I sometimes buy compost in serious bulk.
Last year, for example, a friend and I went halves on a pallet load of compost, which we got at cost price from a retailer who owed us a favor.
That’s 36 bags of 50-liter John Innes compost, which is a lot of compost.
I didn’t use it all, so now the compost I am using is at least a year old.
That’s against some of the advice online, which recommends using compost in several months.
I’ll be coming back to my experience – and showing some photos of how the plants are doing – but first, let’s take a detailed look at how long compost lasts.
What is compost? (We need to understand this first!)
Compost is simply broken down organic material.
In the soil, it mostly benefits plants indirectly.
The compost improves the soil structure, and fungi and microorganisms feed off the compost. The fungi extract nutrients and swap them with plants in return for carbohydrates.
Compost continues to break down in the soil – but it does so over time.
In fact, it can take years to fully break down.
If compost takes years to break down, it’s obviously going to last longer than a few months.
That said, there are three disadvantages to leaving it for too long.
Compost loses volume over time
It’s simply amazing how fast compost heaps shrink when you first build them.
I had a compost heap that was 6 feet tall – but by the time I came to use it, it had shrunk to two feet in height.
The shrinkage does continue but slows down considerably as the compost becomes more stable. However, it will still slowly decrease in bulk over time.
If you leave it too long, you are feeding microorganisms in the compost, rather than in the soil.
The trick, then, is to leave your compost long enough to become a nice brown crumbly material, and for it to mature, and not so long that it diminishes too much in size.
How long? Well, that’s a matter of considerable controversy.
(That may be because of a lack of research – I haven’t found a single helpful study on compost longevity on google scholar.)
However, some of the best composting companies leave their compost to mature for at least a year, while researchers who study mushroom compost often leave it to age for more than a year.
I’d suggest it depends on how and when you are using your compost.
If you are using it for tender young seedlings, for example, I would leave the compost to mature for longer – possibly as long as a year. That will allow excess salts, which could harm plants, to leach from the compost.
If you are using it on soil, however, you can probably get away with using it much more quickly.
That’s especially the case if you are putting compost on bare beds in autumn or winter, as the compost will have several months to break down.
Nutrient value of compost
People probably worry too much about nutrients leaching from compost.
Many nutrients in mature compost, including nitrogen, magnesium, potassium and sodium, are in a mostly insoluble form – which means they are not easily washed out by water.
What’s more, the older the compost the fewer nutrients are leached.
However, some nutrients will be lost, and the amount can depend on the type of compost and how the compost was made.
For example, vermicompost (worm castings) loses nutrients faster than other types of compost.
The study I linked to above also found that compost loses more nutrients if it gets too wet, and the longer it stays wet the more nutrients it loses.
Microorganisms in the compost
Another factor to consider is the quality of the compost.
Compost is full of microbes, and some studies suggest that these microbes have benefits for the soil.
For example, a recent study found that certain bacteria in compost (when made in hot conditions) can improve both plant growth and how nutritious the plant is.
If compost is stored correctly, these microbes are likely to remain active.
As we shall see, if the compost is not stored correctly, these microbes are likely to become inactive and carry less benefit.
However, compost will still increase organic matter in the soil, and feed the microbes that benefit soil structure and growing plants.
How should you store compost to maximize quality and longevity?
Follow these three tips, and your compost should last longer:
- Store in a cool, shady location: Temperature encourages microorganisms to break down organic material in compost. So if the temperatures are hot, the compost will lose bulk more quickly.
- Ensure the compost container is ventilated: A lack of air can cause anaerobic (without air) composting to take place. This can cause it to become smelly.
- Avoid excess moisture: If the compost is too moist oxygen will be forced out, again leading to anaerobic composting. As we have seen, it compost gets very wet it can lose a lot of nutrients.
At the same time, if you want to keep beneficial microorganisms in the compost, you’ll need some moisture.
How long does bagged compost last?
Bagged compost can last well over a year. If it is kept dry, microbial activity will reduce, but it will still feed the soil. If it is kept moist, though, volume may reduce over time.
However, the compost I am using from last year has not reduced significantly in volume.
I’ve also grown a load of chilli plants in the compost, and some of them are doing very well indeed.
In fact, although it’s only April, I’ve already got chillis on some of the plants.
(Do note I use a heated propagator and grow lamps, and pre-germinate the chillis before putting them in compost.)
So if you have a ton of old compost, don’t be afraid to use it.
While it might lose some benefits over time, it will still help your soil and plants.
How long does compost last in pots?
That depends on:
- the type of plant
- the size of the pot
- the amount of compost used
- the type of compost used
- the environmental conditions in which the pot is kept.
Usually, compost can provide nutrients to plants for several months to a year, depending on the quality of the compost and the plant’s nutrient needs. However, some plants are more demanding than others!
For example, tomatoes and cucumbers are hungry and deplete compost more quickly. Other plants are less demanding – and may not even react well to rich compost.
Conditions such as temperature, humidity, and sunlight can also affect how long compost lasts in pots. In warm, sunny conditions, the compost may break down more quickly, while in cooler, shadier conditions, the compost may last longer.
Can you revive compost?
There’s a lot of talk about reviving old compost, but little research to back it up. However, logic tells us you’re not going to increase any organic matter, but it does seem likely that microorganisms will repopulate compost that has become dry.
How long does compost last in the garden?
Compost in the garden can last for several years, depending on the quality of the compost and the environmental conditions.
How long does it to make compost (and how to speed it up)
12 different types of compost and their uses
How to use compost for maximum effect