As cost of K fertilizers is relatively high, this experiment was conducted to study the effect of applying RWW on soil fertility, yield quantity and quality of greenhouse cucumber and tomato. Uniform 30-day old cucumber seedlings and 45-day old tomato seedlings were transplanted to two multi-span greenhouses (1000 m2) on December 12, 2012, at Dair Alla Research Station in Central Jordan Valley (JV). The transplanted seedlings were subjected to 5 RWW treatments of 75% of the traditional amount of K fertilizer farmers of the JV apply during the growing season, 100% of K of the traditional amount of K-fertilizer of which 25% were applied before transplanting and 75% were applied during the growing season, 125% of the traditional amount of K-fertilizer where 25% were added to the soil before transplanting and 100% were added during the growing season, traditional amounts of N, P and K chemical fertilizers, and traditional amounts of N and P chemical fertilizers only. The results showed that RWW can effectively substitute K-chemical fertilizer and can also improve some soil fertility parameters by the end of the growing season. For example, increases in available K and organic matter in the RWW treated cucumber beds were 25–71% and 2–11%, respectively.