You’ve seeded your plants and they’ve grown past the first couple of nodes and so far they look fine. All of the sudden the youngest leaves turn yellow and it slowly spreads from there. While the root of the problem may be hard to fix, you can easily reverse the effects of chlorosis.
Chlorosis, a lack of the green pigment chlorophyll, generally happens due to iron deficiencies. The veins of the leaves will keep their green, but the thinnest part turn yellow. Above pH = 7.2 iron becomes insoluble, and iron deficiencies can even begin above 6.5. Over fertilization, especially of phosphorus, calcium, zinc and copper, can also cause chlorosis by competing with iron absorption.
Chlorosis due to iron deficiency can be easily confused with manganese deficiency, because they both show the same symptoms: yellowing leaves. Only the observant farmer can tell the two apart because it has to do with timing. If it’s iron deficiency the yellowing starts with the newest, youngest leaves. Since iron doesn’t mobilize around the plant, older leaves will take longer to loose their chlorophyll and turn yellow. While manganese deficiency also causes chlorosis (loss of chlorophyll), it starts to happen on the innermost and older leaves first, and then progresses outward.
For soil growers, an iron deficiency is hard to fix at the source; your soil is already either overloaded with nutrients or has a high pH. Try a different potting soil or mixture next time, and if you’re outdoors, you may need to treat your soil so lower the pH and clay content. For the meanwhile, the precious chlorophyll needs iron to keep your cannabis plants alive.
To supplement iron use a foliar feed with chelated iron. Chelated iron can come in many forms, and most are decent for foliar feeds. Foliar feeding provides iron right to where it needs to be, and it can turn your plants around in a day. Apply the spray at night or in the early morning, definitely not during the day. To stave off the iron deficiency in unhealthy soil use a dilute spray every three days or more during the vegetative cycle, and sparingly during flowering especially at the end. Manganese also comes in chelates for foliar sprays. Foliar feeds in general are great for boosting growth because they provide otherwise immobile nutrients directly to the leaves. Foliar feeds with fermented seaweed provide a great source of calcium, and the enzymes of fermentation also aid growth.
We don’t recommend adding iron chelated with EDTA directly to the soil. EDTA is such a powerful chelating agent it can pick up heavy metals that were otherwise immobile. You can add chelated iron in other forms to soil but the problem doesn’t likely stem from a lack of iron, but an imbalance that prevents roots from absorbing it. Adding chelates can help, but instead of investing in the large amount of iron chelate for soil, you’ll want to either lower pH or fix the nutrient imbalance to properly tackle the problem.
One of the most important aspects of flowering is flushing. You need to flush out nutrients from the root zone, and this normally leads to your leaves turning yellow around the buds. Feel free to trim these leaves, but leave a few green ones. Use foliar feeds to fix chlorosis during the vegetative stage, but yellowing during the later stages of flowering is perfectly normal, happy growing!