Why do we age compost?

Simple question, not so simple answer.

I'm currently doing a deep dive on the process of aging compost and what is really happening to give us a great end product.

Currently, I understand we don't want to put fresh piping hot compost on our beds as it needs time to settle out and mellow. But what is happening during this mellowing period? Are there pockets of anaerobic bacteria and fungus that further break down the compost product releasing plant-available nutrients? Are we waiting for that primary bacterial decomposition population to die back after blooming from the composting process?

Does anyone have some insight to help sort out the why to aging compost?



Potential Reasons

Reduce nutrient leaching.

Prevent injury to plants from nitrogen burn.

Allow PH to balance in compost.

Reduce the load of harmful pathogens. (Source

Increase friability

How to tell if my compost is finished?

Many sources I have read indicate that the compost should smell earthy, not be producing heat, be broken down to where you can not tell what originally made up the compost, and should not have a lot of big chunks to it. It should look, essentially, like dirt! It should be crumbly and friable, and the pile should be smaller than when it first started. If you smell something sour or ammonia-like, it most likely is not aged enough.

A test that can be done to determine if your compost is 'finished' or not is to take a sample of soil into a container and attempt to germinate radish seeds in it. If you get 75% or more germination, your compost can be considered "finished". (Source)


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This probably doesn't answer your question, but when I go to the compost pile to make a withdrawal, I have a purpose in mind. If I'm building a seed bed, I want only the finest compost, so I scoop the top aside and draw from the bottom. If I want a top-dressing later in the season, I just scoop stuff out of the middle and whatever I get is probably good enough. That's really all the concern I put into aging my compost. but I'm not turning it and stuff, so the people who're into that must have a different set of policies.

I just did a Google (AI) search for "biochemistry of composting?" and one of the references was from the Cornell Waste Management Institute:

https://cwmi.css.cornell.edu/composting.htm

The search linked to a neat chapter of a book, "Composting in the Classroom" attached.  Nice little reference.

On page two there is a chart showing the second mesophilic phase of decomposition in the pile, during which lignin and other resistant compounds are broken down.  Likewise, there are nice charts on page six and eight.  Later it mentions that "fungal species are predominantly mesophilic" so that makes sense about needing cooler temperature and longer time to break microscopic woody bits down.

I used to be concerned with a 'curing' phase after the thermophillic stage when I do hot composting, but now I don't bother. I apply it right after the hot phase without worry. The main case, as I understand it, for curing is to reduce phytotoxicity. Phytotoxicity occurs when certain compounds are still present after the hot phase. Some of these compounds may take time to break down and not necessarily heat. But if you are using common materials like weeds, leaves, grass clippings, foodscraps, and livestock manure; and you don't regularly apply pesticides or use non-organic feed with your livestock. These compounds causing phytotoxicity should be a non-issue.

https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/phytotoxicity

Regarding nutrient leaching. This could be a problem with your soil if it has a reduced cation exchange capacity. If your soil is high in organic matter, your cation exchange capacity should be fine to handle nutrients breaking down from a hot compost. If your soil is low in organic matter, you could use biochar to increase your cation exchange capacity.

On our farm, we like to use hot compost to apply after crops are harvested because those veg beds usually need more nutrients immediately. We 'mostly' use cold 'cured' compost the beginning and end of season, or for a midway boost for long term crops like cabbage. But I've used hot compost on our beds midway through a crop's growth stage with no problems. So we're not really sticklers, we just use what we have. If you have healthy soil with a diverse soil biology, it should be able to handle 'uncured' compost that has gone through the thermophilic phase.

Hope that helps!