Which forms of nitrogen do plants primarily take up from the soil solution?

Plants primarily take up nitrogen from the soil solution mainly in the forms of nitrate and ammonium. Nitrate (NO₃⁻) is a negatively charged ion that is highly mobile in the soil and readily available for plant uptake through root absorption. Since many plants favor nitrate due to its accessibility, they convert it into amino acids and proteins after uptake.

Ammonium (NH₄⁺), on the other hand, is a positively charged ion, and while it is less mobile than nitrate, it is still an important source of nitrogen for plants. Ammonium can be directly incorporated into organic molecules or converted to other nitrogen forms through biochemical processes.

The other options include urea, which is a nitrogen source that must first be converted to ammonium by soil microorganisms before it can be absorbed by plants. Nitrite is an intermediate form of nitrogen that is typically not taken up by plants; rather, it is quickly converted to nitrate through nitrification. Phosphate is not a nitrogen source but rather a separate macronutrient essential for plant development. Thus, the correct answer emphasizes the two most significant and readily absorbed forms of nitrogen by plants, which are nitrate and ammonium.