Epsom Salt is Magnesium Sulfate. Magnesium is needed by plants in trace amounts. The key word there would be 'trace amounts'. It encourages growth by increasing chlorophyll production. But...
I have sago palms that were brought from Japan in the 1960's. I have never fertilized them. I think everyone is going a little crazy with the "must fertilize" thing. If the leaves are yellowing (and you aren't over watering), fertilize. Otherwise, it's fine.
And that goes for every other plant you own. I rarely fertilize and when I do, I dilute to half to quarter strength.
PS: Russell says he lives in North Carolina (not California). There are some incredible sago palms living on (and off) the coast of the Carolina's, Georgia and Florida that were planted 200 years ago.
