Question
I would like to know that how much plant wheat cultivars need Mg and how much we sould apply of MgSO4,7H2O in the field?

Answer
Depends on your Soil test, Ebi.  You probably have PLENTY of Mg in your Soil.  Unless you're using it up at a furious rate.  If your pH is too high, you could be growing it in pure Magnesium and all your crops would exhibit Magnesium deficiency symptoms.  Where are you, and EXACTLY what does your garden grow?

rsvp,

L.I.G.