Nice sago! They love fertilizer and anything high in nitrogen is great! The worst time to fertilize is in the fall because nitrogen gives them a push to make a new flush and if it grows indoors, the new fronds will be much longer than the old ones and the plant wont look very proportional. Early spring is probably the best time to fertilize and you can fertilize it probably until early August.
Good luck!
-Alex

I think where you live you could get away with fertilizing any time of year as long as these stay outside. But the yellowing is not from lack of fertilizer, it is just old leaves, and I would just cut them off.