Does magnesium sulfate in aqueous solution form magnesium (hydrogen-)carbonate when hydrogen carbonat is present?
The white precipitate is more likely insoluble white calcium sulfate.
Per a reference on the making of fertilizer, to quote:

Calcium, magnesium, and sulfur are also important materials in plant growth. They are only included in fertilizers in small amounts...

Net ionic reaction:
$\ce{ Ca^2+ (aq) + SO4^2- (aq) -&gt; CaSO4 (s) }$