We have measured the ${\mathit{M}}_{4,5}$ edges of Er in submonolayer coverages of Er on Si(111), using linearly polarized x rays. The observed dichroism has been related to the surface crystal field and interpreted quantitatively in the hypothesis of an axial perturbation. The free ion $^{4}$${\mathit{I}}_{15/2}$ ground state is split over $≊${}25 meV, the lowest level being of $‖${}\ifmmode±\else\textpm\fi{}15/2〉 symmetry. The results are compared to those of a previous experiment on Dy/Si(111).