Using Finished Compost and Where to Put It and How Much to Use

Turning kitchen scraps into compost is only part of the process. After six months of tending my bokashi set-up in my flat in Bristol, happily watching my kitchen waste turn into… something soil-like, I realised I didn’t actually know what to do with it. There’s lots of advice about how to build a heap and maintain carbon: nitrogen ratios, but frustratingly little information about what’s actually important: using it.

There’s a big difference between finished and unfinished compost. Use it wrong and you can damage plants, use it right and it’s transformative. Whether you’re growing beans in window boxes on a balcony or vegetables in a garden, homemade compost can take your plants to the next level. Here’s how to actually use compost.

## Why finished compost is better

Its science is useful to understand why finished compost works so well, and why using unfinished compost is bad advice. There are two key changes that occur during composting: stabilised pH levels and a balanced carbon:nitrogen ratio.

**Stable pH**: Finished compost has a pH that is mostly between 6 and 8 (Gardening Know How), which is just where most plants like it. The pH will stay constant over time as the microbial activity that creates compost complete decays.

**Carbon to Nitrogen Ratio**: Carbon to nitrogen ratio will level out at about 10:1 to 15:1 (Cornell Composting). Because of this your compost won’t pull nitrogen from the soil to finish decomposing like it will when you add fresh organic matter high in carbon. This is why putting cardboard or raw veg scraps straight onto your soil won’t improve things for your plants.

**Nutrient Content**: Finally, compost provides nutrition in a form that’s easy for plants to access. This doesn’t just come from nitrogen, phosphorus and potassium. Compost adds organic matter to the soil, which helps create spaces for air and water. This improves drainage in clay soils and water retention in sandy soils. Plants also get nutrition from the organic matter itself as soil microbes break it down, and those microbes need feeding too.

When people advise against using compost directly on plants what they really mean is don’t use unfinished compost on seedlings. Because the microbial process of decomposition isn’t complete, using it directly on plants can damage them.

One exception is wormeries. Much of the material that goes into, and comes out of a wormery is unfinished compost. But because it’s been processed by worms first, it’s safe to use. Their casts are actually excellent plant food and are fully mature compost.

## How to tell if compost is ready

The usual advice is you’ll know when your compost is ready because it looks dark and crumbly. News flash: it sometimes looks dark and crumbly but still isn’t ready. Here’s how to tell for sure.

**Texture**: Mature compost should be dark brown to black and crumbly. It should hold together when you squeeze it, but fall apart if you poke it. Ideally you shouldn’t be able to identify individual components of your compost. Woody stalks and egg shells are about the only thing I can still recognise in my finished compost. If your compost is still hot to the touch it’s not ready.

**Smell**: Compost shouldn’t smell like anything. If it smells sweet or ammonia-like or you can identify specific foods in there it’s not ready. I learned this lesson the hard way and once killed off a bunch of herbs by pouring smelly compost around their roots.

**Wait**: The rule of thumb I’ve seen( The Spruce) is that once your compost is dark crumbly and smells earthy, let it sit awhile before using it on anything thats prone to be susceptible to disease or rot. Seedlings definitely fall into this category for me. I let all my compost sit for at least a month after it reaches this stage.

**Grow Something In It**: If you’re still unsure, you can always try growing something in it to test. I sow cress seeds on a mix of seed starting compost and regular potting soil. If the seeds sprout fine, and the seedlings look healthy it’s ready. If they don’t sprout or they struggle, it needs more time.

It can take anywhere from months to years for compost to finish, depending on your system. Composts that you have less control over like hot piles and wormeries take longer because you can’t turn them as often. Bokashi composting has the shortest finishing time because it’s basically anaerobic digestion. Most of my compost is finished in less than 2 months.

## Using compost effectively

Knowing how and when your compost is ready means you can put it to use. Here are my top tips for actually using compost.

**Plants**: Vegetables, herbs, annuals and fruit trees all benefit from the addition of compost. You can dig it into soil before planting anything new, or top-dress around established plants. There are some differences in application rates and methods depending on the plants you grow, but good compost is good for everything.

**Feed seedlings**: Instead of buying seed starting mix I use half finished compost and half regular potting soil to grow seedlings in. It’s not essential if you’re recycling polystyrene trays or something like that, but gives me something to do with my potting soil.

One exception to this rule are Mediterranean herbs like lavender, rosemary and thyme. They don’t like rich soil and will actually do better without compost. Water with beer to keep slugs away and they’ll thrive in conditions that would kill off less-hardy plants.

How to Use Finished Compost Diagram

1. Dig It In: If you have vegetable beds or flowerbeds you can simply turn your compost into the top 6-8 inches of soil before planting. I dig mine in using a garden fork, which lets me mix it in without completely overturning soil layers. The more turned over soil is good if you’re restarting a garden from scratch. For beds that are already established mix in fewer spadesful of compost.

Digging compost into soil is great because it improves the whole bed. If you’re container growing though, digging compost in just isn’t an option.

2. Potting Mixes: As above, compost is an excellent ingredient in potting mixes for starting seeds or repotting houseplants. Some sources say you can use straight compost but mine doesn’t work well alone. It’s too rich and doesn’t drain well enough for most plants. I mix one-part compost to two parts regular potting soil, adding some perlite to improve drainage.

3. Top-Dressing Beds: Giving your beds and borders a dusting of compost is like feeding without turning the whole thing over. It’s an easy way to improve soil quality slowly over time. I top-dress my established beds every spring. Trees, shrubs and perennials get a few inches around the dripline. These plants don’t need much annual nutrition so I keep leafy vegetables and herbs well fed with extra compost.

Some plants don’t like rich soil. I leave a few inches between compost and the stems of my plants to avoid attracting slugs and reduce the risk of rot. Plants that prefer drier soil, like lavender can be harmed by compost that increases water retention.

4. Mulch: Speaking of improving soil over time; compost works as mulch too. It won’t suppress weeds as well as something like straw or wood chips but can help if you don’t have many weeds. As the compost breaks down further it feeds soil and improves drainage. Avoid piling it against stems to prevent rot. I give trees, shrubs and perennials the full two to three inches while leafier veggies and herbs get smaller amounts.

Any plant will tell you if your compost isn’t ready. They might not grow right or wilt unexpectedly. Seedlings are especially good at displaying compost-related issues, so start with them if you’re unsure.

## Final Thoughts

Use your compost! Seriously. It’s only worthless if you let it sit around. Even if you’re not confident enough to use it on your favorite plants, mix it into potting soil for seedlings. They don’t stay seedlings for long and will soon be large enough to let you know if your compost is amazing or not.

## Want to learn more?

Here are some more places you can learn how to use compost from trusted experts: