Nitrogen fixing bacteria take nitrogen gas from the air and convert it into usable nitrogen in the forms of ammonia, ammonium, nitrates and nitrites. This process is known as what?
fixation
assimilation
nitrification
denitrification

The table below shows the elements that mainly comprise each of the four layers of earth: elements in layers layer main elements crust oxygen, silicon mantle iron, magnesium outer core iron, nickel inner core iron which of these elements make up the layer that is 0.2 to 1.1 percent of earth's total diameter?