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Tillage, Soil Depth, and Precipitation Effects on Wheat Response to Nitrogen

USDA-ARS, Columbia Plateau Conservation Research Center

C. R. Rohde

Oregon State Univ., P.O. Box 370, Pendleton, OR 97801

* Corresponding author.

ABSTRACT

Wheat (Triticum aestivum L.) response to N in semiarid climates is highly influenced by precipitation and tillage, leading to inefficient use of fertilizer and greater potential for groundwater contamination. To improve fertilizer recommendations, we measured white winter wheat response to 45, 90, 135, and 180 kg N ha^–1 for 10 crops grown in a wheat-fallow rotation with three tillage treatments, one conventional (moldboard plow) and two stubble-mulch (offset disk, subsurface sweep). The soil was a Walla Walla silt loam (coarse-silty, mixed, mesic Typic Haploxeroll) with three depths to bedrock (208, 132, and 111 cm). Grain yield averaged 5.51, 4.07, and 3.55 Mg ha^–1 with above- (147), near- (88), and below-normal (61 mm) growing-season precipitation (GSP). Nitrogen increased yield 3.0, 1.9, and 0.4 Mg ha^–1 in these precipitation regimes. Grain yield was not affected by soil depth when GSP was above normal, but 10 to 20% less on the shallower soils when GSP was below normal. There was a trend towards higher yield with conventional tillage when GSP was above normal. The amount of applied N required for optimum yield was >135 kg N ha^–1 with above-normal GSP and <45 kg N ha^–1 with below-normal GSP. Excess N decreased grain yield when GSP was near normal, but not when above or below. Applied N increased straw yield curvilinearly, with little influence of tillage or soil depth. Straw/grain ratio averaged 1.53 and 1.70 on the 208- and 111-cm soils, respectively. Nitrogen uptake tended to be greater in conventional than in stubble-mulch tillage, and was not affected by soil depth. Grain protein exceeded the desirable level for white wheat only when applied-N was above that needed for optimum yield. Precise selection of the amount of N to apply each year was difficult because of the strong influence of GSP on yield and the need to apply N before GSP is known.

Joint contribution of the USDA-ARS and Oregon State Univ. Oregon Agric. Exp. Stn. Technical Paper no. 9096.

Received for publication December 6, 1989.

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