natural clay pellets comes free and easily available are excellent. These were tried with northiana, rafflesiana,campanulata, veitchii(L/L and intermediate).
northiana example...seeds grown since Jan,2007. Germinated in peat moss and later transfered to pur natural clay pellets.
Burnt clay works exceptionally well on northiana and campa., Probably after the burning process the increased presence of nitrates and other oxides of iron and manesium encourages plant growth. It is also widely used in agricultural farms or nurseries.