Can it be a K lockout or a Mg deficiency?

ur underfeeding she's very hungry so u should feed it with bloom fert around 1-1.2k ppm for this week and adjast it to the next

Nh4+ . It's lowering pH as more and more nitrogen is still being released even if the plant doesn't need it. The ratio of nh4+ increases. Skewing ph with it. Normally organic nitrogen would convert to nitrate No3- But conversion is and rate of is linked to ph, so as ph keeps becoming more and more acidic, less and less nh4+ is converted to no3-. This will just keep accumulating in the soil. Organic nitrogen can take weeks to months to decompose. So thay organic nitrogen you added 2 weeks into flower is just getting started. Nh4+ sticks to soil like glue. 4x5x more water to release from soil through leeching. No3- readily leechable. Normally you would jist flush away all the nitrates leave behind all the ammoniacal. Soon as ph goes back towards 7, the rate of nitrification will x5 from 6.0 If you didn't wash out all the sticky nh4+.

It's not so much about "oh its K"...... it's obviously K. It's more about how did K become deficent. If there is one thing I've learned is that it's ever rarely deficiency alone. More often than not it's a combination of running low along with more difficulty with uptake either because of stress or pH fluctuation. Before you do anything I'd advice getting a solid idea of what your ph is. One glance at your temperature metrics and I'm not surprised 72F daytime and 62F nights. I'm surprised your plant is even photosynthesizing at that temperature, almost 0 cellular respiration is happening overnight at that temp. Low root zone temperatures below 68°F will significantly reduce potassium uptake and overall nutrient absorption. Just the fact your also using organic nitrogen makes it more difficult too, Both ammonium and potassium ions are positively charged (cations) and compete for the same transporters on plant roots and cell membranes. When ammonium is readily available, it can hinder the uptake of potassium, leading to a deficiency even if potassium is present in the soil.

Severe potassium deficiency...........7.0 pH won't "lock out" potassium. True lock out only occurs below 5.5 or above 7.6. Remember too, what is showing in the leaves now, is a direct reflection of what was happening in the root zone 2-3 weeks ago. Get some proper fertilizers that are specifically designed for cannabis.....using "middle flowering" does not seem to be adequate and I have no idea what "EM" is ...........but that doesn't seem to be cutting it either. Wood ash is a poor choice for supplementing K as it is usually very alkaline and will cause major pH issues if used too heavily in soil. Organically, kelp is the best source of potassium and works quickly. Plants don't "focus energy" anywhere or only on healthy leaves, all energy gets used equally wherever active growth is occurring. Plants will "recycle" elements from damaged leaves to use elsewhere, removing leaves pre-maturely just because thet might look ugly deprives the plant of this source of pre-made goodness.........only remove leaves once they have gone completely yellow. Always remove dead leaves/prunings from your grow space, leaving them lying around is a great way to encourage diseases and pests.

7.0 won't lock out K. More likely a deficiency. Yes, it looks like a k-deficiency. Might have a N-deficiency, also. devil's advocate -- is that the tallest cola by at least a few inchces? might be getting too much light... if it isn't taller, that's not the case. leaf pattern (veritical spacing) looks a bit tight at top. Too much light could cause a leaf to cannibalize itself beyond rate of upkeep.

The leaf in your photo shows classic signs of a potassium (K) deficiency. Why: The leaf is yellowing from the edges inward while the veins remain lighter. You can see brown, necrotic spots along the margins and tips, which is very typical for potassium deficiency. It usually starts on older, lower leaves, exactly like in your picture. Possible causes: Insufficient potassium in your nutrient mix. Nutrient lockout due to pH imbalance (check that your pH is in the right range: soil ~6.2–6.5, hydro/coco ~5.8–6.2). High levels of calcium, magnesium, or sodium can also block potassium uptake. What to do: Check pH of your feed and runoff to avoid lockout. Ensure your fertilizer has enough K (especially important during flowering). If using organic nutrients, add a supplement like wood ash, kelp extract, or PK booster. Remove heavily damaged leaves, they won’t recover but the plant will focus energy on healthy growth. If untreated, potassium deficiency can lead to weaker buds, reduced aroma, and smaller yields.