PROBLEM TO BE SOLVED: To provide a method for diagnosing magnesium deficiency in a plant, using an infrared absorption spectrum, which does not need experiment or intuition of an agriculture worker.SOLUTION: A method for diagnosing magnesium deficiency in a plant comprises: a step for preparing a water extract of a plant leaf; a step for measuring an infrared absorption spectrum of the water extract; a step for calculating a difference spectrum based on the infrared absorption spectrum of the water extract and an infrared absorption spectrum of water; a step for correcting a base line of the difference spectrum, for obtaining a correction spectrum; and a step for determining that the plant is magnesium deficiency, when there is a peak having a peak top in a wavelength area of 9.0 μm or more and 9.5 μm or less, in the correction spectrum, and a wavelength of the peak top is equal to or more than a reference value.SELECTED DRAWING: Figure 2
