By interference contrast microscopy and atomic-force microscopy, we observed for the first time a periodic roughening transition occurring on the surface of crystal aggregates of NH₄Cl grown in agarose gel. The experiments show that the period of the roughening transitions depends on both NH₄Cl concentration and agarose gel density. We suggest that this phenomenon arises from the competition of the interfacial growth and the nutrient transfer, a common but usually ignored effect in many growth systems.