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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.

Some questions:

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)?

Thanks

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You are missing some basic facts about what plants are made of.

Up to 80% of the mass of living plant cells is simply water. Ther are no "nutrients" at all in that, and it should be obvious where it comes from.

(Note: plants which live in dry environments don't have spines just make them hard to eat. When the temperature at falls at night, water vapour in the air tends to condense at the sharp points of the spines, and is absorbed by the plant).

More than 50% of the remainder is carbon, which comes from carbon dioxide in the air via photosynthesis. The chemical reactions involved need an energy source, which is either solar radiation or artificial light.

Of the remaining few percent of the total mass of a plant, nitrogen is a significant component. Air is about 80% nitrogen, but plants can not use that directly. It has to be converted into compounds that dissolve in water and are absorbed mainly by the plant roots.

One natural process which does that is the chemical reactions which are a side-effect of lightning strikes, but the most significant source is bacteria living in the soil which convert nitrogen gas into chemicals that plants can use. In fact some plants (e.g. legumes and clover) have colonies of these bacteria living inside the plant roots. (Compare that with the fact that more than 50% of the cells in a living "human body" are not human cells at all but many different species of micro-organisms, and without them the human would soon be dead.)

For your questions about light, the basic point is that even when filtered through a glass, direct sunlight contains much more energy to power photosynthesis than light levels in natural shade, or typical indoor environments. The light level in full sunlight out of doors is around 10,000 lux. A glass window reduces that to around 1,000. But the light level in a typical house or office building away from any patches of "direct sunlight" shining through the windows is about 25 to 50 lux.

In other words, a plant which is kept in a room that has enough light for humans to function efficiently for 10 hours a day, is getting the equivalent of about three minutes of full outdoor sunlight each day, or about 30 minutes of sunlight in a greenhouse.

Before LED lamps were available, it was difficult to provide the energy that plants required with artificial light, because about 99% of the electrical energy used by a non-LED light is converted to heat not light, and heat is no use for photosynthesis.

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