Immature compost in a Dalek bin.

5 Reasons to Let Your Compost Mature for Longer

My local council has a free compost scheme. The food and garden waste they collect from households is turned into huge quantities of compost. A portion of this is taken to one collection point, and anyone can come and help themselves. 

When you arrive at the collection point, the compost is often still hot. Sometimes you can see the steam rising off the compost like mist, and as you start to shovel the compost into sacks the acrid smell of ammonia lifts to your nose.

When I first started gardening I used to apply this compost straight to my raised beds. I wasn’t the only one. One friend even enthused about the potential for the compost to warm the soil up after the end of the winter. 

Unfortunately, the results were never quite what I wanted. Then I left a couple of sacks over the winter for use in the spring. The compost was then completely different. It was darker, crumblier and had a pleasant, earthy smell. 

And when I tended to my friend’s bed for his wife after he sadly passed away, I found his beds had produced poor, spindly plants. By winter, though, and after an application of mature compost, his kale, chard and purple broccoli plants were strong and vigorous. 

We are trained by modern life to seek speed. The internet – and indeed this website – is full of tips for achieving compost faster. You can find methods for turning waste into compost in just 14 days. 

The thing is, though, that while material may look decomposed, it’s not necessarily great for your garden – yet. In this article, we’ll look at why compost is best left to mature and stabilise before using. 


The stages of composting

The stages of composting image.

In the Science of Compost we covered the three phases of composting. While it simplifies matters, it’s a useful view of composting. We start with the mesophilic phases, when mesophilic bacteria get to work and the compost starts to heat up. 

If you have a well constructed compost heap, with enough volume, air space and a good mixture of both nitrogen and carbon, your heap will enter the thermophilic phase. This is the fastest phase as thermophilic bacteria rapidly break down material. 

The final part of composting could be the most important. 

It can actually be broken down into two parts – cooling and maturing (also called curing).

First the temperature drops down as thermophilic activity reduces. Crucially, though, just because the compost has cooled down it is not finished. 

After it has cooled down, other decomposers, such as fungi, worms and beetles, really get to work, further improving the quality of the compost. Finally, you end up with a stable compost that will improve your soil. 

Let’s look at the benefits of allowing enough time during this stage…

The benefits of maturing compost

Breaking down tougher materials

During the early stages of composting, bacteria break up easy-to-digest material. However, not everything is easy for those bacteria to digest. 

You’ll probably notice when you come to use your compost heap that it still has woodier materials such as branches and wood chips. The lignin in woody materials is much harder to digest than other structures. 

As compost cools down, though, other decomposers get to work. Fungi, for example, has developed enzymes that break apart the lignin in wood. 

The longer you leave your compost, the more of the woody material will be broken up, leaving you with a finer, higher quality compost. 

Not too much nitrogen…

Plants need nitrogen, but too much nitrogen can burn their roots. Immature compost can be too high in nitrogen, especially if you added a lot of hot material such as chicken manure. 

By allowing time for the compost process to finish, and for the excess nitrogen to either be released into the atmosphere or be used up by bacteria, you ensure that finished compost has the right level of nitrogen for your soil. 

Removing competing mico-organisms

Hot compost is full of microorganisms. These never completely disappear, of course, but as compost matures they reduce in number. However, if you add compost which is still immature, these microorganisms can compete with plants for both nitrogen and oxygen

Safer for plants

If you don’t allow compost long enough to mature, you could find it is phytotoxic. Phytotoxins are substances that can both slow down seedlings and damage plants. 

For example, immature compost may be high in ammonia. In the right quantities, ammonia is a good source of nitrogen, but if the quantities are too high they can leave brown spots on leaves or cause leaf burn. 

During the composting processes immature compost also contains of high levels of organic acids and salts, which can also damage plants. 

The incredible value of worm castings

Worms hard at work in immature compost.

Worms do an amazing job, and the value of worm castings is greater than its simple nutritional value.

For example: 

  • Worms can help plants withstand diseases. For example, research by Cornell University found vermicompost has beneficial microbes that help protect seeds from a common plant disease. 
  • Worms can remove toxins and diseases like Salmonella from compost
  • Worm castings contain minerals like phosphorus, magnesium, potassium, and calcium, which are used by plants for growth.

Fortunately, worms absolutely love compost. The longer you leave your compost, the more time you allow worms to do their work. 

So, how long does it take compost to mature?

There’s two big differences between commercial composting and composting at home. 

  1. Commercial composting (in theory) will be done under ideal condition, allowing finished compost to be achieved faster. 
  2. Home composting does not have the financial pressure to achieve compost as quickly as possibly. 

Even though commercial compost should be ready when bought, many gardeners have complained that bulk bought compost is still raw and hot. So it’s possible that some commercial composters are simply not allowing enough time for the compost to mature. 

If you’re composting at home, you have the luxury of waiting. After all, you don’t have a business which needs to optimise use of time, space and resources.

So how long?

The RHS recommends allowing 6 months to two years before using compost. 

There seems a big difference between 6 months and two years, but there are many variables that go into compost. 

For example, your finished compost can be affected by the starting C:N ratio, by moisture levels, by the air temperature, whether your compost has air pockets and so on. 

It also depends on how and when you are going to use your compost. You can be a little bit more blase about a rough layer of compost applied in autumn, as it has all winter to break down before you plant. If you are using compost for seeds, your compost needs to be much finer.

Knowing when your compost is ready

Dark, mature compost in a raised bed.
Finished compost, ready for planting in.

What if you’re in a hurry, or want to be more precise? 

There are all sorts of tests to ensure that compost is stable. Many of these are complicated, involving laboratory equipment, but there are some easy ways to test it yourself too.

Check the temperature

A compost thermometer.

If you’ve been using a hot composting method, use a compost thermometer to ensure the compost has reached air temperature. Then fill a pot with soil, and use it to germinate a seed. 

Test it with seeds

If a strong, healthy seedling emerges, your compost is probably ready.

Check the smell

A sour smell may mean the compost is still high in ammonia. An earthy smell indicates the compost is likely ready. 

Check the consistency

The finer the compost the better. What you really don’t want is half rotted sections. Don’t sweat too much about a few bits of wood or woodchip, though – these can be sieved out. 

Measure the PH levels

If you’re up for something slightly more technical, Cornell University suggests measuring the PH levels of compost. Stable compost should have a PH level of between 6 and 8.

Growing in your unfinished compost

While I may preach patience, I don’t always practice it! 

I’ve found one way to deal with my impatience while waiting for compost to finish is to grow in it. 

That may sound crazy, given everything we’ve already talked about, but I’m not talking about adding the compost to soil here. Instead, I’m talking about planting directly into your compost while it continues to mature. 

Here are 6 reasons why it’s okay to grow in your compost pile:

  1. Your soil is safe from damage – all you will risk is a few left-over seeds/spare plants. 
  2. You will be maximising the use of space in your garden. 
  3. Some plants seem to be happier growing in unfinished compost than others.
  4. Growing in your compost may give you early warnings of serious problems. 
  5. Herbicides seem to start breaking down when you start growing in compost. If there are any herbicides, it’s best both to know before adding to your soil. Growing in your compost heap can also help break down the herbicides. 
  6. It forces you to give your compost more time to mature. 

Plants I have tried include: 

  • Potatoes: It’s been success every time!
  • Tomatoes: I simply took cuttings from my greenhouse plants and pushed them into my compost heap, where they grew like wildfire. I did have to use a lot of green tomatoes, but that’s still impressive for late tomatoes in my climate. 
  • Pumpkins: This failed – but so did every pumpkin and squash plant that year. One to retry!
  • Field beans: I used this for compost which contained some manure infected with herbicide. As expected, they didn’t grow well, but the compost was fine by the time I used it. 

As always, experiment, plant and try again. (And don’t forget to let me know the results…)