Graduate Thesis Or Dissertation
 

Fertilizer requirements of wheat grown under conservation tillage

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  • Nitrogen (N), phosphorus (P), and sulfur (S) fertilization trials with Stephens winter wheat (Triticum aestivum L.) were conducted on the Columbia Plateau under minimum and no tillage conditions. The crop rotation was either wheat-summer fallow (summer fallow) or wheat-wheat (recrop). Nitrogen variables included: N rates, N timing, and N source treatments. The N fertilizer rate required to maximize yield depended on the amount of precipitation received, soil nitrate levels, and yield potential. Increasing precipitation increased yield potential (within limits partly determined by the available water holding capacity of the soil) which increased the N requirement for maximum grain yields. Soil nitrate varied in its ability to reduce the requirement for N fertilizer. At low soil nitrate levels, an increase in soil nitrate caused a corresponding decrease in N fertilizer requirement; however, at moderate to high soil nitrate levels, N fertilizer requirements were reduced less than the increase in soil nitrate. Nitrogen timing treatments consisted of N applied in the spring of summer fallow (SS), at seeding (SD), and in the spring of the crop year (SC). In a wet year (1983-84), SC and SD applied N gave similar yield results. In a dry year (1984-85), SS and SD applied N gave similar yield results. Overall, N applied at SD gave the highest yields. Urea, ammonium chloride, ammonium sulfate, and urea phosphate were compared as N sources. Urea phosphate tended to increase yield more than urea plus phosphorus. Ammonium chloride tended to lower protein content and increase 300-kernel weight more than urea and ammonium sulfate, possibly because of chloride effects on plant water potentials. Responses to phosphorus fertilizer were not accurately predicted by soil P test levels. Cold soil temperatures at planting (due to high elevation and/or late planting dates) in addition to high yield potentials, seemed to correspond to P responsive sites better than soil P test levels. At experimental sites where S soil test levels were above 2 ppm in the top 30 cm, wheat did not respond to S fertilizer. When S soil test levels were below 2 ppm in the top 30 cm, a response to S fertilizer was recorded approximately 17%, of the time.
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