Flushing should be done periodically if you feel that you haven’t been following basic watering techniques. Should you see signs of overfeeding, (curling or misshaped leaves, leaves with brown or yellow tips, or leaves that are crunchy dry) flush the soil. The use of a drip tray under the pot is good idea. But be careful the plant doesn’t stand in water for long lengths of time, as this will reduce the oxygen content of the soil with the possibility of root rot occurring.
General Rule of Watering
A good rule to determine how much water is needed at every watering is to determine how much wastewater run-off occurs. At each watering, calculate the wastewater run-off from each pot to be approximately 10-20% of the water applied to the soil.
The idea is to make sure that the soil does not build up too much soluble plant food generated by the naturally occurring beneficial microorganism colonies and at the same time not flush all of the nutrients away. Remember, wastewater run-off should not be allowed to soak back up into the pot’s soil. It may be used to fertilize outdoor plants.
General Rule of Flushing
If you haven’t been following the recommended watering technique outlined above, you should flush/leach your plant's soil every month with water.
Exactly how little water can be used and still have a good flush? A light periodic flush every three weeks is a good place to start. Lets say your plant is in a 5-gallon pot. Water the soil with enough water until you see at least 25 to 50 percent of the 5-gallon pot capacity come out of the bottom and run away. In this case it would be 1.25 - 2.5 gallons of water. Calculate it this way; 5 (pot size in gallons) times by .25 (percent of flushed run-off water needed) = 1.25 (1 gallon of water you want to run out the bottom of the pot never to be seen again).
If you think you might need more water than a 25 percent flush run-off, increase to 50 percent or more and log that information into your log book so you can remember what you did for your next grow.
Should you see the plant have a direct benefit from leaching/flushing you may want to increase the amount of water applied at each watering.
How are you supposed to know how much flushed wastewater run-off has exited your plants pot without measuring the run-off from every pot? I might suggest the use of my old standby watering technique for flushing. Lets use the 5-gallon pot as an example again. Water the plant’s pot slowly and softly (sprinkler/shower attachment) until all of the soil in the pot is moist. Stop watering when the wastewater is just starting to exit the pot. Now go to the next pot and do the same until all of your pots have very moist soil. This is the point where we will measure out the 25 percent water flush, and as shown in the previous paragraph, it’s 1.25 gallons of water. Now, evenly pour the 1.25 gallons of water into the pot without disturbing the soil’s surface. Once again, you might need to use the sprinkler/shower attachment to your watering can to keep the soil surface undamaged. I think you will now find that 1.25 gallons of wastewater will have exited the bottom of the pot.