Crop rotation is important because:
Some plants use a lot of nitrogen.
It increases soil fertility.
Repeated planting of the same crops decreases soil quality.
All of the above.

your answer is "the 1950's"

a physical change is being made.

the nitrogen cycle provides nitrogen to the ecosystem from the atmosphere, ground and oceans. nitrogen is an important component of complex molecules such as amino acids and nucleotides, which lead to the creation of proteins and dna, the building blocks of all life. plants absorb nitrogen in the form of ammonium, nitrate ion and, on rare occasions, as amino acids. animals receive nitrogen necessary for biological processes from feeding on living or dead organic matter.