The phrase "top drest" refers to the act of spreading fertilizer on the surface layer of soil. There are different synonyms that can be used in place of "top drest." One of the common synonyms is "top dressing." Other suitable synonyms include "mulching," which refers to adding organic materials to the soil surface, and "dusting," which involves sprinkling small amounts of fertilizer on the soil surface. "Broadcasting" is another synonym that refers to spreading fertilizer uniformly across a wide area. Ultimately, the choice of synonym for "top drest" would depend on the specific context, as each word has a slightly different implication and connotation.