Not positive on the cause of your leaf burn but these are some possibles.
Light Burn: Even though your light is set at 220 PPF it might be too intense for young plants or too close to them. Young cannabis plants are more sensitive to intense light.
Nutrient Burn: This can happen even with organic soils if the nutrient concentration is too high for the young plants. Nutrient burn typically shows up as yellow or brown tips on the leaves.
Watering Issues: Overwatering or underwatering can both stress plants, leading to burnt-looking leaves. Ensure that the soil is moist but not waterlogged. It's also important to let the soil dry out slightly between waterings.
pH Imbalance: Cannabis plants prefer a soil pH between 6.0 and 7.0. An imbalance can hinder their ability to absorb nutrients properly, leading to nutrient deficiencies or toxicities.
Pests or Diseases: Check for signs of pests or diseases, as these can also cause leaf burn.
To address these issues, consider the following steps:
Adjust the light height or intensity.
Test the soil for nutrient levels and pH, and adjust if necessary.
Ensure a consistent watering schedule, allowing the soil to dry out slightly between waterings.
Inspect plants regularly for signs of pests or diseases.
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