A weakly modulated large-area two-dimensional square photonic lattice slab was fabricated through optical induction technique in a photorefractive photovoltaic LiNbO(3):Fe crystal. Bragg-matched diffraction technique was used to characterize the square photonic lattice slab. Interestingly, linear discrete diffraction typical for waveguide arrays was observed in such a square photonic lattice slab, indicating that the lattice slab can be viewed effectively as a one-dimensional waveguide array. Furthermore, discrete soliton was demonstrated in the photonic lattice slab due to a saturable self-defocusing nonlinearity arising from the bulk photorefractive photovoltaic effect of LiNbO(3):Fe.