We try to distinguish between 'flushing' and 'leaching' -viewed from a pre-harvest context.
Towards the end of the flowering cycle, in the run-up to harvest, many hydroponic growers of consumable crops stop feeding their plants nutrients altogether. The logic here is simple. An excess of nutritional elements in the plant tissue can adversely affect the taste and aroma of the crop with some growers even reporting a ‘metallic’ taste when too much phosphorus and potassium are present in the plant tissue.
In order to combat this, we must start at the root zone—after all, this is the central point of food for your plants.
Leaching uses just water—the lowest EC water (purest) you can get your hands on—preferably reverse osmosis. Low EC water is known as a ‘hypotonic solution’—it contains less solute (lower EC) compared to the cytoplasm of the cell and draws minerals out of your plant. Use two to three liters of water to every liter of medium.
Leaching is a little like ‘washing’ your growing media through, carrying with it lots of soluble mineral salts stored up in the root zone. Growers usually leach throughout the last week before harvest. Leaching can also be used to correct build-ups of excess nutrients in the root zone at other point during the growth or bloom cycle.
Flushing is quite a lot like leaching, only with the addition of a special chemical that will bond to residual, insoluble salts in the root zone. (Remember, insoluble salts don't register on an EC meter.) The insoluble salts, though useless in terms of plant uptake, can still cause undesirable effects in the presence of soluble nutrients.
The theory of both leaching and flushing is to reduce osmotic pressure in the root zone, causing excess salts to be given back up by the plant in an effort to restore balance after a loss off salts in the root system.
Try using a product like Floraclean through it at least once and remember not to re-circulate! You must drain to waste.
Special thanks to Carlos Aguilla and Adam Williams.