Thanks for starting this thread. I like the microscope pictures. I used to only make lazy man’s compost (everything in a heap, turn once after a year, use after another year). Last year I started making Johnson-Su compost (mainly inspired by the desire to make something decent out of the two products that I have in abundance, wood shaving/sawdust (I am a carpenter) and grass cuttings). It humbled me to realise that composting is not just composting, which I had kind of always believed. The smell, texture, everything about the new compost “feels” better. Under the microscope it is full of life, fungi and humic like substances. Not all compost is equal!

The main point that I question in @slaskow text is that the nutrients at the end of composting are “used up” and the NPK value is practically 0-0-0. Assuming the C:N ratio is high enough (<30:1) so there is not an enormous amount of ammonia gases, where have the nutrients gone?

This said, I realise that humic substances are what are left when practically all the energy (stored in the organic bonds of the start material) are used up. The energy value of humus for microbes is nearly zero, hence stable. Do the plants then need a lot of energy to get the nutrients from compost, or are the raw nutrients held by the humic substances and just waiting to be picked off?

So the energy from the original organic material is used up, but surely not the nutrients. I don’t understand where they could go (apart from CO2). I realise that I come at the edge of my understanding.