Soil Fertility and Plant Nutrition
What the exam asks
Soil Fertility and Plant Nutrition tests essential nutrients, deficiency symptoms, fertilizer grades, nutrient-use efficiency and integrated nutrient management (INM). Expect calculations of fertilizer quantity and matching of symptoms to nutrients.
Core rules
- Essential elements are macro (N, P, K, Ca, Mg, S) and micro (Fe, Mn, Zn, Cu, B, Mo, Cl, Ni). Carbon, hydrogen and oxygen come mainly from air and water.
- N supports vegetative growth and protein; P supports root development and energy transfer; K supports water relations, quality and stress tolerance.
- A fertilizer grade such as 18-46-0 gives percent N, percent P2O5 and percent K2O. P and K are usually expressed as oxides, not elemental P and K.
- INM combines organic manure, biofertilizers and mineral fertilizer to supply nutrients and maintain soil organic matter.
- Deficiency symptoms: N pale older leaves; P purplish/undersized; K leaf-margin scorch; Zn interveinal chlorosis; B brittle growing points.
Worked example
A recommendation is 120 kg N ha−1. Urea contains 46% N. Urea required = 120 ÷ 0.46 = 260.9 kg ha−1, about 261 kg. If bags hold 50 kg, bags = 261 ÷ 50 = 5.22, so buy 6 bags for the hectare. Reasoning: divide the nutrient dose by the nutrient fraction; do not treat urea as 100% N.
Traps
Do not confuse P with P2O5 or K with K2O; do not add N percentages of DAP and urea without calculating each product's nutrient content. Organic manure is not waste disposal: its nutrient content is low but it improves soil. Deficiency symptoms can resemble herbicide injury, so confirm with soil or plant tests.