i'm not sure exactly what form the nitrogen is present in guano, castings, etc., but potassium, magnesium, and calcium are all present as carbonates. just because they're organic doesn't mean they're not salts, and the counterions - carbonates, for example - can cause lockout in excess quantity by making the desired ions unavailable. This is when a flush is productive, usually in between two major stages of growth, between vegging and flowering, for example. In any case, it sure doesn't hurt.