American agriculturist who originated the method in which an expert demonstrates, farm by farm, new agricultural discoveries and technologies.

Knapp graduated (1856) from Union College, Schenectady, N.Y., and taught school for several years. In 1866 he moved to Iowa, where he was by turns a farmer, livestock breeder, banker, professor (from 1879) and president (1884–86) of Iowa State Agricultural College and Farm at Ames. In 1882 he helped draft a bill that was a precursor of the Hatch Act of 1887, which provided for the dissemination of practical and scientific agricultural information and allocated money to each state to establish agricultural experimental stations.

In 1886 Knapp moved to Louisiana, where he guided the establishment of rice as a staple crop. During the boll weevil invasion of Texas (1904), Knapp, as a representative of the U.S. Department of Agriculture (USDA), supervised a demonstration that proved the effectiveness of good farming techniques in weevil control. Thus he originated the program of the Farmers Cooperative Demonstration Work of the USDA, in which representatives of the department, usually known as county agents, worked with farmers to familiarize them with the findings of agricultural scientists. This system greatly improved the productivity of American agriculture in the 20th century.