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Permaculture • Homesteading • Organic Gardening • Self Sufficiency • Sustainability

How Much Compost Do I Need For My Garden?

We all know that adding compost is a crucial element of keeping your garden rich and ensuring that your plants are nurtured and have the nutrients to stay healthy. Soil that is high in organic matter can help plants to grow strong, but how do you know how much compost you need?

Bags of Compost

You can either apply compost as a top dressing to enrich the soil or mix it in with the soil that is already present to amend its composition. Both have different benefits and drawbacks and require different amounts of compost. In general, you have to add up to half an inch of compost if you are adding a top dressing, and up to two inches for soil amendment.

How Do I Know How Much Compost I Need?

The online calculator below will help you work out how much compost to buy based on the area that you want to cover. You will need to input the depth that you wish to achieve and the total overall area, and then the calculator will tell you how much compost you should purchase.

For example, if you want to cover four feet by four feet adding 1/2 inch of compost top dressing, you will need .67 cubic feet, so you should buy one 40-pound bag which contains about .75 cubic feet of compost. You would then have some left over.

It’s best to aim for a little more, rather than a little less, as you can always add a bit more compost, or save it for another project. However, being able to calculate with a reasonable degree of accuracy will save you from buying excess, or ending up unable to finish your project without another trip to the garden center.

What Happens If You Add Too Much Compost To A Garden?

Although compost is great for the garden, you need to make sure that you are adding it in the right quantities. Adding too much compost could get very pricey, but it might also be harmful to the plants.

Compost contains a lot of phosphorus, and in high concentrations, this could be harmful to a plant. It makes it much more difficult for plants to effectively absorb other nutrients they need, such as manganese and iron. This can, in serious cases, kill your plant.

You, therefore, do need to pay attention to how much compost you are using and avoid adding too much at once. Overall, you should only be adding between 25 and 30 percent at the most. More than this could be harmful.

If you are laying the compost on top, rather than mixing it into the soil, you should aim for no more than around half an inch. That might not look like a lot, but it can make a big difference to both the structure and nutrient content of the soil once it has worked its way into the ground.

Adding Compost Top Dressing

It is also important to note that compost is not the same as fertilizer, although it will result in some of the same outcomes, and many gardeners use it like fertilizer. The difference lies mostly in how accessible the nutrients are.

In fertilizer, the nutrients are immediately accessible to the plant, and it can begin absorbing them immediately. In compost, these nutrients are present but stored within organic matter. It takes far longer for compost to release its nutrients than fertilizer.

However, if you are using a lot of compost, you will probably want to reduce how frequently you fertilize the soil, at least for a while. The plant should have access to plenty of nutrients as a result of the compost, even if they are being released more slowly. Too much rich soil could be harmful.

What Is Top Dressing Vs Soil Amendment?

Top dressing and soil amendment are two different processes, although both can involve using compost to enrich the soil.

Top dressings are spread on top of the soil, while soil amendments are dug into it and mixed with it. Top dressings tend to be easier to apply, but they will take longer to have an effect, as they have to be combined by weather and insect activity, rather than being mixed in by you.

When you apply a top dressing, you usually spread a fairly thin layer of compost across the surface of a bed or lawn. If you are covering plants (e.g. grass), this needs to be thin enough not to block their light and risk damaging them.

Over time, the rain will then wash this organic matter into the surface and further down, helped by worms and other invertebrates. It will mingle with the soil, gradually enriching it and releasing nutrients to the plants’ roots.

Compost Amending

By contrast, soil amendments are mixed directly into the soil. Obviously, this involves disturbing the top layer, so it isn’t usually suitable for enriching lawns and other established plant areas. You will need to dig up the existing soil to around six or eight inches down, mix it with some compost, and then redistribute it.

Doing this enriches the soil more quickly, but it does involve quite a lot more work. It may also use more compost, although this depends on how deep you would make a top dressing if you chose that method instead.


You should calculate the amount of compost that you need based on your garden size and the depth of compost that you wish to add to the soil. Doing this before you start will ensure that you can purchase the right amount of compost for your project, rather than getting too much or too little. It will also reduce the risk of accidentally adding too much and damaging your plants.

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