Jean-Baptiste Boussingault (1838), considered the father of modern agricultural chemistry:
Conducted quantitative greenhouse sand-culture experiments showing that clover and peas accumulated nitrogen even when grown in sand without added nitrogen, proving they could assimilate nitrogen from the atmosphere.
Later, Hellriegel and Wilfarth (1888) proved this fixation occurred inside root nodules via symbiotic bacteria.
Martinus Beijerinck (1888) isolated and cultured the symbiotic bacterium Rhizobium leguminosarum.
Artturi I. Virtanen (Nobel Prize 1945) worked on the biochemistry of nitrogen fixation and silage preservation (AIV method).

Nitrification is a biological two-step aerobic oxidation of ammonium ($\text{NH}_4^+$) to nitrate ($\text{NO}_3^-$):

• Ammonia oxidation: $\text{NH}_4^+ + \frac{3}{2}\text{O}_2 \rightarrow \text{NO}_2^- + \text{H}_2\text{O} + 2\text{H}^+$ (catalyzed by obligate autotrophs like Nitrosomonas, Nitrosococcus, Nitrosospira).
• Nitrite oxidation: $\text{NO}_2^- + \frac{1}{2}\text{O}_2 \rightarrow \text{NO}_3^-$ (catalyzed by Nitrobacter, Nitrospina, and Nitrospira).
  Complete ammonia oxidation (Comammox) carried out by certain Nitrospira species oxidizes ammonia directly to nitrate.

Assertion (A) is true: Gypsum ($\text{CaSO}_4\cdot 2\text{H}_2\text{O}$) is the most widely used chemical amendment for sodic (alkali) soils due to its low cost, widespread availability, and moderate solubility.
Reason (R) is true and correctly explains (A): Sodic soils have high exchangeable sodium percentage ($ESP \gt 15\%$), which causes severe clay dispersion, crusting, and poor permeability. Gypsum supplies soluble $\text{Ca}^{2+}$ ions that replace adsorbed $\text{Na}^+$ from the clay colloid:

The displaced soluble $\text{Na}_2\text{SO}_4$ is then removed from the root zone by leaching with good-quality irrigation water, restoring soil structure and permeability.

Assertion (A) is true: Continuous application of ammonium-based fertilizers (e.g., ammonium sulphate, urea) is the primary agricultural driver of soil acidification because nitrification of $\text{NH}_4^+$ releases 2 protons per molecule:

Reason (R) is false: The major acidifying effect occurs when the generated nitrate ($\text{NO}_3^-$) is leached downwards accompanying basic cations ($\text{Ca}^{2+}$, $\text{Mg}^{2+}$, $\text{K}^+$), leaving excess $\text{H}^+$ behind in the soil profile. Conversely, if nitrate is taken up by plant roots, the plant exudes bicarbonate/hydroxide ($\text{HCO}_3^- / \text{OH}^-$) to maintain electroneutrality, which partially counteracts acidity. Therefore, leaching of nitrate, rather than non-leached taken-up ions, causes net soil acidification.