That article in the Washington Post on Sunday did call the bomb in Karachi a "fertilizer bomb." But that need not mean the bomb only contained fertilizer (indeed, I don't believe fertilizer by itself would explode). People claiming expertise on explosives have claimed an ANFO (ammonium nitrate fertilizer plus fuel oil) bomb would not have done the damage the OKC bomb did. I am no expert on explosives, and thus cannot confirm this claim, but, if it is true, it is presumably equally true of the bomb in Karachi.
I strongly suspect the Karachi, OKC, and East African embassy bombs all contained the mixture of ammonium nitrate, TNT, and nitroglycerine detailed on that notebook page entitled "Explosivija za Oklahomu" (Serbo-Croatian for "Explosive Used in Oklahoma") found at the house in Kabul last year. I strongly suspect such a mixture would have had the stronger secondary blast that apparently occurred at least in the OKC and East African bombings. Does anybody know if there was such a secondary blast in Karachi?