Sulfate is the doubly charged anion SO4 2-, the normal source of sulfur for plant roots. Plants reduce it internally to build sulfur-containing amino acids and other compounds. Many fertiliser salts carry their metal partner as a sulfate, so sulfate often enters a feed as a passenger.
In practice
Magnesium sulfate, potassium sulfate and the micronutrient sulfates (manganese, zinc, copper) all add sulfate. Calcium sulfate (gypsum) is only sparingly soluble, so concentrated calcium and sulfate stocks precipitate together. That is one of the two main reasons mineral feeds are split into A and B parts. In the final dilute solution they coexist without trouble. Tap water can also contribute meaningful sulfate; a water report will show it.
Reports may give SO4 or SO4-S. S = SO4 × 0.334.
Don't confuse with
- Sulfite (SO3 2-): a preservative and reduced form, not a fertiliser.
- Sulfur: the element; elemental sulfur powder must be oxidised by microbes before plants can use it.
Sulfur (S) is a secondary nutrient taken up as sulfate, used in the amino acids cysteine and methionine; deficiency yellows new growth fairly evenly.Magnesium sulfate heptahydrate, sold as Epsom salt, is the common hydro magnesium source, supplying about 9.9% magnesium and 13% sulfur.Potassium sulfate (K2SO4) supplies roughly 41-45% potassium and 17-18% sulfur with no nitrogen, useful for adding K while tapering N in late flower.A micronutrient is an essential element plants need only in trace amounts, below a few mg/L: iron, manganese, zinc, copper, boron, molybdenum, chlorine and nickel.Manganese (Mn) is a micronutrient that splits water in photosynthesis and activates many enzymes; low pH can raise it to toxic levels.An A/B nutrient is a mineral feed split into two concentrates so that calcium (part A) is kept apart from sulfate and phosphate (part B), which would precipitate together.Tap water is municipal or well water used for irrigation; its EC, alkalinity, hardness, sodium and disinfectant vary widely, so it should be tested before use.A heated vessel that vaporises elemental sulfur to suppress powdery mildew and mites in veg rooms; it carries fire, gas and timing risks and is never used in flower.Calcium (Ca) is an immobile secondary nutrient that builds cell walls and membranes; it travels only in the transpiration stream, so deficiency appears in new growth.
