Evaluation of soil nitrate as a predictor of nitrogen requirement for sugar beet grown in a mediterranean climate

Rational N fertilization is important to reduce environmental impact of fertilization and to increase profitability in crop production. Am evaluation of preplant soil nitrate concentration in soil to estimate optimum N rate for autumn-sown sugar beet (Beta vulgaris L.) was done by performing 33 N response experiments in different locations between 1989 and 2000. For each location and year, the N rate for maximum beet and sucrose production and the economic optimum N rate for adjusted beet production (yield adjusted for sucrose concentration) were estimated. Nitrate N concentration in the soil (0-30 cm deep) before fertilization ranged between 6 and 156 mg kg(-1). This concentration allows one to distinguish between N responsive and nonresponsive sites when no particular leaching (tile-drained soils in rainy winters - more than 470 mm from October to January - or furrow irrigated) or water stress limitations exist (rainfed locations with less than 350 mm annual rainfall - less than 170 mm during spring). A critical value for the preplant soil nitrate was determined, above which no response to N fertilizer can be expected. This value was 39 mg kg(-1) for beet production and 33 mg kg(-1) for adjusted beet production. Preplant soil nitrate also provided an accurate estimation of the optimum N rate for adjusted beet production under the same conditions (R-2 = 0.93). It is concluded that soil nitrate before planting can be a useful method for assessing N fertilizer rate in production of sugar beet under a Mediterranean climate in soils where leaching is not favored by tile draining or where yield potential is not excessively limited by water supply.