it is only necesary to flush if your using chemical nutes..the reason is to remove chemical and salt buildup in the plant and in chemical it also helps to begin getting rid of chlorophyll, this is possible because the nutes just sit in the soil and are immediately soluble by the plant..if you grow with organic nutes it is physically imposible to flush the soil because the soil itself feeds the plant through microbial interaction and water will not stop this..their is also no negative taste or properties when using organic nutes so no reason to flush even if you could.