I am assuming this question stems from your other post in which you cite that you accidentialy doubled a concentration. The plant will only draw from available
nutrients what it needs so, technically, you can't over fert. Howver, in large, or specific enough concentrations, the nutrients can cause damage to the plant on the celluar level, inhibiting or even destroying it's ability to photosyntheize or preform mitosis. So you can't "over fert" but you can have a
nutrient solution that's too "hot", or in which a large concentration of a particular element is having an unwanted chemical reaction binding/or restricting another element or , as aformentioned, interfering with biological a biological process. also excessive nutrient can cause ppm/TDs to increase to a level that causes nutrient lockout, ie brown leaves, etc, etc. Matt- hydroponica.blogspot.com