Two long term field trials were conducted at EL-Serw Research Station (heavy clay soils), Domaine Governorate ( North of Nile Delta ) during four growing seasons (1995/1996 - 1998/1999) to compare the effect of potassium sulphate ( SOP) and potassium chloride ( MOP ), each in two rates (70 and 140 kg K20 / ha ) under surface irrigation system . The crop sequence of the four growing seasons at the first trial (EL-Sera-1 ) was berseem-rice, berseem-rice, wheat-rice and berseem-rice, while it was wheat-sorghum, sugar beet-rice, berseem-rice and rape-sorghum at the second trial (EL-Serve-2). Soil samples were collect-ed from each plot before starting the trials and yearly after summer crops to follow up the concentration of total soluble salts ( TSS ), chlo-ride and the available K. The obtained results indicated that: 1. The grown crops showed sporadic responses to the applied potash fertilizers , may be due to the heavy clay soils of this area with high K content . Positive significant responses by SOP application were de-tected in one out of the two winter seasons for wheat, two out of four winter seasons for berseem and only one out of six summer sea-sons for rice. Most of these effects were found under low salinity, and there were slight advantages in favour of SOP for most crops 2. MOP showed no signification effects on the grown crops ; exception was found with rice grain yield which was adversely affected by the high MOP rate under high salinity condition during one out of two seasons, and berseem which responded positively only to the low MOP rate at both sites.
3. MOP also showed no adverse effects on rice; the compulsory grown crop in this area, especially under lower salinity; may be because it is grown on flooded soils and this consequently prevent salt and CI ac-cumulation .
4. At EL-Senv-1, with low salinity , both K rate and source showed no marked difference in both soil TSS and CI concentrations, due to the leaching process under rice cultivation as preceding crop before col-lecting the soil samples, but when the preceding crop was sorghum. the TSS and CI concentration were slightly higher . This was con-firmed at EL-Serw-2. with high salinity of the soil samples taken after rice, however application of MOP induced higher soil contents of both TSS and CL than SOP .