PROBLEM TO BE SOLVED: To determine an appropriate fertilization amount based on the fact that a slow-release fertilizer is supplied at the time of fertilization, regarding the fertilization amount of the additional fertilization performed after supplying the slow-release fertilizer in one period. A fertilizer application rate determination device (1) acquires a vegetation-related value relating to vegetation of a crop before topdressing, and uses a fertilization application derivation formula that outputs a fertilization application as one of the inputs. Based on the acquired vegetation-related value, derive the fertilizer application rate during provisional additional fertilization, and further obtain the accumulated temperature from after the initial fertilization in this period to the time of additional fertilization, and obtain the nutrient elution rate based on the obtained accumulated temperature. The residual amount of slow-release fertilizer is derived from the nutrient elution rate, and the residual amount is subtracted from the fertilizer application amount during topdressing to determine the final fertilizer application amount during topdressing. [Selection diagram] Figure 1【課題】１つの期の中で緩効性肥料を供給した後に行われる追肥の施肥量に関して、施肥時に緩効性肥料が供給されることを踏まえた適切な施肥量を決定できるようにする。【解決手段】施肥量決定装置１は、追肥前の農作物の植生に関する植生関連値を取得し、植生関連値を入力の１つとし施肥量を出力とする施肥量導出計算式を利用して、取得した植生関連値に基づいて仮の追肥時施肥量を導出し、更に今期の初期の施肥後から追肥時に至るまでの積算温度を取得し、取得した積算温度に基づいて養分溶出率を求め、養分溶出率から緩効性肥料の残存量を導出し、追肥時施肥量から残存量を減算して最終的な追肥時施肥量を決定する。【選択図】図１
