(c)Because during nitrification process ammonium compounds are oxidized into nitrate in two stages $(i) $ these are acted upon by the nitrite bacteria (Nitrosomonas) and oxidized into $NO_2$ and $(ii)$ the nitrite thus formed is again acted upon by the nitrate bacteria (Nitrobacter) and further oxidized into nitrate $NO_3$ and these are absorbed by plants. Sometimes Nitrogen fixing bacteria (Rhizobium leguminosarum), lives in the root nodule of the plants of leguminosae family can fix direct atmospheric nitrogen. Some other bacteria like cyanobacteria and photosynthetic bacteria can also fix the nitrogen in the soil. The denitrifying bacteria can convert the salts of nitrogen directly into atmospheric nitrogen.