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Sometimes you're better off looking on gardening forums for this kind of info. Forums require zero qualifications to post so you end up with a lot of "I heard" or "my friend said" type growers. You can see in this thread alone how people will tell you what will happen if you don't flush, then a few posts later say they've never tried not flushing. How someone can form an opinion about something they've never done is beyond me. Growing weed is pretty new and full of overnight pro's, gardening forums have more experienced grower's that don't reference a youtube video they watched last night to give people advice. This is one reason flushing plants is bullshit.
http://masteringhorticulture.blogspot.com/2012/01/mobile-vs-immobile-nutrients.html
What are plant-immobile nutrients?
By now, the answer to this question is obvious.
Plant-immobile nutrients cannot be translocated from older tissue to a new one due to the nature of the elements and sometimes other conditions. In other words they are stuck where ever they landed the first time. They have reached their destination. Deficiency symptoms for these elements are observed in the young plant parts. Calcium is an example of plant-immobile elements. It plays an important role in cell expansion. When calcium is deficient, the young shoots and flower buds exhibit the devastating effects. If the condition is not corrected the shoots and bud get aborted eventually.
Examples of plant-immobile nutrients are: Iron (Fe), Calcium (Ca), Manganese (Mn), Zinc (Zn), Copper (Cu), and Boron (B).