Because it feeds it! Simple as that.

it also improves the texture and areation.

And it feeds all the soil organisms that process nutrients into forms plants can use, and that keep the soil aerated with their tunneling.

...and thats one point you will get in your project/exam question.

LOL.

but we didnt say why rotted :o)

Oooh, now that would be a three point answer :-)

well some questions could be extended to 15 marks......

mini essays really

Sorry - I'm not writing a min-essay on well-rotted compost! This guest will have to do a little more research if he/she wants 15 marks. :-/