The subsequent nitrogen supply of soils varies year by year. The previous crop and any manure given play a major role. Measuring methods such as Nmin-examinations in spring provide a basis for the assessment of the first fertilizer application.
In order to measure the fertilization at the stage of shooting and earing the N-tester provides support. A specific N-fertilization is possible with this method since the measurement reflects the immediate nitrogen supply. Fertilization trials show that the N-tester gives a tight recommendation on nitrogen which delivers however with the correct distribution satisfying yields and qualities.
For the farmer the N-optimum serves as an orientation. In order to find out the N-optimum nitrogen enhancement trials were used in order to determine the net revenue curve. The net revenue increases with enhanced N-quantities until a maximum is reached. The N-optimum is the amount of nitrogen at which the curve reaches its peak. Increasing nitrogen prices and decreasing grain prices shift the fertilization optimum to lower expenditure amounts.
From more than 150 N-enhancement trials in Germany from 1996 until 2007 a net revenue curve was calculated from which a fertilization optimum was derived. If the price of calcium ammonium nitrate (CAN) increases from 270 to 475 EUR/t the fertilization optimum is reduced by 26 kg N/ha. A wheat price of 150 EUR/t is assumed here. Profit reduction due to the raised N-costs are in that case 110 EUR/ha. (graph 1)
If the wheat prices changes, the impact on the net revenues per hectare is more considerable than with increased N-costs.
With a CAN-price of 405 EUR/t the maximum net revenue is 980 EUR/ha, assuming a wheat price of 150 EUR/t. If the wheat price amounts to 250 EUR/t, the net revenue increases with the same KAS-price to 1780 EUR/ha. Graph 2 shows that the fertilization optimum increases by 20 kg N/ha in this case.
Both graphs show that the wheat price determines the revenue. The fertilization optimum alternates in the depicted situations only in the area from 20 until 30 kg N/ha. Considerable nitrogen savings are therefore also under different conditions not useful.
Experience has shown If the nitrogen application per hectare is reduced due to increased fertilizer costs, lower raw protein contents in quality grain cultivation have to be expected. Insufficient qualities decrease the wheat price additionally. The herewith lost revenue per hectare is usually larger than the saved costs in fertilization.