NPK

Nitrogen drives vegetative growth and chlorophyll production — plants show yellowing, stunted growth when N is deficient. Phosphorus drives root development and energy transfer (ATP); deficiency shows as purplish leaves and poor flowering. Potassium regulates water use, disease resistance, and fruit quality; deficient plants show scorched leaf edges and weak stems.

Typical annual demand per hectare: corn 150–250 kg N, 60–100 kg P₂O₅, 150–200 kg K₂O; wheat 100–150 kg N, 40–60 kg P₂O₅, 60–100 kg K₂O; tomato 150–300 kg N, 100–200 kg P₂O₅, 300–500 kg K₂O. A soil test measures existing supply; the crop's demand minus the soil's supply equals the fertilizer recommendation.

Reading NPK labels: a 50 kg bag of 20-10-10 contains 10 kg of N, 5 kg of P₂O₅ (= 2.2 kg of actual P), and 5 kg of K₂O (= 4.15 kg of actual K). The P and K numbers are oxide-equivalent, not elemental — this is a common source of confusion. Secondary nutrients (calcium, magnesium, sulfur) and micronutrients (iron, zinc, boron, manganese, copper, molybdenum) are also required in smaller amounts, but most yield responses come from correct NPK management.