Topic: Plant Nutrition
Plants require nitrogen for growth, but they cannot utilize the abundant $N_2$ in the atmosphere directly. They must absorb inorganic nitrogen from the soil.
Steps:
Understanding the Question: Which specific chemical form of nitrogen do plants primarily take up through their roots from the soil solution?
Key Formulas and Approach: Nitrogen in the soil is converted by bacteria through nitrification. Most plants prefer the form that is most stable and available in aerated soils.
Detailed Solution:
Although plants can absorb ammonium ($NH_4^+$), Nitrate ($NO_3^-$) is the most prevalent and mobile form of nitrogen in most agricultural soils.
Bacteria in the soil convert ammonia into nitrite and then into nitrate.
Once absorbed, the plant reduces nitrate back to ammonium internally to incorporate it into amino acids.