When growing in a soilless mixture like sunshine, peat moss or Coconut fiber flushing is extremely important as it is the only way to remove built up fertilizer deposits in any medium, These built up deposits effectively make the medium toxic.
When this happens the roots start to die off and the plants starts to show signs of deficiency (no roots no food). Most people try and remedy this by adding more fertilizer, only adding to the problem.
The best way to stop this from taking place if to flush regularly, for instance when growing in soil, it is best to follow the four step feeding schedule
Fertilize, Water, Water, Water.
When doing this you allow at least one third the liquid you pour into the top of the container to drain out of the bottom.
This runoff will generally contain a very high concentration of built up fertilizer I personally have seen this runoff be has high as 4500 PPM (Parts per Million) which in soil is extremely high.
Flushing is probably the single most important part of growing in soil.
When growing in a hydroponic, aeroponic, NFT type system flushing is again very important but done usually once a week while making a nutrient solution change the reservoir is emptied and a large portion of water is run through the container holding each plant washing away any built up salts that have accumulated in and around the plant.
Salt based ferts are very concentrated and as the water evaporates or is taken up by the plant what remains gets stronger and stronger eventually you will end up with a salt like coating on the pots and inside the reservoir that’s bad for plants to say the least so flushing it away once a week helps remove it and keep your plants roots happy and healthy
This is especially important in systems using Rockwool, as it is notorious for holding in salts and you can actually see the stem canker and deform from high concentrations of salts . Hydroton, hydrocorn,lecca clay balls go by many names but its another very basic growing medium and is another medium that in prone to salt build up and a visible one because of its nature any white lines building up are salts and need flushing
Another good idea when doing a weekly flush is adding a “ZYM”.
Adding a Zym to your flushing schedule will help break down old root decaying root matter which can lead to root rot, and turn it back into a usable food for the plant.
There is an interesting string on thcfarmer.com that shows just how divided this topic is among gardeners. I personally fall into the pro flushing camp and that’s mostly based on talking with dozens of people a day over the course of a decade hearing the different technics and their results. part of working in a Hydroponic store is listening to your clients and if necessary changing your views to take their experience into account.
Read the thread here