Four factorial field trials were carried out in El-Serw Agric. Res. Station during 1994 and 1995 seasons on wheat, clover, rice and cot­ton. This study aimed to determine the optimum rates of K-fertilization, and evaluate the irrigation water quality from two main sources at Dam­mietta governorate. Results indicate that the optimum rate is 48 kg K20/fed. for wheat, clover, rice and cotton to obtain the maximum yields of 2.245, 10.545, 3.340 ton/fed. respectively and 9.475 kentar/ fed. The water of EI-Serw main drain is classified as moderate in proper­ties and has no adverse effect on crop yield in many cases when com­pared with El-Shoka canal water. The higher values of TSS, SSP and Adj, SAR for such drain water of 162.5%, 1 64.3%, 358% in crease over the normal canal water, respectively. This shows that salinity and alkalinity problems would increase on the long run usage. Data revealed that po­tassium fertilization efficiency depends on irrigation water quality since it increases by using drainage water for winter crops especially clover. On the other hand, use of canal water increases yields of summer crops especially cotton. That may be attributed to the indirect effects of Na+ and K+ balances and their relations.