[NTG-context] activating font specific ligatures

Hi, I'm currently adding some otf-fonts (MinionPro,MyriadPro from Designer8.2) to ConTeXt. I use the following font feature for MinionPro: \definefontfeature[minion][script=latn,liga=yes,kern=yes,tlig=yes,trep=yes,onum=yes] Most ligature work with this setting, but ch ck ct ij IJ sp ss st do not. Also the ligatures for small caps (some are defined) do not work at all. Is there a way to manually select the ligatures of a font? I like to activate the ch-,ch-ligatures (c_h E085, c_k E086), but not the too playful ct-,sp-,st-ligatures. I can live without all this, but it would be nice to use the possibilities of the font. Any hint is welcome. Greetings, Peter