Compost is ready when it’s brown, dark, and smells like earth. It’s when the bacteria are no longer active, that you have mature compost capable of growing plants, which is people often question how long it takes to have mature compost.
Does more mature compost produce taller plants? Even though we have limited evidence, there is an experimental trial where researchers found that older compost increased growth in bean plants. They provided the bean plants with the same amount of water and sunlight, and the only variable was the age of the compost.
However, and this is my opinion, we should also recognize that compost creation involves diverse variables, so the quality should also play a role in plant growth since the compost I create might be different from the compost you create.
What is Mature Compost?
Mature compost looks, feels and smells like rich, dark earth rather than rotting food waste. This is ready-to-use compost that decomposed completely and can now nurture the soil and grow plants.
You can also use compost as mulch or a top dressing, but according to composting experts, that type of use doesn’t require the compost to be as mature.
Immature compost may contain substances damaging to plants, including acids and pathogens. Plus, immature compost is still decaying.
The bacteria still require both nitrogen and oxygen, which means these elements are still being used to decompose organic waste, so they’re unavailable for plants.
If you’re using compost, you will want to use mature compost to grow plants.
How Mature Should My Compost Be?
According to the Institute of Food and Agricultural Sciences of the University of Florida, mature compost is anywhere between 1 and 12 months old.
I believe you should aim for 5-6 months because they mention that compost applied to grow plants should be more mature. Some people use compost as mulch, so in that case the compost doesn’t need to be as mature.
Other sources like the Royal Horticultural Society suggest that compost can take between six months and two years to reach maturity.
However, that also depends on other variables, including the size of the materials you have included in the compost system, management, weather – basically it depends on how proficient you are at setting up the ideal environment for compost to decay.
If you live in a fairly inhospitable environment with a short warm season, then it’s likely that it will take longer for you to have ready-to-use, mature compost.
When Is My Compost Mature?
If you got a warm pile, that’s turned regularly, then knowing when the compost is finally mature is easier because it won’t heat anymore, even if you turn it. You just have to wait several weeks, and it’s finished.
Leaving it alone for several weeks will allow microbes that operate at lower temperatures to apply a finishing touch to the heap. It will also allow other larger organisms such as earthworms which are incapable of tolerating high-temperatures to get back into the heap.
This is important because these will improve the quality of the compost, which will improve the quality of the soil itself.
With cool piles, the story is slightly different.
Figuring out whether your compost is mature or unfinished isn’t as easy, and it takes experience to figure when it’s done. Nonetheless, most cool piles are ready in one year.
With finished compost made with completely shredded materials, none of the original materials will be recognizable. Shred the materials as much as you can, otherwise it will take years to decompose, especially if you have egg-shells, peanut shells, twigs, and wood chips, which take longer to degrade than pear cores and moldy bits of bread.
Look at the compost and evaluate the look, feel, and smell of it. You shouldn’t be able to find slimy things, including ordinary kitchen waste such as carrots, potatoes, corn, and bell peppers. Garden waste should also be unrecognizable, except for the occasional woody stem and fall leaves.
If you’re still able to pick out and name a lot of easily biodegradable materials, then the compost needs more time. Additionally, the volume of the pile should be reduced by half.
Compost Maturity Test
There are scientific methods of testing compost maturity.
One method involves placing the compost into two containers and sprinkle it with radish seeds. If 75% of the radish seeds germinate and grow into radishes, then you have finished and ready to use compost.
The reason you should use radish seeds is because they germinate and develop quickly.
Another method is less convenient but involves calculating the germination rates of seeds using a “control group”. However, this takes a while.
The phytotoxins in unfinished compost can prevent seeds from germinating or kill the sprouts during growth. So, if you achieve a plausible germination rate, you can consider your compost safe to use.
Still, this is something most composters do not do, and instead draw on their experience to know when compost is ready based on its look, feel, and smell.
Though, it’s important to remember that unfinished compost can stunt or kill plants.
Is Older Compost Optimal For Growing Plants?
The term “maturity” comes from recognizing that the age of the compost makes a difference in how it performs in a soil.
It also describes the difference between an old and a young compost.
However, age is not the only factor that determines how compost performs, and whether it has properly matured. An old compost that was badly planned and maintained is less mature than a young compost that was carefully planned and well aerated.
Therefore, we should also take other variables into account, but assuming you have a well planned and maintained compost pile – age plays a role and having a compost that went through a full composting process does help.
Experts suggest that high-maturity composts are useful for soil health and increasing nutrient efficiency, while low-maturity composts are useful for nutrient supply.
High maturity composts have complex organic molecules that foster diverse microbial populations that play crucial roles in the nutrient cycle, transforming nitrogen from organic forms to inorganic forms and decomposing organic matter, then predating on each other while slowly releasing a trickle of nutrients plants can assimilate over time.
Having a diverse microbial population means that you can prevent negative effects, such as losing soil carbon, or having excess nutrients in the soil tied up in organic forms.
Low-maturity composts have a greater proportion of nutrients readily available because they contain simple organic molecules that are relatively easier for microbes in the soil to break down and pass on to plants.
This type of compost is useful in urgent scenarios where plants need a fast nutrient supply (functions similarly to fertilizers), but you can’t use too much, otherwise you risk losing excess nutrients because the plants won’t have the capacity to take up everything.
So, which one is better at promoting plant growth?
More mature composts provide you with stable forms of carbon combined with a steady flow of slow release nutrients that allow your plants to receive a continual supply of nutrients over time.
They also improve soil structure by increasing the amount of water the soil is able to store, increasing its ability to hold onto nutrients in a plant available form, and stimulating plant growth.
The bottom line is that a more mature compost is better capable of producing an optimal soil environment for plants to grow.
However, we should also note they can be used simultaneously, so you can get both the benefits of the fertilizing effects of the low maturity compost, and the soil health and nutrient efficiency effects of the high maturity compost.
If you have any questions or comments about this or any other blog post, don’t hesitate to get in contact with me.