Is it salt buildup.
My last indoor grow (northern lights) had some small salt rock looking things sitting right on top of the soil.
What soil was used? A lot of bagged potting soils use moisture crystals for water retention. Maybe thats what you were seeing?
Excuse me for being a vague but I spend 40+ hours a week staring at a computer monitor so I will keep this short and simple.
When you flush arent you trying to flush out the fertilizers youve used? Or is it both. Also it seems like that would kill the plant giving it that much water?
There are two reasons for flushing.
1. To remove excess nutes and minerals right before harvest to produce a cleaner better tasting less harsh smoke.
2. To flush out excessive salts, nutes, chems and beer infused urine (you were skeptical but you were also drunk and it seemed like a good idea at the time) from your grow media to prevent or resolve lock out issues.
Salt buildup is most often a concern amongst hydro and chem fert users because of the use of synthetically bonding minerals and salts. Every time you feed a little more salt is left behind when the plant absorbs the nutrients it needs hence nutrient lock out due to salt build up. Its like ordering a #5 combo with fries everyday and only eating the burger while tossing the drink and fries on the floor. Sure, you filled an immediate craving for "food" but you left a lot of shit behind that you eventually have to deal with. Prolly not the best analogy but if you are as lit as me it may make sense.
Don't worry about the large amount of water, I flush with 5 times the volume of my media. e This is just about the only time during your grow that excessive quantities of anything are ok.
And what about all the buds? I was told to flush with 3 times the pot capacity 2 weeks before harvesting and not to give it anymore fert after that. Once again I will be that close to harvesting so 18 gallons of water at once seems like it would make my buds keel over and die. Thanks for your help and understanding in advance.
If your soil came with nutrients added then flush two weeks prior to harvest.
If your soil was made of organic matter then maybe flush only one week prior to harvest.
If you want to test your patience then flush one week prior to harvest then wait another week without watering at all to allow the plant to dry out and to help guarantee proper maturity. This is what I have been doing and it works out great for me.
Good luck and I hope I have been of some help.......
