Procurement GuideBy Dr. Ramesh Patel, Chief Agronomist
Zinc Sulphate Monohydrate 33% vs Heptahydrate 21%: The Complete B2B & Agronomic Guide
Published on 2026-08-1814 mins readFCO 1985 & ISO 9001:2015 Verified
Executive Summary & Key Takeaways
Evaluate chemical purity, moisture retention, anti-caking properties, and commercial cost per unit of Zinc. Discover why 33% Monohydrate is the global choice for bulk buyers and NPK blenders.
Table of Contents (Jump to Section)
Zinc is an essential micronutrient required for enzyme activation, auxin synthesis, and cellular elongation in high-yielding crops. When sourcing zinc compounds, institutional buyers, fertilizer distributors, and compound blenders consistently evaluate two primary chemical forms: Zinc Sulphate Monohydrate (ZnSO4 · H2O) and Zinc Sulphate Heptahydrate (ZnSO4 · 7H2O).
1. Chemical Composition & Elemental Zinc Density
The fundamental difference between the two grades lies in the water of crystallization bound within the molecular lattice. Zinc Sulphate Monohydrate 33% contains only one water molecule per zinc sulphate unit, yielding a guaranteed minimum of 33.0% elemental Zinc (Zn) and 15.0% Sulphur (S). In contrast, Zinc Sulphate Heptahydrate 21% contains seven water molecules per unit, diluting the elemental zinc concentration down to 21.0% Zn and 10.0% S.| Technical Parameter | Zinc Sulphate Monohydrate (33%) | Zinc Sulphate Heptahydrate (21%) |
|---|---|---|
| Chemical Formula | ZnSO4 · H2O | ZnSO4 · 7H2O |
| Molecular Weight | 179.47 g/mol | 287.54 g/mol |
| Elemental Zinc (Zn) | 33.0% min | 21.0% min |
| Total Sulphur (S) | 15.0% min | 10.0% min |
| Physical State | Off-white micro-granules / fine powder | Transparent snow-white crystalline granules |
| Water Solubility | 100% Soluble (540 g/L at 20°C) | 100% Rapid Dissolution (960 g/L at 20°C) |
| Bulk Density | 1.35 – 1.45 g/cm³ | 1.15 – 1.25 g/cm³ |
| Hygroscopicity | Very Low (Anti-caking stable) | High (Prone to deliquescence in humid climates) |
2. Technical Specification & Heavy Metal Limits (FCO 1985 Standards)
Under the statutory Fertilizer (Control) Order (FCO 1985, Government of India) and international FAO quality mandates, high-purity agricultural zinc must strictly adhere to maximum permissible trace heavy metal limits to prevent agricultural soil contamination:| Quality Parameter | FCO Statutory Standard (33% Mono) | FCO Statutory Standard (21% Hepta) | Auron Lab Guaranteed Assay |
|---|---|---|---|
| Total Zinc (as Zn) | 33.0% min | 21.0% min | 33.2% - 33.5% |
| Lead (as Pb) | 0.003% (30 ppm) max | 0.003% (30 ppm) max | < 8.0 ppm (AAS) |
| Cadmium (as Cd) | 0.0025% (25 ppm) max | 0.0025% (25 ppm) max | < 3.5 ppm (AAS) |
| Arsenic (as As) | 0.01% (100 ppm) max | 0.01% (100 ppm) max | < 2.0 ppm |
| Water Insoluble Matter | 1.0% max | 1.0% max | < 0.15% (Crystal Clear) |
| pH of 5% Solution | Not less than 4.0 | Not less than 4.0 | 4.5 – 5.5 |
3. Commercial Freight & Container Loading Economics
For domestic bulk truckloads (FTL) and international maritime export, freight cost is charged per gross metric ton, not per unit of active zinc. Because Zinc Sulphate Monohydrate 33% carries 57% higher zinc concentration per bag, bulk buyers achieve dramatic logistics savings:- •Reduced Bagging Costs: Sourcing 100 MT of active zinc requires only 303 MT of 33% Monohydrate versus 476 MT of Heptahydrate—saving over 3,460 HDPE bags.
- •Ocean Freight Savings: Sourcing 33% Monohydrate reduces total 20ft FCL container requirements by 36%, significantly reducing CIF port landed costs in the Middle East, Africa, and the Americas.
- •Warehouse Optimization: Takes up 35% less palletized floor space while eliminating moisture seepage in humid coastal warehouses.
4. Agronomic Application Protocols & Crop Response
Both forms deliver bioavailable divalent zinc (Zn2+) to crops, but their physical forms lend them to distinct field application methods:- •Basal Soil Broadcasting: Broadcast 8 to 10 kg per acre of Zinc Sulphate 33% Granular during final land preparation or puddling for Paddy, Wheat, Maize, and Cotton.
- •Foliar Spray Application: Dissolve 3 to 5 grams of 33% Monohydrate per liter of water (0.3% to 0.5% concentration). Spray during active vegetative flushes in Citrus, Apples, Grapes, and Tomatoes.
- •Drip Fertigation & Hydroponics: Both grades are fully water-soluble and pass 100-mesh drip filtration without emitter clogging.
5. Frequently Asked Questions (FAQ)
- •Q: Can Zinc Sulphate 33% be mixed directly with DAP or NPK fertilizers?
- •A: In dry bulk blending, granular Zinc 33% can be blended with DAP if particle size (SGN) is matched. In liquid foliar tanks, avoid mixing concentrated zinc with orthophosphates to prevent zinc phosphate precipitation.
- •Q: What is the shelf life of Auron Zinc Sulphate Monohydrate?
- •A: Auron 33% Monohydrate has an active shelf life of 36 months when stored in unopened dual-layer HDPE bags with LDPE moisture liners in a cool, dry warehouse.
Direct Manufacturer SourcingAuron Fertilizers · Ahmedabad, Gujarat
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