This study determined the effect of potassium (K) sources and rates on plant growth and yield for hydroponic cabbage, celery and lettuce, and also identified deficiency symptoms that would be helpful in treating deficiencies. We used seven treatments: one control treatment with 164.8 $mg{cdot}L^{-1}$ K concentration in nutrient solution and six additional K treatments achieved by adding $K_2SO_4$ and KCl to get higher K of 321.8 $mg{cdot}L^{-1}$, 482.7 $mg{cdot}L^{-1}$, and 643.7 $mg{cdot}L^{-1}$, respectively. Increasing the K concentration in the nutrient solution greatly affected the growth and yield of cabbage, celery and lettuce, especially the yield quantity, and the Ca and Mg content of plant tissues. Increasing the K concentration to 482.7 $mg{cdot}L^{-1}$ improved the yield of cabbage, celery and lettuce while a further increase of K to 643.7 $mg{cdot}L^{-1}$ did not result in any additional benefits. On the other hand, the Mg concentration, but not Ca, decreased with an increase in K supply but the concentration was still at a sufficient level. Supplying the K in the form of KCl resulted in a relatively higher yield for celery but not for cabbage and lettuce, irrespective of the amount of K fertilization. In addition, supplying the K in the form of $K_2SO_4$ resulted in higher tip burn incidence compared to KCl, especially at high K concentrations.