How To Use Compost In The Garden (3 Important Applications)


Did you know that yard waste like leaves and grass clippings account for up to 20% of the waste sent to landfills? What a waste! Instead, you could use them to make fertile compost that provides many benefits to your garden and yard in three important applications.

You can use compost in the garden by 1) amending or topdressing new and existing garden beds, 2) mixing it into a potting mix or a raised bed, or 3) using it as a mulch. For most applications, you need finished compost – compost that is dark, crumbles easily, and smells like fresh earth.

However, it’s important to remember that compost is not a miracle solution and more is not always better. The optimal concentration of organic matter in your soil is only between 2 to 5%. 

It’s always important to get a soil test done regularly to monitor nutrient levels in your soil. Compost contains a lot of the micronutrients plants need, but the concentrations of nutrients differ between different sources and may not include enough (or maybe too much) of one or more nutrients that your soil is lacking.

You may need to use another amendment instead of or with compost to give your soil what it needs.

How To Use Compost In The Garden

So first off – why compost? Compost adds many benefits to your garden, including:

  • Adds macro- and micronutrients to your soil that slowly release over months or years to feed your garden. Synthetic NPK fertilizers don’t contain the micronutrients that plants need and even slow release fertilizer don’t last as long.
  • Improves soil structure, making the soil easier to work with. Compost loosens clay and silt soil, preventing compaction, improving drainage, and allowing roots to spread. Compost helps sandy soils keep water and nutrients for longer.
  • Feeds beneficial microbes, insects, and worms. Insects and worms aerate the soil as they burrow and feed on the microbes. The microbes aid plants by helping roots gather more nutrients and support plants against diseases and pests.
  • Prevents erosion and decreases runoff. Good topsoil is precious, and you want to keep it in your garden bed, not flying off in the wind. Nutrients like nitrogen and phosphorus are necessary for plant growth, but too much and they’ll run off when it rains to pollute nearby waterways, causing toxic algae blooms. Compost holds onto more water and encourages healthy root systems to hold down soil and use up those nutrients. You can also reduce or even eliminate the use of synthetic fertilizers, which are the primary cause of nitrogen and phosphorus pollution.
  • Diverts kitchen and plant waste from garbage dumps while keeping money in your pocket, if you make your own, since you don’t need to buy synthetic fertilizer for your garden.
compost bin
Compost adds micronutrients to the soil and helps to feed earthworms, among other benefits for your garden.

How can you best use it in your garden? Here are three important applications for compost in your garden:

  1. Apply a layer of compost on top of your garden (called topdressing) or mix it into the top few inches of soil to add nutrients and improve the soil structure.
  2. Like with amendment, compost adds nutrients and helps with drainage. You can also refresh used potting mix by mixing it with compost. Compost should only make up ⅓ to ½ of your mix, and when using it for potting mix, screen it to remove any remaining debris.
  3. Adding a thick layer of plant-based compost (3 to 6 inches deep) can help suppress weeds and conserve soil moisture while still providing all the benefits of compost. In the fall, you can also use all those dead leaves in your yard as a mulch to cover the soil over the winter. By spring planting, most of it will have decomposed to finished compost.
pottings soil mix plant in container
You can use compost as part of a homemade potting mix.

How Do You Know When Your Compost Is Ready?

Depending on your methods, compost can take as little as a month or as long as a year. Factors that affect this length include the size and type of organic matter in the pile, how often you turn your pile, and whether you use hold or cold composting.

compost bin
Make sure you turn your compost once in a while to aerate it, and keep it wet enough to encourage decomposition.

The hotter the temperature, the faster the decomposition, and the only way to get your compost pile hot enough is by turning it often and making sure it’s moist. Many gardeners plan to use hot composting, but end up actually using cold composting since they don’t turn their pile.

Your compost is ready, or “finished”, when it: 

  • Looks black or dark brown, 
  • Smells like earth,
  • Has no visible plant or animal parts (except for hard debris that takes longer to decompose like twigs and fruit pits), and
  • Crumbles easily in your hand.
Compost
Finished compost looks like this: black or dark brown, with an earthy smell. You can’t really tell what it was made from.

You know your compost isn’t ready when it:

  • Has visible plant or animal parts,
  • Feels still warm,
  • Smells sour or like ammonia, and/or
  • Has lots of large lumps remaining.
compost
Unfinished compost has visible plant parts (such as the leaves above), feels warm (meaning it is still decomposing), and may smell sour or like ammonia.

How Much Compost To Put In Your Garden

When starting a new garden bed, apply up to 3 to 4 inches of plant-based compost. 

For existing garden beds, apply 1 to 2 inches of compost. 

If you’re filling a raised bed, fill it with ⅓ to ½ compost (and no more). 

pallet raised bed
Fill a raised bed 1/3 to 1/2 of the way with compost (no more!)

If you’re intensively growing vegetables (using succession planting and close planting), then reapply compost every time you turn over the bed. 

If you’re using manure or mushroom compost, use only 1 inch of compost. Don’t use manure or mushroom compost for deep mulching or for high tunnels and greenhouses, since it’s higher in nitrogen and phosphorus and soluble salts will build up in the soil. Only use plant-based compost for these purposes.

When ordering compost, you can estimate how many cubic yards of compost you need by using a calculator like this one by McGill Compost.

What Happens If You Use Too Much Compost?

Adding compost to your garden beds comes with a lot of benefits, but there is a limit. The best percentage of organic matter in the soil is between 3 to 5%.

Soil should ideally be at most 5% organic material. The rest is mineral, water, or air.

Above that, and you will set yourself up for the excess nutrients and threat of nitrogen burn that plagues synthetic fertilizers. Problems include: 

  • High nitrogen can burn plant roots and inhibit the uptake of calcium, magnesium, and potassium. In severe cases, the plants will die. In less severe cases, you can water shallowly daily over a week to wash the nitrogen deeper into the soil without causing runoff.
  • High calcium, magnesium, potassium, and sodium can increase your soil’s alkalinity. Highly alkaline soil restricts the availability of many nutrients, causing deficiencies when there’s plenty in the soil. Most vegetables and flowers need a slightly acidic to neutral soil. Changing pH levels is finicky work.
  • High phosphorus inhibits iron and zinc, and interferes with the beneficial relationship between mycorrhizal fungi and your plant roots, which would otherwise help your plants gather more nutrients from the soil, among other benefits. The only thing you can do to fix it is by not adding more and waiting, as it can take up to 5 years to resolve.
compost pile grass
Using too much compost will cause problems, such as excessive levels of nutrients.

Nutrient deficiencies and excesses are very difficult to identify and many are very difficult to fix. It’s a lot easier to avoid the problems in the first place by adding compost conservatively and testing your soil regularly to check the existing nutrient levels. 

When To Put Compost On Your Garden

The best time to add compost is during fall or spring bed preparation.

If you add the compost in the fall, then the compost mulch will protect your soil from the cold over the winter, and the microbial decomposition will pick back up as the soil thaws. Your garden beds will be ready to plant earlier as you don’t need to wait to rework the soil.

wooden raised garden bed
The best time to add compost to your garden is fall or spring. Be sure to allow enough time (4 weeks or more) for manure to decompose before applying it.

You can also apply compost when you’re preparing the garden beds in the spring. Apply finished manure compost at least 4 weeks before you plant. 

Can You Add Compost To Your Garden After Planting?

Yes, you can add plant-based compost to your garden after planting. You can apply it as a mulch or as an amendment. 

If you’re applying it as a mulch, then you’ll want a layer that’s between 3 to 6 inches thick. 

rows of potato plants
You can add compost to your garden after planting. One good use of compost in the garden is for hilling potatoes as they grow taller.

If you’re applying it as an amendment (to add or refresh nutrients), then apply a 2 to 3 inch layer on top of the soil, working it into the top couple inches when it’s away from plant roots, but near plants, leaving it as a layer on top. 

Don’t pile up compost against trunks or stems as it can cause rot.

Can I Put Compost On Top Of Soil Or Should Compost Be Dug In?

You can leave compost on top or you can dig it in, it really depends on your garden bed and climate. Leaving compost in place on top mimics how it happens in the natural world – plant matter dies, falls on the ground, and composts in place as earthworms and other decomposers mix the compost deeper. 

earthworm
Earthworms will help to break down plant matter, but you can also let it compost and then dig it into the garden.

But many horticultural experts recommend mixing the compost into the top few inches of garden soil. If you have clay soil, then you definitely want to dig it in, as that will help improve its structure a lot more. 

Mixing in the top few inches will also help water move between the different textures of compost and existing garden soil so you won’t have water sitting between the layers. You can still do this with no-dig practices, as you can disturb the top couple inches of soil without disrupting most of the ecology below the soil. 

In desert areas where the soil is sandy and alkaline, you will need to dig in compost because of factors like high winds and a lack of earthworms.

Can Plants Grow In Compost Only?

Some plants may be able to grow in compost only, so long as the pH level matches their preferences and they need little root structure. But in most cases, plants will struggle as their needs aren’t met.

Problems that prevent plants from growing in only compost include:

  • Fast Drainage: Using organic matter helps keep moisture, but it also helps soil drain quicker. With just compost and nothing else to help keep moisture, you’ll be watering it constantly.
  • Shrinkage: As the compost decomposes, it takes up less volume. After a season, you’ll find the soil level has dropped a lot, and what remains may be compacted.
  • Lack of structural support: Plant roots use the soil structure to anchor themselves into the ground. Without that support, they won’t be able to grow straight or hold themselves steady.
  • Too much nutrients: Excess nutrients are just as harmful and tricky to diagnose as deficiencies. Too much nitrogen can burn plant roots. Other excesses cause deficiencies in other nutrients.

Mixing compost with other components (like topsoil in a garden bed or coconut coir/peat moss and vermiculite in potting mix) helps bring out compost’s good qualities, while helping to ameliorate its drawbacks. Compost is not meant to be used on its own.

compost
A compost pile is a great way to recycle yard and garden scraps, and compost has its uses in the garden, but you need soil (and minerals) together with compost to grow.

If you have a compost pile lying on the ground and your plants can reach their roots through to the topsoil below, then that’s growing in a compost mulch rather than only compost, and more plants will grow just fine.

Does Compost Keep Weeds Down?

You can use compost as a mulch to prevent weeds, but it may not be the most effective method. You will need to add more compost as the season progresses as the compost breaks down, and weeds germinate in the soil/compost.

weeds
Seeds from weeds can germinate in soil with compost, so try using compost with mulch as weed control.

However, when weeds pop up, it should be much easier to pull them as the soil will be nice and loose. Using compost with another mulch will be more effective.

Only use plant-based compost as a mulch and apply it 3 to 6 inches deep.

Conclusion

Compost feeds your plants, improves your soil structure, and feeds the beneficial microbes in your soil, but only when used in moderation. Remember, only add 2 to 3 inches of plant-based compost at one time (for manure, use 1 inch), or 3 to 6 inches of plant-based compost if deep mulching.

This article can help you to decide whether to use a container for compost or not.

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Jon M

Hi, I'm Jon. Let's solve your gardening problems, spend more time growing, and get the best harvest every year!

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