Compost is just amazing and like everything else involved with growing food; complicated. I’m reading Steve Solomon’s chapter about it and for my own sake I’m going to attempt to summarize it.
It starts with organic matter that hasn’t been broken down by the microorganisms in the soil. This organic matter will eventually break down under any circumstance but it will break down at different speeds and end in different ways according to a variety of factors. One of those factors is the carbon to nitrogen ratio (or C:N) of the organic material. Cardboard, for example, has a lot of carbon and not a lot of nitrogen. Therefore, it’s carbon to nitrogen ratio is high. Bonemeal, on the other hand, has a lot of more nitrogen and therefore it’s carbon to nitrogen ratio is low. To read more about this visit this site.
Now there’s this thing called humus (pronounced youmus, not hummus, the chickpea spread) which is the stable residue of decomposed organic matter. It’s C:N ratio is usually 12:1. Humus is like the structure of good soil. It gives it the ability to hold moisture and nutrients.
If organic matter with a higher C:N ratio than 12:1 is added to the soil, microorganisms in the soil will “burn” it’s carbon for fuel and save the nitrogen until the 12:1 C:N ratio is reached. At the end of this process the soil will have more humus and be healthier. At this point any left over nitrogen can be used by plants. Apparently, the soil holds the organic matter nutrients from the plants until the microorganisms have all they want. At this point the soil has become more fertile.
If organic matter with a lower C:N ratio than 12:1 is added to the soil, the microorganisms will convert the excess nitrogen to ammonia gas until the stable 12:1 C:N ratio is reached. The ammonia is then converted by other bacteria into water-soluble nitrates that make plants grow fast. The only problem is there won’t be any humus left after the conversion and that means less fertile soil. This is because nitrates cause soil microbes to multiply and attack humus.
This is why it’s usually best to add decomposed organic matter to soil instead of just adding organic matter especially when you want to grow something right away. Even though you will get more humus out of a higher C:N ratio, during that microorganism snack time, you’ll be basically getting nothing for the plants until snack time is over. And to make matters worse, the higher the C:N ratio, the longer it takes for snack time to be over and the longer it will take for a plant to reap the benefits of the added organic matter and the soil will actually be worse than it was before adding the organic matter, but only temporarily.