Coco coir is a popular growing-medium for cannabis, as it offers many benefits such as high water retention, good aeration, and natural pest resistance. However, coco coir also has some drawbacks, such as its tendency to lock out iron at high pH levels. To treat it, you need to do the following steps: Check and adjust the pH of your water and nutrient solution. The optimal pH range for coco coir is 5.5 to 6.5. If your pH is too high, you can lower it by adding some acid, such as lemon juice or vinegar. If your pH is too low, you can raise it by adding some base, such as baking soda or lime. Flush your coco coir with plain water or a mild nutrient solution to remove any excess salts or contaminants that may interfere with iron uptake. Apply an iron supplement to your plants according to the manufacturer's instructions. You can use a foliar spray or a soil drench, depending on your preference and convenience. A foliar spray is faster and more direct, but it may cause leaf burn if applied under strong light or high temperature. A soil drench is slower but more thorough, but it may cause nutrient runoff if applied too much or too often. Monitor your plants for signs of improvement or worsening. You should see some improvement within a week or two after applying an iron supplement. If not, you may need to repeat the treatment or adjust the dosage. If you see any signs of worsening, such as leaf burn or nutrient toxicity, you may need to stop the treatment or flush your coco coir again.