1. Once crops are harvested for human consumption, the natural supply of nutrients in the soil must be "re-filled". This is why farmer's add nutrients to their soils.
3. Most organisms, including humans, can't use the free gaseous form of nitrogen found in the air. We need other organisms and natural processes to "fix" nitrogen into a solid form that plants and animals can use. Most of the nitrogen that plants and animals use is made by bacteria.
I could only answer questions 1 and 3 for you but I hope I was able to help!
