PROBLEM TO BE SOLVED: To provide a method for diagnosing potassium deficiency in plant, which does not require experiment or intuition of an agriculture worker and uses an infrared absorption spectrum.SOLUTION: A method for diagnosing potassium 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 acquiring a correction spectrum; and a step for in the correction spectrum, when there is a peak in a wavelength area of 9.57 μm or more and 9.89 μm or less, and when an area ratio expressed by a formula (1) is equal to or less than a reference value, determining that the plant is potassium deficiency. The area ratio=(a first peak area having a peak top in a wavelength area of 9.16 μm or mor
