil. Try aged cow, chicken or goat manure but make sure it has been broken down and looks like compost. Never add fresh manures to your soil as it can burn the plants.
Can You Put Compost on Top of Mulch? | Summary
It is best not to put compost on top of mulch. Layer cardboard, newspaper, compost and then mulch on the soil to create a no dig garden bed. Use a thick layer of cardboard, 3 inches of compost, a thin layer of cow manure and then 2 inches of mulch on top. Leave the mix for 4 weeks and you can plant straight into the compost layer or dig down to the soil.
I am an accredited practicing dietitian, experienced gardener and a dedicated cook. I love writing and sharing my experience so you can learn from my successes and mistakes.