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Residual Effects of N Fertilization on Dryland Spring Wheat in the Northern Plains. I. Wheat Yield and Water Use¹

Data are frequently lacking to quantify the residual effects of fertilizer applications on soil nutrient availability, especially in regions of limited rainfall. Therefore, the effects of fertilizer N on yields of continuous dryland spring wheat (Triticum aestivum L.) grown on Typic haploboroll soil were determined in a field study at Mandan, North Dakota. Rates up to 540 kg N/ha applied either (i) all in 1 year, (ii) one-third the total rate in each of 3 years, or (iii) one-sixth of the total rate each year for 6 years of the experiment. Average dry matter (1967–1972) at different stages of growth and average grain yields were increased significantly by increased N rate, but only dry matter at tillering was significantly affected by rate x timing interaction. With 270 and 540 kg N/ha applied in only the first year, residual effects significantly increased grain production in 4 of the following 5 years. At total rates of 135 kg N/ha or above, total grain yield for the 6-year period was greatest for the 6-year fertilization treatment and least for the 1-year treatment. Average annual grain yields for the 540 kg N/ha rates were 1,700, 1,660 and 1,880 kg/ha for the 1, 3, and 6-year treatments, respectively, and 1,050 kg/ha for the zero-N treatment. Weight of grain produced per kilogram of fertilizer N applied usually decreased as N rate increased. At the 135 kg N/ha rate or higher, grain production per unit of fertilizer applied was greatest for the 6-year treatments (annual fertilization). Root weight and development in the 6th year were related to the availability of soil N as reflected by fertilizer treatment.

Total water use was significantly influenced by N rate or frequency of fertilization in only 2 of the 6 years. Water use efficiency (WUE) increased as N rate increased, irrespective of fertilization frequency. Highest WUE was obtained by 6-year fertilization treatment and for most treatments WUE decreased .when annual fertilization ceased. Although residual N derived from previous fertilization seems to be very important for subsequent crop production, availability of N from residual sources was not sufficient to maintain maximum crop production over a number of years.

Key Words: Fertilizer N carryover • Method of N fertilization • Water use efficiency • Fertilizer use efficiency

² Soil Scientists, USDA Northern Great Plains Research Center, Mandan, N. D. 58554.

Received for publication November 13, 1976.