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Nitrogen Fertilizer Requirements of Irrigated Safflower in the Northern Great Plains¹

Alternaria carthami-resistant safflower varieties have been developed for irrigated production in the Northern Great Plains. Data were needed to determine the irrigation and fertilizer N requirements of this crop. Seven rates of N fertilizer in 45 kg/ha increments to 270 kg/ha of silty clay soil types, and two rates of K, 0 and 45 kg/ha, were applied as subplots in a split-plot, randomized irrigation by N rate study in field research on safflower, Carthamus tinctorius, ‘Sidwill.’ Irrigation treatments were applied at specific plant growth stages. At only one of three sites was seed yield increased by irrigation. In that instance irrigation at early bud stage produced maximum seed yield. Pre-irrigation and a water table at a depth of 1.5 m at Sites H7 and H8, and above normal precipitation at Site H8, may have limited yield response to irrigation. Test weight of seed was lowered and N, P, and K concentrations in safflower residue were increased by irrigation.

Yield and protein content of the seed increased with increasing fertilizer N rates. The 270 kg/ha fertilizer N rate decreased seed oil content on the site with the highest residual and mineralizable soil N. Test weight response to N rates varied among sites. Increasing rates of N fertilizer increased the N and K but decreased the P concentrations in the safflower stubble. Statistically significant optimum yields were obtained when the sum of residual NO₃-N in the 0 to 1.22 m soil depth, mineralizable N from organic matter and previous crop residue, and fertilizer N equaled 274 kg/ha.

Key Words: Carthamus tinctorius • Sidwill • Oil content • Protein • Mineralizable N.

² Associate professor, Soils, Southern Agric. Res. Ctr., Route 1, Box 131, Huntley, MT 59037; director and soil scientist, U.S. Northern Great Plains Res. Ctr., P.O. Box 459, Mandan, ND 58554 (formerly at Sidney, Mont.); associate professor—Safflower Breeding, Eastern Agric. Res. Ctr., Sidney, MT 59270; and research specialist in Soils, Southern Agric. Res. Ctr., Route 1, Box 131, Huntley, MT 59037, respectively.

Received for publication February 17, 1981.