When funds ran out to pay him, he returned to Kansas State Agricultural College and acquired his master's degree in botany and plant cultivation.

When the Division of Vegetable Physiology and Pathology became part of the Bureau of Plant Industry in 1901, Carleton was put in charge of the grain investigations section (head cerealist).

Due to his belief that improvements in yield would be most likely from the introduction of new varieties, he concentrated on a program of experimental trials of various seed stocks.

In 1925, while studying an infestation of pink boll weevils in the Peruvian cotton crop, he died of heart disease complicated by malaria.

He was a Fellow of the American Association for the Advancement of Science and received the distinguished French Ordre National du Mérite Agricole.