ECONOMIC OPTIMAL AMOUNT OF NITROGEN-FERTI LIZATION FOR SUGAR-BEET - CRITICAL OBSERVATION OF DIFFERENT METHODS EXEMPLIFIED ON 39 FIELD TRIALS IN LOWER SAXONY

Data were taken from 39 field trials with stepwise nitrogen fertilizat ion in Lower Saxony (FRG) from 1980-1992. The compared economic parame ters are the corrected revenue, the corrected revenue plus quality pay ment and the latter plus varying shares of C-sugar beets. Farmers cann ot influence effects of the year on sugar beet yield. It is assumed th at farmers organize the sugar beet acreage in a way that they can fulf ill the contract with the sugar factory in years with low sugar beet y ield. In years with high sugar beet yield, above the A-/B-quota some C -sugar beets are produced. To define the amount of C-sugar beets, two models with different assumptions were developed to calculate the infl uence of the year on sugar beet yield. Model A defines the line betwee n B- and C-sugar beets at levels of 14% or 20% below the highest yield of each field trial. In model B this line is at the yield without nit rogen fertilization. It is shown that 20% lower revenue, quality payme nt and 43% higher fertilizer price had no important effect on the opti mal amount of nitrogen fertilizer. Whereas including of C-sugar beets in model A and B resulted in 34-50 kg N/ha lower fertilizing optimum, the level is at 80-90 kg N/ha. This makes clear that the corrected rev enue is not a satisfactory parameter to estimate the optimal amount of nitrogen fertilizer if C-sugar beets are not involved. The models A a nd B differ hardly in their results.