Fertilizer Materials and Soil Amendments
Core concepts
Understand fertilizer grade notation (N-P₂O₅-K₂O), how to read a label, and the properties of common materials: urea (46-0-0, volatile if surface-applied without incorporation), ammonium nitrate, ammonium sulfate (acidifying, supplies S), MAP/DAP (P sources with different N contents and pH effects), muriate vs sulfate of potash, and micronutrient chelates vs sulfates. Know amendment functions: lime raises pH and supplies Ca/Mg; gypsum supplies Ca and S without raising pH and can improve sodic soil structure; elemental S acidifies slowly via microbial oxidation. Understand salt index and seed-placed fertilizer limits.
Traps
Questions confuse gypsum with lime (gypsum does not neutralize acidity) and test whether urea volatilization losses are recognized. Another trap is assuming all P fertilizers behave identically - MAP and DAP differ in N content and in the pH they create near the granule. Watch for chelate vs sulfate questions (chelates stay available at higher pH) and for salt-index questions about germination damage.
Rule of thumb and worked example
To find pounds of product needed: lb nutrient ÷ (% nutrient ÷ 100). Worked example: need 120 lb N/acre using urea (46-0-0): 120 ÷ 0.46 ≈ 261 lb urea/acre. That also supplies 261 × 0.46 = 120 lb N and nothing else. For a blend, compute each nutrient's product mass separately and check the total blend sums to the intended rate.