You are confusing heavy metals with salts.  Ammonium Sulfate is a salt, as is liquid N, and Pottasium Chloride.  They will all temporarily increase EC.  What will really increase EC is poor subsoil drainage, where water evaporates off the surface rather than perculate through the soil downward carrying excess salts with it. 

Look at your base saturation numbers.  high Na levels will really cause high EC.  My brother has some alkali 8.1Ph soil that has a water table just a foot below the surface, and had high salts.  EC in bad spots were 4 and above.

Seperately, A neighbor's employee turned a corner to fast with a semi and lost a plastic tank full of liquid N that spilled into a good corn field.  Even with several truckloads of saturated dirt removed, nothing currently grows in that spot, it is too salty.  The EC three years after the accident was 4.0  IIRC.

Nearly all commercial fertilizers are salts and will contribute to total salts until taken up by the plant and incorporated into organic matter or until it is leached away by downward water movement.   It is the byproducts from industrial power plants sold as fertilizer and municipal sludge (organic plant food) that is laced with dangerous heavy metals.

Subsoil drainage, and not irrigating with salty water, and gypsum will all help with high EC.