How Much Compost Do I Need?

If you’re getting into compost, you might be wondering how much you’ll need to feed your garden, and whether you need to supplement your own supply by buying commercially available compost.

When you’re new to composting you probably won’t have any usable compost for at least six months. How much you have will depend entirely on how much you compost.

Some types of composters, like compost tumblers, can make compost in a few months – but you will still need the organic materials to add to it in order to make the compost.

In this article, I’m going to give you some idea about how much compost you’re going to need, as well as the implications of not having enough, or having too much.

Calculate How Much Compost is Needed

How much compost is needed depends on what you’re using it for.

Do you want to repot some houseplants, or fertilize a large organic vegetable garden? Are you applying it to the top of the soil as a mulch, or using it as a soil amendment?

Below are some general guidelines for adding the right amount of compost in different situations.

  1. Compost as a soil amendment – You’ll want to add 1-3 inches of compost over the area that you are digging it into, either to break up clay soil, or if you want to add moisture retention to sandy soil.
  2. Compost as a yearly fertilizer – Add 1-3 inches over the area of your vegetable patch each year.
  3. Compost as a mulch – Layer on ¼- ½ an inch of compost on the top of the soil. The nutrients will drain into the soil when the plants are watered.
  4. Compost in pots – you’ll want around 50%-70% of what’s in the pot to be soil, and 30%-50% of it to be compost.

If you are ordering soil, you need to work out the cubic inches of the area you need compost for.

For example; if your area is 120×120 inches, that is 14,400 square inches. Area = 120″ x 120″ = 14,400 sq. in. 

For the volume of 2 inches of compost spread over the 14,400 square inches, you will need 28,800 cubic in., about 16.67 cubic feet, of compost. Volume = 2″ x 14,400 sq. in. = 28,800 cubic inches.

There are many online calculators that can help you out if you’re unsure.

What Happens When There Is Not Enough Compost?

Again, this really depends on what you’re using it for. If you don’t have enough to dig into your vegetable patch, then you can always use it as mulch throughout the growing season. 

Your plants will still grow if you just plant them in soil, but you might need to add fertilizer to help them thrive. Compost acts as fertilizer to add nutrients.

If you’re composting at home, and you don’t think you’ll have enough compost for your needs, then don’t worry too much.

Use the compost you do have for the heavy feeding plants and add more compost to the top of the soil when you have it.

Can You Have Too Much Compost? 

While not having enough compost is nothing to worry about, having too much is another matter. It is more of an issue if you use manure based compost, but it is still a problem with plant based compost.

If your top-soil is any more than 5% organic matter, then your plants may grow too quickly and become susceptible to pests and disease.

If you’re using manure based compost, then the problems can get worse. Manure based compost will leave amounts of phosphorus in the soil that will become toxic to plants.

High phosphorous levels will make it hard for plants to take in manganese and iron, and the plants will become deficient in these nutrients.

How Much Compost to Use Final Thoughts

How much compost you will need is both a complex and simple answer.

The simple answer is: however much you make.

If you are growing vegetables and making your own plant based compost, you can add it as mulch or dig it in once a year, or both.

Depending on the size of your compost heap, you may find that you only have enough to use as mulch, and that’s perfectly fine. You can stretch it out by making compost tea, or you can make your own plant-based fertilizer to add extra feed for your vegetables.

The complex answer comes if you want to buy compost for a certain project. This then depends on the size of your plot and what you want to achieve.

Whatever your requirements, too little compost is better than too much. If you think you have got too much, dig in the compost after the growing season in the fall. This way, the compost will be mixed with the soil before you plant out your seedlings in the spring.