chasv77answered grow question 5 years ago If you have excess soil from your planting, start with flooding that, and measuring ph on the runoff.
At three weeks, your root structure may not support flooding the pot, but that is what you'll have to do....at some point.
FYI, I use 3 gal grow bags, and always water/feed with 1 gal fluid, so that there is a slight runoff, as a regular process.