Development of nitrogen fertilizer recommendations for potatoes and sugar beet on the basis of soil testing.

Nitrogen response curves were used to describe yield as affected by various levels of nitrogen fertilization in a large number of trials with potatoes and sugar beet.It was found that the calculation of the optimum amount of fertilizer nitrogen depended on the choice of the response function and that the confidence intervals of the calculated optima were wide.A statistical analysis revealed that the response of potatoes and sugar beet to fertilizer nitrogen depended on the amount of mineral nitrogen already present in the soil, soil type and prior application of organic manures.The results of the statistical analysis were used to formulate recommendations for the use of nitrogen fertilizer in potato and sugar-beet production.The performance of these recommendations was evaluated by applying nitrogen fertilizer retrospectively to the trials considered.It was found that, on average, an optimum fixed rate of fertilizer applied to all trials would not seriously affect yield as compared with a rate of fertilizer based on the mineral N content of the soil of each trial individually.However, as in the latter case less fertilizer nitrogen would be needed to obtain maximum yield, it is recommended to take the mineral nitrogen content of the soil into account.The recommendations could be further refined by taking into account in addition to the mineral nitrogen content of the soil in each trial individually, the effects of soil type and prior application of organic manures.However, this refinement of the recommendations did not improve their performance.