Same times it becomes necessary to add salt in the second extraction to get good recovery on each pesticide.
what is exactly salt action on the second liquid/liquid seperation ?
Thanks for answers

Use your imagination! When salt dissolves in water there remains less space for organic compounds, futhermore it helps to break any emulsions (if otherwise centrifugation is recommended). Of course, this phenomenon can be explained using more sofisticated language. However, no matter how much salt you have added some polar pesticides eg. methamidofos, acephate, metribuzin cannot be recovered by this method. Fortunately, there are other methods available, which overcome this disadvantage. For example ethyl acetate extraction in the presence of anhydrous Na2SO4. Excelent recoveries for a wide range of coumpounds, and you don't need to create a philosophy about "salt action".

The normal, and more likely correct, explanation of the salting out effect is that the salt increases the polarity of the solvent so that a moderatly polar solute is no longer soluble. The energy (Gibbs Free Energy) to keep it in solution is to high, just as there is not enough energy at room temperature to keep a nonpolar compound solvated in pure water.