ASSESSMENT OF SPATIALLY-VARIABLE NITROGEN-FERTILIZER MANAGEMENT IN WINTER-WHEAT

Spatially variable N application may improve N use efficiency, grain y ield, and net returns of winter wheat (Triticum aestivum L.) in fields exhibiting wide ranges of soil characteristics. The objectives of thi s study were to (i) determine the variability in optimal economic grai n yield and in the amount of N required to produce a unit of grain at optimum yield, the unit N requirement (UNR), among landscape positions ; (ii) evaluate landscape position as a criterion for dividing fields into units of equal productivity; and (iii) assess the economic benefi ts of spatially variable N fertilizer application. Replicated N rate ( 0 to 125 lb N/acre) experiments were established on footslope, south-b ackslope (S-backslope), shoulder, and north-backslope (N-backslope) po sitions of the hillslope profile at two eastern Washington farms in 19 90 and 1991. Yield potential among these landscape positions varied by up to 63% and UNRs varied by up to 70%. Landscape position, however, was not an adequate criterion for dividing fields into equal productiv ity units. There was little economic benefit from variable N applicati ons in a hypothetical case analysis if N recommendations were calculat ed assuming a constant UNR; however, if experimentally determined UNRs were used, variable applications increased net returns by up to $14.8 0. The degree of economic benefit depended on the levels of misapplica tion that occurred when a single N rate was applied to a field and the yield responses to N which determined the results of misapplication. Spatially variable N fertilizer management will require accurate estim ates of yield potential, UNR, mineralization and available soil N to b e economically viable.