Geertsma worked in a variety of techniques, including painting, watercolor, drawing, and printmaking. His oeuvre is characterized by strong powers of observation, a refined sense of atmosphere, and a restrained, realistic style. His landscapes and regional scenes, in particular, exude tranquility and a connection with the northern nature and culture.

Due to his long presence and artistic activity in Southeast Friesland, Geertsma played a pioneering role within the regional art world. His work not only constitutes an artistic document of Frisian life in the twentieth century but also bears witness to a deep love for the landscape and the community from which he originated.