Soil Fertility, Plant Nutrition & Fertilizers

Chemical classification of nitrification vs urease inhibitors:

• Chloro-6-(trichloromethyl)pyridine (Nitrapyrin / N-Serve): Classic synthetic nitrification inhibitor suppressing Nitrosomonas bacteria.
• Amino-4-chloro-6-methylpyrimidine (AM): Proven chemical nitrification inhibitor.
• Dicyandiamide (DCD): Widely commercialized organic nitrification inhibitor.
• Phenyl phosphorodiamidate (PPD): An inhibitor of the urease enzyme, NOT a nitrification inhibitor. Hence, (A), (B) and (D) only form the true set.

NBPT pharmacology and kinetics:

• Assertion (A) is true: NBPT (N-(n-butyl) thiophosphoric triamide, sold under the trade name Agrotain) is the leading commercially utilized urease inhibitor coated onto urea to mitigate ammonia volatilization.
• Reason (R) is false: In soil environments, NBPT delays urea hydrolysis by competitively inhibiting urease for typically $7-14\text{ days}$ (up to a maximum of about $10-14\text{ days}$ depending on soil temperature and moisture), NOT for $25\text{ days}$. Hence, Assertion is true but Reason is false.

Parker et al. (1952) introduced the Nutrient Index Value (NIV) to evaluate the overall fertility status of soils in a region by assigning weights 1, 2, and 3 to low, medium, and high soil test fertility classes, respectively:

Interpretation: $\text{NIV} \lt 1.67$ (Low fertility), $1.67 - 2.33$ (Medium), and $\gt 2.33$ (High fertility).