I'm pretty sure that phosphorus is naturally found in nature, and since plants use it as a component of fertilizer, it will eventually be all used up. If you're still concerned, make your own soil. Buy peat moss, black earth, add some compost, and some horticultural sand. Mix them all together in whichever proportions seem best to you to acheive a nice, open mix. Or, just do what I did and buy a bark mulch. Nothing in those but the bark. Just make sure you don't get cedar or pine bark, as those two woods contain chemicals that could be harmful to frogs. Cypress mulch is good though, and also won't rot and break down as quickly in a terrarium. I found that my plants loved the bark mulch, and it doesn't develop the odours that potting soil does from being constantly moist. You can also use just straight peat moss. Don't use artificial stuff, like perlite or vermiculite, because the frog might accidentally ingest some of it and get 'blocked up'.