Hello there, you may find more help for your tree in the regular house plant forums. Ponytail palms are not true bonsai just because they appear to be a large tree in a small pot. I don't have any experience under my belt, however I recently purchased 7 of these - saving them from a "Big Lots" store in Florida. I have done some research and would like to say first.... lay off the watering. These trees store water in their bulbish trunks and some can go a year without receiving water (I imagine based on the size of the bulb). The yellowing of the leaves could be due to improper soil, air pockets, lack of sunlight or possibly over-watering. They really don't need much fertilizer, read up on this before attempting to fertilize, I do know they should receive liquid fertilizer and a very weak solution only every 3 months or so from what I have read about them so far. Again, try the houseplant forums as that is where I will be going for more information. Good Luck.

Hold the water. The new growth is always a lot lighter color, even the ones on my deck in full sun have a pale new growth. The younger ones like a little protection from full sun, you should be OK with full sun in your area though. I've never fertilized any of mine & the biggest is about 12' tall with a base the size of a washtub.
Good luck.
Tally HO!