tanker said:
1) if plants use NPK and other trace, what is the purpose of MgSO4 since it is only magnesium and sulphate. Also, i do read that some people uses potassium sulphate. What is the purpose of sulphate?
The chemicals we are dealing with are compounds - combinations of chemical elements. When they go into solution they split into their positive and negative ions. Take ordinary salt as an example. It's a compound of two elements - Sodium and chlorine make Sodium Chloride (NaCl). Dissolve it in water and you get sodium and chloride ions. Something like Magnesium Sulphate (MgSO4) is made of magnesium, sulphur and oxygen and gives you magnesium and sulphate ions. When we add a compound to the tank, we are targeting elements within that compound, perhaps within an ion, that the plants need.
Magnesium is one of those elements. It's an important trace but you won't find it in AE's trace mix. Same goes for calcium. People with hard water tend to have plenty of these and may not need to worry, depending how the hardness is made up. People with soft water, like me, may need to add them. I use a combination of MgSO4 and GH booster, which contains magnesium, calcium and other sulphates.
Dosing NPK, we use Potassium Phosphate (KH2PO4), which gives us K and P; and Potassium Nitrate (KNO3), which gives us K and N. Some people find this combination gives enough K, but based on the maximum uptake rates it could leave you theoretically short on K. Some people add Potassium Sulphate (K2SO4) to add more K without adding more N and P.
In some ways, the sulphate is just there as a carrier for the Mg or K. But plants do need sulphur too, so it provides that for free.
