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. An 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.