Anhydrous sodium sulphate supplies sulphur in immediately plant-available sulphate form, together with sodium. That combination defines both its usefulness and its limits: sulphate-sulphur is taken up directly with no oxidation step, while the accompanying sodium restricts where the material can be used at rate. This page sets out the composition, the crops and situations where sodium is tolerated or beneficial, the situations where it is not, and the practical points on blending and storage. Industrial uses of the same material — detergents, textiles, glass, pulping — are covered on the chemical products page.
Substance identity
| Field | Value |
|---|---|
| Chemical name | Sodium sulphate (anhydrous) |
| Also written | Sodium sulfate |
| Common names | Salt cake (crude grades), disodium sulphate, thenardite (the natural anhydrous mineral) |
| Formula | Na2SO4 |
| CAS number | 7757-82-6 |
| PubChem CID | 24436 |
| Molar mass | 142.04 g/mol |
| EC number | 231-820-9 |
| Appearance | White crystalline powder or granules |
Composition calculated from the formula (not a claimed analysis):
| Element | Calculation | Content |
|---|---|---|
| Sulphur (S) | 32.06 / 142.04 | approx. 22.6 % S |
| Sulphate (SO4) | 96.06 / 142.04 | approx. 67.6 % SO4 |
| Sodium (Na) | 45.98 / 142.04 | approx. 32.4 % Na |
At a typical commercial assay of 99 % Na2SO4, the delivered sulphur is close to 22 % S. The sodium content is the number that constrains use — roughly one third of the product mass.
Specification
Typical commercial ranges for anhydrous sodium sulphate. Not guaranteed limits; the certificate of analysis governs.
| Parameter | Typical range | Note |
|---|---|---|
| Na2SO4 | 99.0 % min (commonly 99.0–99.5 %) | crude “salt cake” grades run lower |
| Moisture | typically ≤ 0.1–0.5 % | anhydrous product; moisture drives caking |
| Water-insoluble matter | typically ≤ 0.05 % | |
| Chloride as NaCl | typically ≤ 0.3 % | |
| pH, 1 % solution | approx. 6–8 | essentially neutral — see notes |
| Iron (Fe) | typically ≤ 0.002 % | |
| Calcium + magnesium | typically ≤ 0.1 % | |
| Whiteness | typically ≥ 85 % | a detergent-grade parameter, not agronomically meaningful |
Note that several parameters on a standard CoA — whiteness in particular — exist for detergent and textile buyers and carry no agronomic significance. Do not pay for a specification the application does not need, and do not read whiteness as a purity proxy.
Properties
| Property | Value |
|---|---|
| Melting point | approx. 884 °C |
| Density | approx. 2.66 g/cm3 |
| Bulk density | typically approx. 1.2–1.6 t/m3 |
| Solubility in water | approx. 4.8 g/100 g at 0 °C; approx. 19.5 g/100 g at 20 °C; a maximum of roughly 49 g/100 g at 32.4 °C; then a slight decline toward 100 °C |
| pH in solution | near neutral |
| Hygroscopicity | takes up atmospheric moisture and converts to the decahydrate |
Solution reaction is near neutral. Unlike ammonium sulphate, which acidifies soil through nitrification of the ammonium, sodium sulphate has no acidifying or liming effect of its own. Sulphate is the conjugate base of a strong acid and does not hydrolyse appreciably. This is a genuine point of difference when sulphur is needed but a pH shift is not wanted.
Sulphate-sulphur is directly available. Plants take up sulphur as sulphate. Elemental sulphur products must be microbially oxidised first, which is temperature- and moisture-dependent and takes time; sulphate products act in the season of application. The corollary is that sulphate is mobile in soil and leaches, so timing matters on light soils under high rainfall.
Applications
Where the sodium is acceptable or useful:
- Sodium-responsive crops. Sugar beet and fodder beet are the standard example: sodium can substitute for part of the potassium requirement in these species and a positive yield response to sodium is well documented. Several other species of beet-family and coastal origin show similar tolerance or response. Where such a crop is grown, the sodium is not merely tolerated but can be part of the reason for the choice.
- Sulphur correction where no pH change is wanted. On soils already at target pH, or where an acidifying sulphur source is unwelcome, a neutral sulphate source is technically appropriate.
- Sodium-deficient forage situations. Sodium is an essential element for livestock, and forage sodium content can be low; sodium application to grazing swards is an established, if specialised, practice.
- Industrial and formulation use within the fertilizer sector — as a sulphate source, a bulking and free-flow component in blends, and a raw material.
Where it is not appropriate:
- Any soil already sodic or at risk of sodicity, any low-permeability clay soil, and any situation under restricted-quality irrigation water. Sodium disperses clay, collapses soil structure, and reduces infiltration and permeability. That damage is slow to develop and slow to reverse.
- Sodium-sensitive crops, which include many fruit trees, stone fruit and vines, and legumes.
- As a general-purpose sulphur fertiliser at rate. Where sulphur alone is the objective and the accompanying cation is free to choose, the mainstream materials are ammonium sulphate (S with N, acidifying), potassium sulphate (S with K, chloride-free), gypsum (S with Ca, and a positive effect on sodic soils rather than a negative one), and elemental sulphur (slow-release). Sodium sulphate is a specialised choice, not a default one.
Blend compatibility. Sodium sulphate is a dry, near-neutral salt and physically blends readily. Two cautions: (1) match particle size to the other blend components, or the blend will segregate in handling and spread unevenly; (2) the anhydrous salt takes up moisture and forms the decahydrate, which sets hard — in a blend, that means the whole blend sets, not just the sulphate fraction.
Handling and storage
Hazard profile. Sodium sulphate is one of the more benign industrial salts and is not generally classified as hazardous. Dust irritates eyes and the respiratory tract mechanically. The supplier’s current safety data sheet governs classification and controls — a low hazard profile is not an absence of one.
Moisture and caking — the dominant issue. Anhydrous sodium sulphate absorbs water and converts to the decahydrate (Glauber’s salt). That conversion involves a large volume change and produces a hard, set mass, not a damp powder. A pallet that has taken up moisture may need mechanical breaking. Store under cover, sealed, off the floor, in a dry space, and avoid condensation cycles. This is the single most common cause of complaints on this product, and it is a storage failure rather than a quality failure.
Temperature. Because the anhydrous/decahydrate transition sits at 32.4 °C, warehouse temperatures cycling across that point in the presence of moisture accelerate hydration and dehydration cycling and caking. Stable, dry storage is worth more than cool storage.
Incompatibilities. Strong acids (sulphate is stable, but acidic conditions with any sulphide contamination can release hydrogen sulphide). Strong reducing agents at high temperature — sodium sulphate is reduced to sodium sulphide under those conditions, which is exactly the chemistry exploited in kraft recovery furnaces and is not a hazard at ambient. Segregate from acidic fertilizer intermediates as general good practice.
Packaging. Typically supplied in PE-lined bags or FIBCs, or in bulk. Moisture-barrier packaging is not optional for this product in humid conditions.
Notes
- Sodium is not a plant macronutrient. It is classified as a beneficial element for certain species, not as essential in the way nitrogen, phosphorus, potassium, sulphur, calcium and magnesium are. Any recommendation for this product should rest on the sulphur, on a documented sodium response in the specific crop, or on a forage sodium objective — not on a general claim of nutrient value.
- Assess the sodium load before recommending a rate. At approximately 32 % Na, application rates that look modest in product terms deliver substantial sodium. Soil exchangeable sodium percentage, soil texture, drainage and irrigation-water quality all bear on whether that load is acceptable.
- Anhydrous versus decahydrate. Glauber’s salt (the decahydrate) is a separate substance with its own CAS number and contains roughly 44 % Na2SO4 by mass — under half the sulphur per tonne. Specify which one you are buying; the price-per-tonne comparison is meaningless without it.
- Registration. Whether this material may be sold or labelled as a fertiliser input is a jurisdictional question governed by national fertiliser regulations. This page describes chemistry and agronomy, not regulatory status.
- Industrial applications of the same material are covered at
/chemical-products/sodium-sulphate-anhydrous.html.
PART 4 — FALLBACK DRAFTS — NOT USED
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