Nutrient Sources
Core concepts
Know the major source categories: commercial inorganic fertilizers, organic amendments (manure, compost, biosolids), legume biological nitrogen fixation, and recycled crop residues. For organic sources, understand that nutrient availability depends on mineralization rate, which is driven by C:N ratio, moisture, temperature, and incorporation. Materials with C:N above roughly 25–30 immobilize N; below about 20, they net-mineralize. Manure nutrient content varies widely, so book values must be adjusted by availability coefficients (e.g., a fraction of organic N released in year one, with residual credits in later years).
Traps
Questions test whether you credit all manure N as immediately available (you should not) and whether you account for residual N credits from prior legume or manure applications. Another trap is confusing elemental concentration with oxide form (P vs P₂O₅, K vs K₂O). Also be ready to distinguish ammoniacal vs nitrate N sources and their acidifying or volatilization behavior.
Rule of thumb and worked example
Use the C:N rule: >30 immobilizes, 20–30 is roughly neutral, <20 mineralizes. Worked example: 5 tons/acre of manure at 10 lb total N/ton = 50 lb total N. If 50% of N is organic and year-one availability of organic N is 30%, available N ≈ (25 lb inorganic) + (25 × 0.30 = 7.5 lb) ≈ 32.5 lb/acre. Budget only that available portion against crop need and carry a residual credit forward.