I have not grown them, but I have been growing some other plants from Australia (some Proteas are from Africa) so I have read some about them. Many Protea are from very places with very poor depleted soils, and they can be killed by fertilization or even mulch, it seems mostly to be the P (phosphorus) and sometimes the K (potassium). I killed a Grevillea (similar fertilizer intolerance) by spreading lawn fertilizer about 6 feet away once.
Some Protea require acid soils, but there are some that like limy soils. Here is a link about that which includes which species like which.
http://proteaatlas.org.za/grow...
