How good care Voidrake?

I bought Voidrake seeds. But, no idea how choose best "grade topsoils"?

Try trade for blue fox hide.

We have three types of soil that actually have any influence and they all come in three stages (the higher, the better):
- Thanalan soil influence the crossbreeding chance
- La Noscea influences the HQ-chance
- Shroud influences the yield

Now, if you only want to trade it for bluefox hide you'd want as many Voidrakes out of it as possible, meaning using Shroud-soil would be your best option. Using grade 2 or 3 shroud soil will give you 3 voidrakes (instead of just 2 when using any other soil).
If you want to crossbreed Thanalan soil is the way to go.

http://www.ffxivgardening.com/seed-d....php?SeedID=71

This is a good site when you want to get into gardening - it shows you what crossbreeds have been reported possible with and for certain seeds.
As long as you dont care for crossbreeding though Shroud soil is the way to go.