Assessment of fertilizer nitrogen requirement of potatoes and sugar beet

English. This thesis includes six papers on the assessment of fertilizer nitrogen requirement of potatoes and sugar beet. The aim of the investigations described is to improve fertilizer nitrogen recommendations in such a way that high crop yields of a good quality can be obtained while detrimental effects of fertilizer nitrogen application on the environment are minimized.Yield response curves of numerous fertilizer nitrogen trials with potatoes and sugar beet could be described adequately by a modified exponential equation. However, the magnitude of the confidence interval for the economically optimum application rates, as calculated from the response curves, was frequently very high. In 60 % of the potato trials and in 46 % of the sugar-beet trials it was higher than 300 kg N per ha.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 at the end of winter, the soil type, and presence or absence of organic manures applied previously. The performance of the current Dutch nitrogen fertilizer recommendations, which take only the amount of soil mineral nitrogen present at the end of winter into account, was compared with that of recommendations which also take soil type and recent applications of organic manures into account. On average, smaller amounts of fertilizer nitrogen were recommended according to the current method, but the yields obtained with both methods were similar. The current recommendation method is therefore preferable to the refined method.When it is assumed that the probability of a yield deficit larger than 5 % is not allowed to exceed 5 the currently recommended fertilizer nitrogen application rate for potatoes can be reduced by 25 %, but the rate for sugar beet cannot be lowered. The effect of a change in the ratio of cost of fertilizer nitrogen to price of crop produce was much larger for potatoes than for sugar beet. A fivefold increase in the ratio, for instance due to a levy imposed on fertilizers, decreased the optimum for potatoes by 50 %, but for sugar beet by only 20 %. However, it also considerably decreased farm income, especially in the case of potatoes.A dynamic model for the response of potatoes to fertilizer nitrogen is described, the inputs of which are generally available to growers. The model was used to predict the effects of seven levels of fertilizer nitrogen on tuber yield in 61 experiments. When only the input data were used that were available at the time when advice is required, the model correctly predicted the optimum fertilizer nitrogen rate in 84 % of the experiments. This result is about as good as that obtained with the current recommendation method. When it will be possible to reliably predict the mineralization rate at individual sites, it is expected that the model will provide a better basis for practical advice than is available at present.With the model it was calculated that in normal and wet springs, nitrate leaching always occurs. The nitrate leached did not originate from soil nitrogen mineralized in spring or from applied fertilizer, but from mineral nitrogen present in the 30-60 cm soil layer at the end of winter. The total losses of nitrogen, i.e. the amount of nitrate lost due to leaching in spring plus the amount of soil mineral nitrogen accumulated at the time of harvest of potatoes, were similar in a loamy sand and a clay loam. The larger loss by leaching from the sand was offset by the larger accumulation of mineral nitrogen in the loam. It was concluded that little nitrate leaching occurs when the current nitrogen fertilizer recommendations are followed, provided that mineralization in the soil proceeds at an average rate.

Dutch; Flemish. M.b.v. dezelfde proefvelden, die gebruikt zijn voor het opstellen van de huidige bemestingsadviezen, zijn nieuwe proeven uitgevoerd om deze adviezen voor aardappelen en suikerbieten te verfijnen. Hierbij werd rekening gehouden met de invloed van de grondsoort en recent toegediende organische meststoffen op de behoefte van de gewassen aan kunstmeststikstof. Een simulatiemodel is ontwikkeld voor de reactie van aardappelen op stikstof om tot een meer perceelsgewijze advisering van de stikstofbehoefte te komen. Dit kon tevens gebruikt worden om de stikstofuitspoeling te meten