Proč se to děje? Nedostatek draslíku?

Yes, P-K deficiency. EC of the media is to low at this stage of the cycle. It should be somewhere around 2.4 Increase the amount of P-K and switch from Cal/Mag to Mag/Sulfur.

Burning from long term over feeding and a build up of nutrient salts in the substrate. It is not a deficiency. Deficiencies show through the entire plant, not just in random leaves.

​🌟 What You’re Doing Right ​First off, props to you for monitoring your runoff. Knowing your pH (6.3) and EC (1.0) is the absolute foundation of professional growing. You’re using an 11L pot with Light Mix, which is a smart choice for maintaining control, and your current substrate pH is sitting right in the "sweet spot" for nutrient uptake. ​🧐 The Diagnosis: Why the Brown Spots? ​While brown spots can often look like a Potassium (K) deficiency, given your history of high EC, we are likely looking at the aftermath of a nutrient lockout or a Calcium/Magnesium shift. ​Nutrient Lockout Recovery: When your EC was high previously, salts accumulated in the soil. This "locked" the roots, preventing them from absorbing specific elements. Brown, crispy spots on the upper leaves often appear when the plant is transpiring rapidly under lights but cannot move immobile elements like Calcium (Ca) to the top fast enough. ​Exhausted Light Mix: Biobizz Light Mix is excellent, but after 9 weeks, it is completely depleted. If your last flush brought the runoff EC down to 1.0, the soil is now very "clean," but potentially too lean for a plant that still has weeks of fattening up to do. ​Potassium vs. Calcium: Potassium deficiency usually manifests as "burnt" leaf edges (looking like a cigarette burn around the perimeter). These localized brown spots on the upper canopy are the classic signature of past pH fluctuations or a Calcium deficiency triggered by the previous salt buildup. ​🛠️ How to Fix and Manage It ​Stabilize and Feed: Since you’ve flushed the excess salts and your runoff EC is at 1.0, it’s time to reintroduce nutrients—but gently. Resume your flowering nutrients at a lower dose, and critically, add a CalMag supplement. In week 9, the plant’s demand for Calcium under intense lighting is at its peak. ​Check Light Distance: If the spots are strictly on the topmost leaves, check your light height. Heat stress accelerates nutrient evaporation in the leaves, making "calcium spots" appear much faster. ​Potassium Boost: If the leaf edges start to yellow or curl upward, slightly increase your Bio-Bloom or Top-Max, but stay at about 75% of the recommended dose until the plant stabilizes. ​🛡️ How to Avoid This in the Future ​The 20% Rule: Aim for about 10–20% runoff every time you water. This ensures that old salts are pushed out before they can accumulate and spike your EC. ​Enzymes are Key: Use an enzyme product (like Acti-Vera or similar). Enzymes break down dead root material and salt deposits, keeping the "rhizosphere" (root zone) healthy and breathable. ​Don't Starve the Light Mix: Remember that Light Mix is a medium, not a food source. You have to be consistent with your feeding schedule starting from week 3, as the soil provides very little buffer.