Fertilizer & Plant Nutrition – Exam Capsule

Fertilizer Basics

Fertilizer = natural or synthetic material added to soil to improve fertility.
Two broad sources:
• Organic (animal manures, plant residues, compost, bone/fish/blood meal, bio-fertilizers).
• Inorganic/Synthetic (mineral salts).

Major Fertilizer Categories

Direct/Straight: single nutrient (e.g. ammonium nitrate, urea).
Compound: ≥2 nutrients (e.g. mono-ammonium phosphate).
Complete: N+P+K (e.g. NPK 15-15-15).

Common Nutrient-Specific Fertilizers (approx. % nutrient)

N sources: Ammonium nitrate $33.5\%$ , Urea $46\%$ , Ammonium sulfate $21\%$ , Calcium nitrate $15\%$ , Sodium nitrate $16\%$ , Potassium nitrate $14\%N\;44\%K_2O$ .
P sources: Super-phosphate $20\%P2O5$ , Mono-ammonium phosphate $12\%N\;61\%P2O5$ , Phosphoric acid $52\%P2O5$ , CIRP $36\%P2O5$ .
K sources: Potassium chloride/MOP $60\%K2O$ , Potassium sulfate $50\%K2O$ , Potassium nitrate (see above).

Physical Forms

Granules (broadcast/mixed).
Powder (broadcast or dissolved).
Crystalline (water-soluble).
Liquid/Foliar (spray/fertigation).
Slow-release / coated (microbial or chemical controlled release).

Application Methods

Broadcasting (bulk over beds).
Ring around stem (trees).
Placement in polybags/holes.
Fertigation (drip/sprinkler).
Foliar feeding.

Plant Nutrition Essentials

16 essential elements:
• Basic: C, H, O (from air/water).
• Primary macronutrients: N, P, K.
• Secondary macronutrients: Ca, Mg, S.
• Micronutrients: Fe, Mn, Zn, Cu, B, Mo, Cl, Ni.
Concentration ranges:
• Macronutrients $0.02–6.0\%$ (dry wt).
• Micronutrients $0.01–500\,\text{mg kg}^{-1}$ .
Physiological roles grouped into: constituents, enzyme activators, redox components, osmotic/ionic regulators, beneficial stimulants, toxic heavy metals.

Key Functions & Deficiency Clues

• Nitrogen (N)
– Function: proteins, chlorophyll; drives vegetative growth.
– Deficiency: older leaves yellow/pale, stunted.
• Phosphorus (P)
– Function: ATP, nucleic acids; early roots, flowering/seed.
– Deficiency: reddish-purple leaves, delayed maturity.
• Potassium (K)
– Function: stomatal regulation, sugar/starch movement, disease resistance.
– Deficiency: margin scorch on older leaves, short internodes, curling.
• Calcium (Ca)
– Function: cell walls, root/shoot tips, neutralizes organic acids.
– Deficiency: young leaves misshapen/necrotic, root tip die-back.
• Magnesium (Mg)
– Function: chlorophyll core, enzyme cofactor.
– Deficiency: inter-veinal chlorosis on lower leaves (veins stay green).

General rule:
• Mobile nutrients (N, P, K, Mg, Zn) – symptoms on older leaves.
• Immobile nutrients (Ca, B, Cu, Mn, S, Fe) – symptoms on younger growth.

Nutrient Uptake Patterns

Nitrate ( $NO_3^-$ ) moves in soil water → fast uptake.
Ammonium ( $NH_4^+$ ) adsorbed to clays → roots grow toward source.
Urea must hydrolyze to $NH_4^+$ before use.

Fertilizer Requirement Calculations

Fertilizer rate (kg ha⁻¹):
$\text{Rate}=\frac{\text{Nutrient need (kg ha⁻¹)}\times100}{\%\text{ nutrient in fert.}}$
Nutrient supplied (kg ha⁻¹):
$\text{Nutrient}=\frac{\text{Fertilizer rate (kg ha⁻¹)}\times\%\text{ nutrient}}{100}$
Multi-nutrient example (need $20\,\text{kg ha}^{-1}$ N & P):
• Urea $=\frac{20}{46}\times100≈43.5\,\text{kg ha}^{-1}$ .
• Single super-phosphate $=\frac{20}{8.8}\times100≈227\,\text{kg ha}^{-1}$ .
Pot calculation (NPK 15-15-15, pot radius $7\,\text{cm}$ , field dose $200\,\text{kg ha}^{-1}$ ):
• Area/pot $153.96\,\text{cm}^2$ .
• Field density $0.002\,\text{g cm}^{-2}$ .
• Per-nutrient factor $\frac{0.002}{15}\times100=0.0133\,\text{g cm}^{-2}$ .
• Fertilizer needed ≈ $2.1\,\text{g per pot}$ .

Quick Diagnostic Table (visual yellowing)

Yellow older leaves → suspect N, P, K, Mg.
Yellow younger leaves → Fe, Mn, S, Ca, B.
Marginal burn → K.
Purpling → P or Mg.
Inter-veinal chlorosis → Mg (older), Fe (younger).

Organic vs Chemical Fertilizers

Organic: feed soil, add humus, support microbes, slow nutrient release.
Chemical: supply immediately available ions, precise ratios, little organic matter.