So far they are looking good so tell your brother he's doing a good job!!
IME, prepared soils usually have a lifespan of around 5 - 6 weeks before you start seeing deficiencies. I would start supplementing your liquid fertilizers around end of week 4 at 1/4 strength then go half strength at week 6. I use GH FloraNova so I never have to go more than half strength. If your plant is good with 1/2 strength then go with it but if you feel your plant wants more then go full strength. I rarely go with manufacturers advice in terms of feed strength. I use PPM to measure how much nutrient I want in the water. I usually don't go over 1200 ppm in veg and 1000ppm in flower.
In my case, I use prepped soils but I use them as backup to my nutrition program so I start feeding within the first couple of weeks of veg. If the plant is wanting more than I am giving it at the time, the soil nutrition can make up whatever it's wanting at that stage that I'm not giving it. This keeps deficiencies to a minimum limiting the stresses on the plant and keeping it in growth mode rather than recovery mode.
For autos you really want to get everything spot on from the get go as the clock is ticking so you only have so much time to grow your plant as big as you can before it starts flowering. Any stress or shock will delay the growth and reduce the amount of dry weight at harvest. One of the tricks I use in early veg for photos is to let them go an extra day before watering so that the tap shoots out more roots looking for water but it can get a bit tricky if you leave them too long and they stunt. I don't really do this in later veg as by this time I should have a pretty extensive root network developed.
Early root development, IMO, is key to having the throughways for your nutrient to get to your flowers. Bigger roots, Bigger fruits. This is why you want to water the ENTIRE media not just around the root ball that is forming. When you water the entire media in the pot you'll notice that the soil around the root ball will dry out first due to the roots uptaking the easiest water it can find. When that dries out it shoots roots looking for more water. If you only water around the root ball the roots your tap shoots out won't find any water and will die off. Also, capillary action will spread out the water over your media and evaporate it faster meaning you need to water more frequently.
I always recommend newbs and even more experienced growers read this. It answered a lot of questions I've always had but could never really get a definitive answer to. I'm not the author.
(2) Watering cannabis explained by Aqua Man | Bud Builders - Making Friends, Growing Cannabis
You can use this as a guide to when and how much nutrient to apply. I would start applying cal/mag a couple of weeks sooner than the chart indicates.
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