Hi all,
You always end up with MgSO4·7H2O what-ever it was sold as. You can only get the anhydrous form by heating MgSO4·7H2O to drive the water of crystallization off, and then cooling and storing the crystals of MgSO4 in a desiccator, as soon as the salt is exposed to any moisture it will absorb H2O until it becomes the stable heptahydrate form again.
The magnesium oxide content (MgO) is quoted because fertiliser values were traditionally quoted as the oxides, you can ignore it.
I suppose I should have asked, can I use this stuff in the fert recipe in the same way or do I need to adjust the amount?
It doesn't matter where you start from, as long as you know the chemical formula you can work out percentages of each element in a compound easily. In this case (from the periodic table at <
http://www.webelements.com/>)
Element & RAM ("Relative Atomic Mass")
Mg = 24
S = 32
O = 16
H = 1
24+32+(4*16) + (14*1)+(7*16) ~ 24+32+64 = 120 + 14+112= 126 ~ 120 + 126 = 246 (this is the RMM "Relative Molecular Mass" of MgSO4.7H2O from Clive's answer).
So "Epsom Salts" have 24/246 ~ 10% Mg.
10g of MgSO4·7H2O, supplies 1g of Mg.
1g of Mg (from 10g MgSO4·7H2O) in 1litre of H2O = 1000mg in 1,000,000mg = 1000ppm Mg sol.
1g in 10litres = 100ppm Mg
1g in 100litres = 10ppm Mg
cheers Darrel