Compost Magazine

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Compost Maturity Matters: Why Fresh Isn’t Always Best

Would you ever use fresh compost in the garden?

One friend of mine did, and used to wax lyrical about the benefit of free municipal compost. 

“It’s still hot, James – it warms the soil in the spring!”

Much as I loved him, he was completely wrong in this case!

Fresh compost can be high in salts and substances that can harm the plants in your ground. 

One problem is that many guides talk about being able to make compost in a few weeks without mentioning that the compost is completely unsuitable for use at the end of that period. 

Instead, it requires a period of curing and maturing

But what happens to the compost in that maturation process? 

A new study has set to find that out. 

What the study looked at

Open windrows in a field next to a pine wood.

One researcher, John Castain, wanted to uncover the changes that happen to compost when it is removed from a composting shed and stored in open windrows.

He looked at a wide range of factors varying from the acidity of the compost to the available nutrients. 

The compost itself was made from a mixture of manure and wood shavings. 

At the time of testing, the compost had a wildly varying age – anything from 131 days to nearly two years. 

How curing and maturation affected the compost

Carbon and organic material

If you’re an experienced composter, you probably won’t be surprised to find the compost shrank as it got older. 

Organic material shrank, as did the amount of carbon. 

That’s not so good, as one of the benefits of compost is that it increases the amount of organic material in the soil.

This in turn feeds beneficial microorganisms in the soil. 

Carbon is a key component of organic matter and plays a number of roles which vary from erosion protection to helping to store water in the soil. 


As you probably know, nitrogen is a key nutrient plants use for growth. 

The study found that the total amount of nitrogen didn’t change in the maturation process. 

What did change was the form of nitrogen. 

For example, nitrogen in the form of gases was transformed into forms of nitrogen that other plants can more easily use. 

However, other nutrients did not change much in the same time period. 

I find that particularly interesting, as many composters are worried about nutrients leaching when compost is exposed to rain.

Acidity change

A significant revelation was the dramatic drop in the alkalinity of the compost, which fell from a pH of 8.9 to 6.8.

When the pH reaches 8.9, it becomes too alkaline for most plants, resulting in problems like yellowing leaves and stunted growth.

In fact, the ideal pH for soil is usually between 5.5 and 7.5. As the pH falls, it should ensure that the nutrients in the compost become more available to plants. 

Wrapping up

It’s worth noting that the study only looked at uncovered compost. 

It’s possible the results could vary if the compost had not been exposed to wind and rain.

The age of the tested compost also varied dramatically, which makes it difficult to draw any conclusions on how long we should leave compost mature for.

Still, the study highlights the importance of letting compost mature before using it. 

Organic matter and carbon may decrease – but that’s a sacrifice we need to make for a more stable compost that has more nutrients available.

This brings me back to my friend’s advice. 

Taking his advice, I did get myself a few bags of that steaming municipal compost. 

It wasn’t the nicest stuff – hot and with an acid smell. 

However, instead of using it straight away, I put some sacks to the side in the autumn.

When I came back to it the next spring, it was completely different. Cool, brown and with a lovely fresh earthy smell, it was just right for the soil!


Q: What is the difference between compost curing and maturing?

A: Curing is the final stage of the composting process where the decomposition slows down and the compost stabilizes. Maturing refers to the overall aging process, where the compost becomes stable and safe to use in your garden.

Q. How do you mature compost?

Simply leave it until it is ready to use!

Some composters prefer to cover their compost, but this study suggests that leaving in the open is fine (although it is worth noting that it this is only one study, and it is best to base practices on the results of multiple studies when possible.)

Q. What will happen if you leave the compost too long?

Overtime the compost will shrink in size, giving you less compost to use. If it is in the open, you will also finding weeds growing in the compost, requiring additional maintenance. It’s best to add the compost to your soil when it is ready, as the soil and soil microbes can benefit from the continued breaking down of the compost.


Chastain, J.P. (2023). Impact of Storage Time on the Composition of a Finished Compost Product: A Case Study. Applied Engineering in Agriculture, 39(3), 303-313. doi: 10.13031/aea.15312.

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How Long Does Compost Last (and How to Make It Last Longer)

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