Plant cells have large storage units in them, called vacuoles. These vauoles store water in the plant cell. When you mix detergent with water and try to water the plant with the mixture, you have a water concentration in the soil that is lower than the water concentration in the plant itself. The water in the cells of the plant will move across the plant's cell membrane and cell wall in a process known as osmosis, in an effort to mix with the detergent/water solution in the soil. The purpose is to make the water concentration in the soil equal to the water concentration in the plant. The observed effect on the plant will be that the plant droops and has a wilted appearance to it.