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Nitrogen Production by Selected Astragalus Species¹

Astragalus species are widely distributed throughout the temperate regions, and are particularly widespread in the northern hemisphere. The genus has a poor image because of a few toxic species (loco weeds) and its contribution to nitrogen fixation and forage production has not been studied.

A greenhouse experiment was conducted to determine the N-producing capacity of six morphologically diverse Astragalus species: A. asper Jacq., A. cicer L., A. falcatus Lam., A. glycyphyllos L., A. onobrychis L., and A. ponticus Pall., and their effect on the N content of an associated grass, ‘Nordan’ crested wheatgrass [Agropyron desertorum (Fisch. ex Link) Schult.]. The highest concentration of N was found in A. ponticus, a robust, nonforage species. Astragalus cicer was second highest. The lowest quantity was found in A. glycyphyllos. Crested wheatgrass had the highest N content when grown in association with A. onobrychis and the lowest when grown with A. falcatus. Residual soil N was highest with A. cicer.

A field study of soil N from 2-year-old plants of A. alpinus L., A. cicer, A. galegiformis L., A. glycyphyllos, A. odoratus Lam., and A. scorpurius Bge. resulted in a decreasing N gradient laterally from the center of the plant and with increasing depth. The highest soil N was at the 0 to 15 cm depth and 15 cm from the center of the crown. Each species tested had a different associated soil N level. Astragalus scorpurius had the highest soil N level; A. alpinus had the lowest.

Sufficient variation existed in the N levels associated with native and introduced Astragalus species to warrant further investigation of their N-fixing characteristics as well as their forage potential.

Key Words: Milkvetches • Loco weeds • Nitrogen fixation

² Research agronomist, USDA, ARS, Western Regional Plant Introduction Stn., Pullman, WA 99164.

Received for publication February 9, 1981.