Athosanswered grow question 6 years ago You plant is 3 weeks old since breaking soil, in a 20 liter pot. I very much doubt it’s a deficiency, since I can go 7 to 8 weeks in 11 liter pots without deficiencies when not adding nutes. Potassium deficiency would show some chorosis (yellowing) on some part of the leaves, starting from the tip and rust or dark spots, your symptoms are not even similar. What kind of soil are you using? Those burnt tips might even be a sign that the soil is too hot or that your water salt concentration is too high. They even look similar to slight calcium overdose.
Before adding anything, get a EC / PPM pen and measure your tap water, also try to use a pH closer to 6.5; although it’s not a pH issue, you will see better results.
