K − z plot for our USS sources. Green circles and red squares represent sources in the B03 and the S06 fields, respectively. Filled and open symbols represent sources with the spectroscopic and the photometric redshifts, respectively. Solid and dashed-triple-dotted lines represent the best fits for sources at redshifts (z) ≥ 0.5 in the B03 and the S06 fields, respectively. The dashed-dotted, dotted and dashed lines represent best fit lines of the K − z relations for the powerful radio galaxy samples from Bryant et al. (2009); Willott et al. (2003); Brookes et al. (2008), respectively. All the magnitudes are in Vega system.