Step 1: Beijerinck (1888) isolated the bacterium but explained the mechanism, not the first observation. Step 2: Virtanen and Sekhon worked on the biochemistry and soil science much later, unrelated to the first record. Step 3: Boussingault's earlier nitrogen-balance field trials first showed legumes add nitrogen to soil. Step 4: The correct option is J. B. Boussingault.