Apparently you have an acidic matrix being generated in your soil. This could be due to overfeeding. Here is some advice from Royal Queen Seeds: [h=3]"FLUSH YOUR PLANTS[/h]
Many problems with sick cannabis plants can be due to
overfeeding. When your plant can’t take up the nutrients that you provide, salts and minerals will accumulate in the soil over time. This will change the pH level at your plant's root zone, making it more acidic—beyond the small pH window that cannabis has for healthy growth. As a result, your plant is not able to take in nutrients, even if they are present in abundance. When this happens, further feeding only makes it worse.
In almost all cases where your plants show signs of nutrient deficiencies or nutrient burn, you should give your plants a solid flush. Flushing means that you rinse out the excessive salts with pure, pH-balanced water to restore the optimal pH of the growing medium.
To flush your plants, drench the growing medium with water numerous times. It should be ample enough that liquid comes out from the bottom of the container each time. For example, if you grow in 7l pots, flush your plants with 14l of water. When you grow in soil, your water should have a pH of about 6.5pH. After the flush, you can begin giving nutrients again, starting with ½ or ¾-strength doses. You can slowly work your way up from here to avoid putting plants under any additional stress."
