This page covers crystalline ammonium sulphate where it is dissolved rather than spread: fertigation, soluble blends, dissolving tanks and spray-water conditioning. Solubility is high but strongly temperature-dependent, and mixing compatibility has one hard limit that causes most field failures. Particle size grading is covered on the crystal ammonium sulphate page.
Substance identity
Ammonium sulfate · (NH₄)₂SO₄ · CAS 7783-20-2 · PubChem CID 6097028 · 132.14 g/mol · white crystalline solid for solution use; low-insolubles grades only.
Specification
— parameters that matter for solution use; typical commercial ranges
| Parameter | Typical range | Why it matters in solution |
|---|---|---|
| Total nitrogen (N) | 21.0 % min | sets the N concentration of the made-up solution |
| Sulfur (S) | 24 % min | |
| Water-insoluble matter | ≤ 0.05 % | the governing parameter — insolubles block filters and drip emitters |
| Free acid (as H₂SO₄) | ≤ 0.03 – 0.05 % | affects final solution pH and equipment corrosion |
| Iron (Fe) | tens of ppm | iron can precipitate and stain |
| pH, 5 % solution | 5.0 – 6.0 | |
| Moisture (H₂O) | ≤ 0.2 % | affects the accuracy of gravimetric dosing |
Properties
Approximate water solubility: about 71 g per 100 g water at 0 °C, 74–76 g at 20–25 °C, and about 104 g at 100 °C. Two consequences follow. First, the solubility rise with temperature is modest compared with many salts, so heating the tank buys less headroom than expected. Second — and more important operationally — a stock solution made up at the top of its solubility on a warm afternoon will crystallise out overnight as the tank cools, blocking lines and injectors. Make up stock solutions with a deliberate margin below saturation at the coldest temperature the tank will see.
Dissolution is mildly endothermic: the solution cools as the salt goes in, which further reduces solubility during make-up. Add the salt gradually into agitated water rather than charging it all at once. The finished solution is mildly acidic (pH 5–6), which suits most drip systems but should be accounted for where irrigation water is already acidic. Practically insoluble in ethanol and acetone.
Applications
Fertigation and soluble blends. Supplies ammonium-N and sulfate-S in one salt. The ammonium fraction also acidifies the root zone slightly as it is taken up and nitrified, which can help iron, manganese and zinc availability on alkaline irrigation water.
Spray-water conditioning. Dissolved ammonium sulphate is a standard water conditioner for glyphosate and some other weak-acid herbicides in hard water: the ammonium and sulfate ions tie up calcium and magnesium that would otherwise form poorly active herbicide salts. Conditioner is added to the tank and allowed to dissolve fully before the herbicide, at rates set by the herbicide label and the water hardness. Use a low-insolubles grade; coke-oven material is unsuitable.
Industrial solution make-up, including protein precipitation and buffer preparation, where reproducible low-impurity material is required.
Handling and storage
Keep bags sealed — airborne dust and grit contaminate an otherwise clean material and end up on the filter. Dissolve into agitated water; allow time. Keep away from strong alkalis.
Notes
The hard compatibility limit: do not combine ammonium sulphate with calcium nitrate, calcium chloride or other soluble calcium sources in the same concentrated stock tank. Calcium sulfate (gypsum) is only sparingly soluble and precipitates immediately, fouling filters and emitters. Use separate stock tanks with dilution at the point of injection. The same caution applies in less severe form to concentrated phosphate stock solutions in hard water.