Phoenix Roebelenii Brown Leaves

I bought 2 Phoenix Roebelenii 3 weeks ago and left them in their pots in the garden. Last week I noticed that the leaves started getting a little brown. Nott at the tips but all along the frond. I searched the net for answers and found that the pot could be too small. And in fact the roots were sticking out for about an inch above the surface. So I went to my garden centre and ask for advice on repotting Phoenix. I was told the best potting mix for Phoenix is John Innes no.3 so I bought it. I then put my palm into a 6 cm bigger pots and watered well. I also used Miracle-Gro ready to use liquid plant food once.
Now, can anyone tell my what is wrong with my palms? Is it because of the fertiliser. Or maybe they are stressed because i put them straight in the garden. Or is it lack of magnesium? Will they get anny better? What should i do? What is the best fertiliser for for them? I'm so worried and don't want to loose them.
What's more, I've just noticed that the leaves are starting to split:(
I mist spray them every day and always make sure they are moist.
Please, can anyone help me?

looks like sunburn to me... probably they were not acclimated to full sun. By the way, it is nearly impossible for these palms to be in too small a pot unless there literally was no soil in the pot, so I am sure that was not the problem. These are pretty hardy (though no idea what climate you are in) and planting them rarely causes any problems. It is not due to too little fertilizer, though too much can burn plants badly, but they don't burn like in your photo. Best not to fertilize new palms much... let them acclimate first, and then be gentle with fertilizers, particularly granular stuff... liquid fertilizers are a lot safer and can be used earlier.

Thanks:) So it's not that bad then. You made my day. By the way I'm in Cornwall UK. And April was pretty hot and sunny so you are right about the sunburn. I'll keep it in shade.
Thanks for helping:)