In agricultural soils, . . . . . . . . of the total nitrogen is found as fixed ammonium ion

Answer: Option B

In agricultural soils, 10% of the total nitrogen is found as fixed ammonium ions. Ammonium and nitrate ions are the predominant inorganic forms of nitrogen in soils. Ammonium is available in the soil atmosphere in both exchangeable as well as non-exchangeable forms.
Nitrogen is a widely available element in our environment, it is available in different forms like ammonium ions, nitrate ions, nitrite ions or elemental forms.