Soil Fertility and Plant Nutrition
What you must understand
Know the essential elements and their roles. Macronutrients: nitrogen (leaf growth, protein, chlorophyll), phosphorus (roots, energy transfer, early establishment), potassium (water regulation, stem strength, grain filling), calcium (cell walls), magnesium (chlorophyll centre), sulphur (proteins). Micronutrients: iron, manganese, zinc, copper, boron, molybdenum, chlorine. Know deficiency symptoms: N - pale older leaves, stunted; P - purple older leaves, poor roots; K - scorched leaf margins; Mg - interveinal yellowing of older leaves; Fe - interveinal yellowing of young leaves; Zn - short internodes, little leaf; B - hollow stem, poor seed set.
Understand fertiliser analysis (N-P₂O₅-K₂O), straight versus compound fertilisers, and the law of the minimum. Know nitrogen losses: leaching of nitrate, volatilisation of ammonia from urea on dry surfaces, denitrification in waterlogged soil. Understand soil testing, organic matter, green manure, crop rotation and legumes as nitrogen sources.
Traps in questions
- Mobile nutrients (N, P, K, Mg) show symptoms on older leaves first; immobile ones (Fe, Ca, B) on young leaves.
- A 20-10-10 bag is 20% N, 10% P₂O₅, 10% K₂O, not 20% of each element.
- Lime supplies calcium and corrects pH but is not a nitrogen fertiliser.
- Over-application of nitrogen causes lodging and delayed maturity.
- Legumes fix nitrogen only when the correct Rhizobium strain is present.
Rule of thumb
Diagnose by position first: older leaves means a mobile nutrient (N, P, K, Mg); young leaves means an immobile one (Fe, Ca, B).
Worked example
A recommendation is 120 kg N/ha using urea (46% N). Mass of urea = 120 ÷ 0.46 ≈ 261 kg/ha. If applied in two splits, each dose is about 130 kg urea per hectare.