Plant nutrition: What is the chief usable source of nitrogen for green plants under typical soil conditions?

Introduction / Context:
Although nitrogen is abundant in the atmosphere, plants rely on soil-derived, bioavailable nitrogen forms. This question assesses the principal nitrogen form taken up by most green plants in agricultural and natural soils.

Given Data / Assumptions:

• Root uptake mechanisms exist for nitrate (NO3−) and ammonium (NH4+).
• N2 gas is inert and cannot be directly assimilated by plants.
• Typical, well-aerated soils contain significant nitrate produced by nitrification.

Concept / Approach:
In most temperate, aerated soils, nitrifying bacteria convert ammonium to nitrate, which is highly mobile in soil water and readily absorbed by plant roots. While plants can use ammonium, nitrate is generally the chief source under common conditions.

Step-by-Step Solution:

Verification / Alternative check:
Soil fertility texts emphasize nitrate as the main plant-available N in well-drained soils; ammonium dominates more in waterlogged or managed systems with ammonium fertilizers.

Why Other Options Are Wrong:

Common Pitfalls:
Equating fertilizer application form with in-soil prevalent form; nitrification rapidly converts NH4+ to NO3− in aerated soils.

Final Answer:
Nitrates (NO3−) absorbed from soil.