In the context of climate change, survival and productivity of eucalyptus plantations are affected by prolonged drought periods. We evaluated the effects of forest nutrition (with K and Na compared to control) and water availability, and their interactions, in a real prolonged drought scenario (occurred in 2014) on the growth of Eucalyptus grandis trees in Brazil. In a split-plot design installed in June 2010, we evaluated trees submitted to two water regimes (without rainfall exclusion and with 37% rainfall exclusion) and three fertilizer supplies (K, Na and control). The stem diameter increment was measured every 15 days with dendrometer bands and total height, every 6 months in the period from 40th to 61st month. Soil moisture was analyzed weekly and precipitation and temperature daily. The results indicated that nutrition with K showed a higher growth rate; with Na, trees had an intermediate growth rate between K and control treatments. With the artificial exclusion of 37% of throughfall there was a negative effect on the increment growth of K fertilized trees; but in the control and Na treatments the rainfall exclusion showed no significant effect until 58° month. Rainfall influenced significantly the increment growth of the trees, followed by the minimum temperature and soil moisture. K and Na showed interaction with precipitation according to seasons: during dry season, by reducing the growth of the tree trunks with Na and its stoppage with K, and during the rainy season, with increasing growth in diameter and height of the trunk for both K and Na, compared with the control. It is shown the importance of including K in basic fertilization for higher productivity of Eucalyptus forest plantations, however in regions that are more arid and with a greater risk of prolonged drought, potassium and sodium could increase the drought effect.