How do you know a plant needs PK ?

Pk is not only for the last few 3 weeks. That's ripening. Nearly 0 N is needed yes. And just pk on mid ripening..generally the last week. I agree with m0use a well rounded Npk feed is fine. When you come into pre flower, if you're using a 3 part nutrient Grow/Bloom/Micro then you stop the grow mid into pre flower and then start on a mid NPK in PKs favour. Grow NPK 24-6-12 That will then need to be lowered to 16-6-12 for ripening. Just lower N and same in the PK. This is where a 3 part nutrient formula will allow you to adjust your NPK like this. Fattening 12-6-12 is a good start and if needed you can then up PK but doubtful.. coming into ripening 6-18-24 and for the last push on ripening you can just give PK if you want. As N is not needed. And if it is, it'll used what's left in the plants growth. Tis y in some ppl grows you see the fade has yellow behind it. That's the plant taking the tiny amount of N needed. It sounds really complex. But, in once you get nutrients. Where you can somewhat control your Npk. By adding Grow or taking it away ml/l wise..how you'd do that with with trio of nutrients. 100ppm of grow and 300ppm of bloom will give you a 1:3 ratio. Then you can add those NPKS of grow and some more math to get a general NPK using liquid nutrients. I use powders from Green House Feeding. And they have Grow, then Hydroponics/short flowering for pre flower and onwards and then they've a PK booster powder you add to the Hydro/Flowering powders and you can control your NPK much more so. Bit of general math and most importantly is all phenos are dif and may require dif levels of NPK at the same stage compared to another grow. So, there really is no perfect NPK set for all plants or when you are meant to give. As again all phenos grow differently..u might have 1 auto that goes through its stages quicker so, there is really no set NPK as plants moving quicker through fattening say will need dif levels of Npk to the same strain of auto beside it, that make sense. As you're growing fems. You have the luxury of much more control of your NPK and when to adjust to bloom. But the theory applies. Once you're into pre flower. You might have some fems that'll be done in 8 or 11 weeks. But, rated 9 weeks for flowering from breeder. A lot depends on the speed your plant pushes thru the phenos stages. And, that'll come down 2ur setup. Such as light spectrum, DLI, substrate, heat/co2 levels and much more. So, going by the speed of how fast said pheno(s) go thru their stages will depend on when to adjust your NPK. So, there really is no time frame that can be given. You can go by stages, pre flower will be over after the plants tops stop growing. So, when the main tops have stopped stretching wait a few more days to make sure the sides have stopped stretching also. Then you're in fattening stage. npk needs to be adjusted to bloom. So, 6-12-18 / 1-3-6 ripening you can keep it on that for a few days but, then I'd start dropping N completely if u can. Or just give low doses of bloom and higher your pure PK nute. Any N needed will then be taken from your plants stores. Tis why when you see some plants that have lovely fade. And yellow behind it. Its a sign the plant dipped into the leaves to get the tiny N needed. It really is an art my pal. On my 4th crop now and I'm still getting my head round it. So, I suggest you just stick with a basic trio and add your extra PK when you come into fattening in the way of bloom boosters r whatever you're using. Just get a few grows under you. Then worry about fine tuning to get that A grade bud. Still can be achieved but, jst get a good bloom nute that can be given in mid pre flower r after. BTW, looking at ur plants. Your PH is slightly off. They're the early signs that ur sub is getting too low of an input ph wise.. Sry for the very complex answer but, its a very hard Q to answer, as I can't, nor can anybody give you npk levels. As only you by seen your plant every day will know its speed thru varies stages, therefore there is no set NPK for a certain week my friend. Feel free to mes me and explain what you're using and so on and I can help some more, r m0use here will be happy to help. There really is so much to NPk when to add and so on. But, I've yet to come across a set thing, as when to add pk. Yes, it tells you when to add at stages. But, that stages NPK needs depend on its speed thru stages. Good luck