🥈Secondary Nutrients, Micronutrient Fertilizers, and Application Methods

Opening: The Hidden Hunger in Indian Soils

A groundnut farmer in Gujarat applies plenty of urea and DAP every season, yet his pods remain hollow and poorly filled. The cause? Sulphur deficiency — a secondary nutrient that most farmers overlook. Across India, 50% of soils are deficient in zinc and 25% in sulphur. These “hidden hungers” — secondary and micronutrient deficiencies — silently cap yields even when primary nutrients (N, P, K) are adequately supplied. Understanding these nutrients, their fertilizer sources, and correct application methods is essential for both crop productivity and competitive exams.

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Secondary Nutrients

Secondary nutrients (Ca, Mg, S) are required in moderate quantities — less than N, P, K but more than micronutrients. They are equally essential for healthy plant growth.

Gypsum (CaSO₄.2H₂O)

Gypsum serves a dual purpose: it is both a source of calcium and sulphur, and a critical soil amendment for reclaiming alkali soils.

Application rates:

Agricultural example: In groundnut cultivation, gypsum applied at 250 kg/ha at flowering significantly improves peg penetration and pod filling because calcium is essential for cell wall development in the pegging zone, while sulphur improves protein quality.

Why sulphur matters in pulses: Sulphur is a key component of amino acids methionine and cysteine. Its deficiency leads to formation of indigestible protein in pulse crops, reducing their nutritional value.

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Micronutrients

Micronutrients (Fe, Zn, Mn, Cu, B, Mo) are needed in very small quantities but are equally essential. Deficiency of even one micronutrient can severely limit crop yields.

The Problem of Fixation

When metallic micronutrient salts are applied to soil, they are often transformed into non-available forms due to chemical reactions. The solution is chelation.

Chelating agents — organic compounds like EDTA, DTPA, and CDTA — hold metallic ions in their cyclic molecular structure, creating metal chelates. Think of chelation as placing a metal ion inside a protective molecular cage.

Micronutrient Fertilizers at a Glance

Agricultural example: In paddy-wheat systems of the Indo-Gangetic plains, zinc deficiency is the most common micronutrient problem. Applying zinc sulphate at 25 kg/ha to the rice crop corrects the “khaira disease” (zinc deficiency in rice) — characterized by dusty brown spots on leaves.

Special note on Molybdenum in pulses: Mo is a constituent of leghaemoglobin and the nitrogenase enzyme in root nodules. Even 3 g of ammonium molybdate per kg of seed can dramatically improve nodulation and nitrogen fixation in chickpea, lentil, and other legumes.

Special note on Boron: The gap between deficiency and toxicity is very narrow. Use boron frits (slow-release form) rather than borax directly to prevent boron toxicity.

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Methods of Fertilizer Application

Choosing the right method affects nutrient availability, efficiency, and cost. The following methods are arranged from simplest to most specialized:

a. Basal Dose

• Fertilizer applied at the time of sowing/transplanting.
• Phosphorus and potassium are always applied as basal dose because they are immobile in soil and slow-releasing. Placing them near the developing root zone at sowing ensures early availability.

Agricultural example: When sowing wheat, the entire dose of P and K (e.g., DAP + MOP) is drilled at sowing, while nitrogen is split.

b. Split Doses of Nitrogen

• Nitrogen is applied in multiple doses to match crop demand at different growth stages, reducing losses and improving Nitrogen Use Efficiency (NUE).

c. Deep Placement

• In paddy, only ammonium form (NH₄⁺) is placed in the reduced (anaerobic) zone to prevent leaching and volatilization losses ^{UPPSC 2021}.
• In the reduced zone, ammonium remains stable and is not converted to nitrate (which would be lost through denitrification).

d. Starter Dose

• In legumes and vegetable seedlings at transplanting, apply NPK at 1:2:1 ratio.
• The higher proportion of phosphorus supports root establishment during the critical transplanting phase.

e. Foliar Application

• Spraying nutrients directly on leaves for rapid absorption.
• Best for micronutrients (Fe, Zn, Mn) and for the last dose of nitrogen in seed crops (improves seed quality and germinability).

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Suitability of Fertilizers for Different Conditions

This is a frequently tested topic. The following table summarizes which fertilizer works best under which condition:

Elemental sulphur transformation:

S —(Thiobacillus spp.)—> SO₄^2-

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Crop-Specific Nitrogen Management

Key Research Finding

De Datta and Gomez (1975): N-efficiency was highest in the first rice crop and declined progressively with successive crops. However, K-efficiency increased sharply over years, especially in the dry season.

Timing of P and K

• Plants require more phosphorus at early stages (root development).
• Plants absorb potassium continuously up to harvesting, and K-fertilizer becomes available slowly.

Rabbing

Rabbing = heat treatment applied to soil by burning refuse placed over it. This traditional practice destroys soil-borne pathogens and releases nutrients from organic residues.

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General Recommended Fertilizer Doses (N:P:K)

The NPK Imbalance Problem

The extreme imbalance (especially in Punjab) is a direct consequence of heavy urea subsidy and reflects serious soil health degradation from nitrogen over-application.

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Nutrient Based Subsidy (NBS) Scheme

The Core Problem

• Fertilizer subsidy is the 2nd largest subsidy after food subsidy.
• Urea remains excluded from NBS, keeping its price artificially low and encouraging overuse.
• This exclusion is the primary reason the NPK ratio keeps worsening.

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Summary Table

Summary Cheat Sheet