Water is certainly one of the most critical inputs in crop production in many parts of the world particularly in the arid and semi-arid regions, i.e. Egypt, in the near future the required development of irrigated agriculture is necessary to cope with the increasing food demands from increasing population and water scarcity in Egypt. It has thus become necessary to explore new water sources that can meet current or future demand for irrigation supply. The main objective of this study was to determine the maximum amount of seawater irrigation needed to produce a good yield and study the combined effect of diluted seawater (I₁, I₂, I₃, I₄, I_5, and I₆) and rates of compost addition (0.0, 4.0, 8.0 ton fad.^-1) on plant growth, crop production and soil properties. Sandy soil Lysimeters were planted by salt-tolerant plants (barley) followed by sunflower crop under the same treatments. Results concluded that all diluted seawater prepared was suitable for irrigation crops according to the calculated criteria of water quality. The values of soil pH, Ec (ds/m) and ESP at the end of the two seasons were taken the same trend, which increased significantly with increasing the salinity of irrigation water or increasing the rate of compost addition as individual factors but in combination among them appeared insignificant variation in most studied properties. Also, available macronutrients residual in experimental soil were affected by the antagonism relationship between the studied factors, positively by addition of compost and negatively with salinity water supply. It was noticed that the interaction between treatments has positive role in reducing the hazards of the salinity irrigation water, which the biological yield of barley was decreased to maximum percentage (28.95%) at using the highest salinity irrigation water (I₆). But, it was obtained the minimum reduction percentage (5.9%) if combined with the highest rates of compost addition compared with the control (I₁). The residual effect of highest compost addition was more clearly on the parameters studied of sunflower cultivated in the second season which hadn't reduce the hazards of high salinity irrigation water (I₆) alone but optimized with the values parameters studied particularly seed yield by 16.3% at the treatments (I₆ × C₃) compared with the control (I₁).