I have started out my journey of growing things with strawberries and cacti. But I feel I miss some of the basics of how the growth cycle works. So I have a few questions.
First of all, the plants needs energy and nutrients to grow. The energy comes from sunlight through photosynthesis. The nutrients (presumably the most important of which is Nitrogen) comes from the soil either naturally or through the use of fertilisers.
1.) Since Nitrogen is present in animal and plant proteins, any compost made from organic materials would presumably have plenty of Nitrogen already? In this case why would a fertiliser be necessary?
2.) In regular soil that's been lying around doing nothing for years, would you expect it to be low in Nitrogen levels, and therefore require fertilisation?
3.) I have been reading about cactus potting mixes and I understand they need to be well draining, so they have grit/perlite/stones etc added, and the natural habitat of many cacti would simply be sand. But in these cases where does the cactus get its nutrients from? If I planted a cactus in a pot of sand, or stones, where's the Nitrogen? Would it grow? If not how can it in the desert?
4.) For the human body I can easily calculate my protein (and therefore Nitrogen) needs based on rates of muscle protein breakdown and the need to replenish this, are there such considerations for plants too? How might I go about such a calculation?
5.) When it is recommended to have "full sun" and be "under glass", are these two things mutually exclusive. In other words if I was growing in a greenhouse would I still have to take the plant out into the full sun everyday or would it be fine getting full sun through the glass (obviously the glass is going to reduce the intensity a little and filter out the UV)?