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The adequate amounts of potassium (K) fertilizer application may play an essential role in the growth and tolerance of plants against the drought stress. In this study we investigate the effectiveness of potassium fertilization on the growth and tolerance of soybean plants [Glycine max (L.) Merrill.] to drought stress. Treatments were arranged in a randomized block design in a 2 × 3 factorial: two potassium fertilizer levels [40 mg kg-1 of K (low) or 160 mg kg-1 of K (high)] and three irrigation regimes [100% of pot capacity - PC (well watered control), 50% of PC (moderate stress) and 25% of PC (severe stress)] with four replicates. Leaf relative water content, cell membrane stability, plant growth, and morphophysiological indexes were recorded after 18 days of exposure to drought stress. The appropriate supply of potassium fertilizer improved leaf membrane stability and minimized the water loss from leaf tissue of soybean plants exposed to drought stress. The adverse effects of drought on leaf abscission and pod abortion rate could be mitigated by adequate K supply. The appropriate supply of potassium fertilizer alleviates the negative effects of drought stress and maintain shoot growth and the water status soybean plants, and therefore, the proper management of potassium fertilization may confer greater drought tolerance. [ABSTRACT FROM AUTHOR]