You’ve probably spent the better part of the last gardening season keeping your compost pile healthy. Once it’s fully matured, what will you do with all that compost?
Sprinkle over topsoil, use it to germinate seeds, or mix it into the soil to enrich the nutrients available to plants?
As compost is a soil amendment, it’s always best to mix it with soil or apply it as a top layer over your soil.
Which method you use, doesn’t really matter. Both get extra nutrients to the roots of plants.
What you do need to figure out though is just how much compost to add to your soil.
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Ideal soil to compost Ratio
The soil to compost ratio differs based on what you’re growing, and where you’re growing the plant. With ground soil for flower beds or border plants, the maximum compost to use is 30%. In containers, only 25% of a soil mix should be composted to allow for sufficient aeration. As a potting mix for container plants, a good ratio is 4 parts soil to 1 part compost. In vegetable gardens, one-fifth of an inch of compost for each inch of soil depth. Trees and shrubs only need about 10% compost, which is a 9:1 ratio – 9 parts soil to 1 part compost. Preparing new lawns is best done by tilling in 1 to 2.5 inches of compost to a depth of 6 inches. On an established lawn, you only need one eighth to one-quarter of an inch of compost raked into the grass after aerating it.
Soil to Compost Ratio for Raised Bed Gardens
Raised bed gardens always do better with a soil amendment. Compost is ideal, but in lower quantities than you’d use with your ground soil for flower beds or vegetables grown in ground soil.
Containers do well with an overall soil to compost ratio of 3:1 or 75% soil to 25% compost. If you’re adding a potting mix containing peat moss or vermiculite to improve drainage, you can use more compost, up to the maximum of 30%.
The reason you need less compost in raised bed gardens is because the conditions are more compacted so there’s less aeration.
A common method for using compost in raised garden beds is to mix 30% compost with 60% topsoil and mix in a 10% potting mix that contains peat moss, vermiculite, or perlite.
Soil to Compost Ratio for Ground Soil
For all types of plants, the maximum overall ratio of compost to soil shouldn’t exceed 30%. It can be higher, but there’s really no point unless you’re working with dirt that has no nutrient value whatsoever.
There are of course certain plants that can’t tolerate ground soil such as the Monstera Deliciosa, in which case, in which case you’d need to use an alternative potting mix and apply the compost through that instead.
Provided you have good soil to start with, aim for no higher than 30% total volume of compost.
New Lawns
For new lawns, it’s best to till between 1 to 2.5 inches of compost into the top 6 inches of soil before adding lawn seed.
On an established lawn, one-eighth to one-quarter inch of compost can be raked into the grass after aerating it.
Vegetable Gardens
Up to 30% of compost can be added to ground soil being used for growing vegetables.
If you’re using a rototiller, aim to till an inch of compost to a depth of five inches. If you’re adding it manually, aim to add a fifth of an inch of compost for each inch of soil.
Trees and Shrubs
Use soil to compost ratio of 9:1 – 9 parts soil to 1 part compost.
Established trees can be top-dressed with up to a half-inch of compost.
How to mix soil with compost
To use compost, you can till it into the soil or layer it over your soil. Which method is better is a subject of debate.
Using compost as topsoil is the simplest method as all you do is sprinkle the compost over the soil and let the rainwater wash the nutrients down.
Think of this as feeding your soil with compost tea.
The other method is to till your compost into the soil.
The benefit of this is the speed of delivery. Nutrients reach the roots faster.
Depending on what you’re using your compost for, you may not need to mix it at all.
An example of this is using your compost as lawn fertilizer, in which case, it’s best to aerate the lawn first, and then rake the compost over the lawn as a top dressing.
The simplest method to use your compost is to enrich aging soil. Soils lose their nutrients over time. Mixing in compost lets you reuse your potting mixes, saving you money and it contributes towards growing healthier plants, too.
Make sure to read our article about the best compost for garden.
Frequently Asked Questions About Soil and Compost Ratio
Can plants grow in compost without soil?
It’s highly unlikely because soil is different from compost. The core purpose of compost is to amend the soil. It’s rarely suitable for soilless gardening. Where compost differs the most is in its richness of micronutrients. Growing with compost only is likely to introduce more problems than it solves. Compost is highly degradable, which is why it works for fertilizing soil. On its own, it drains far too fast for most plants, lacks the structural support for any decent-sized plant roots to get established in, and once they’re established, the compost will shrink as it degrades. For those reasons, always use compost to enrich your soil and never try to replace the soil with compost.
Do compost ratios need to be altered for vermicompost?
Vermicompost will have a higher amount of nitrogen, phosphorus, and potassium because of the worm castings. In any type of compost, manure is always beneficial. As with most things in life, too much of a good thing is a disaster in the waiting. In the case of vermicompost, if you add too much to already healthy soil, there is a risk of adding too many nutrients, which will have the reverse effect and actually damage the plant. Before mixing your compost and soil, remember you’re adding organic fertilizer to the soil. It has nitrogen, phosphorus, and potassium, plus multiple macronutrients. If you’d apply fertilizer sparingly, then lower the amount of compost you use.
Daniel has been a plant enthusiast for over 20 years. He owns hundreds of houseplants and prepares for the chili growing seasons yearly with great anticipation. His favorite plants are plant species in the Araceae family, such as Monstera, Philodendron, and Anthurium. He also loves gardening and is growing hot peppers, tomatoes, and many more vegetables.