Final answer:
Urea constitutes 50% of all non-protein nitrogenous (NPN) substances, with urea being the primary compound used in agriculture and medical diagnostics to assess kidney function.
Step-by-step explanation:
The compounds that constitute 50% of all non-protein nitrogenous (NPN) substances are primarily urea (CO(NH₂)₂), with other substances like ammonia (NH₃) and nitrate (NO₃) making up the rest of the composition. Urea is a common NPN component due to its widespread use in agriculture as a nitrogen fertilizer. It provides an easily accessible source of nitrogen for plants, which is vital for their growth and development. On the other hand, in medical diagnostics, NPN substances like urea, uric acid, and creatinine are used to assess kidney function. Elevated levels of these substances can indicate renal dysfunction or failure.