We study the photo-desorption occurring in H2O:CO:NH3 ice mixtures irradiated with monochromatic (550 and 900 eV) and broadband (250-1250 eV) soft X-rays generated at the National Synchrotron Radiation Research Center (Hsinchu, Taiwan). We detect many masses photo-desorbing, from atomic hydrogen (m/z = 1) to complex species with m/z = 69 (e.g., C3H3NO, C4H5O, C4H7N), supporting the enrichment of the gas phase. At low numbers of absorbed photons, substrate-mediated, exciton-promoted desorption dominates the photo-desorption yield, inducing the release of weakly bound (to the surface of the ice) species; as the number of weakly bound species declines, the photo-desorption yield decreases about one order of magnitude, until porosity effects, reducing the surface/volume ratio, produce a further drop of the yield. We derive an upper limit to the CO photo-desorption yield, which in our experiments varies from 1.4 to 0.007 molecules photon-1 in the range ∼1015-1020 absorbed photons cm-2. We apply these findings to a protoplanetary disk model irradiated by a central T Tauri star.