Compost is simply just organic matter that’s been broken down into a stabilized form, to provide nutrients as a fertilizer to soils.
Good compost is really high in organic matter. Organic matter is the broken-down material – a lot of carbon…can consist of humic matter that is good for soils, and it’s good because it does a few things – one is: high organic matter holds water really well.
Good organic matter also holds nutrients better. So, when you add nutrients whether they’re in the compost, or you add them as a fertilizer – especially in mineralized form – those nutrients will be held by the compost – by the humic matter in the organic matter, in the compost – so that it’s available when the plant needs it. It won’t be as easily leached out by watering or by rain.