Answer: Yellow leaves can be caused by many things including lack of nitrogen, insufficient light, water-logged soil (plant roots need oxygen to thrive), dry soil, or iron deficiency. If the older bottom leaves are yellow, but new growth is green, it's usually a lack of nitrogen. If new leaves are yellow, with green veins, it's usually a lack of iron. Soil should be kept moderately moist (but not wet). Finally, transplant shock can contribute to yellowing. If new growth shows up as green, that might be the problem. Try to isolate each of these possibilities one at a time to determine the problem.
I'd start by examining your watering practices. Watering every day can be a problem. It's best to water deeply, and then as infrequently, as possible. Don't water just because plants wilt in the afternoon heat. Transpiration (water loss from the leaves) can be more rapid than the roots can uptake water to replace it during hot weather. If the plants are wilted in the afternoon, wait until morning to water. They may recover. If still wilted in the morning, they need water. Also, I wouldn't fertilize in the midst of summer. It's generally too stressful for the plant.
Iron chlorosis is a problem in the desert. Overwatering, combined with heavy summer rains, can promote root rot, fungal diseases and iron chlorosis. Chlorosis is recognized by new leaves that are yellow, while the veins remain green. If the condition is severe, the entire leaf may be yellow. Queen palms, pyracantha, bottle brush and silk oak trees often suffer from chlorosis.
Although iron may be present in the soil, it is not always in a form that plants can use. Overly wet soils are depleted of oxygen. (As water fills in the minute spaces between soil particles, air moves out.) Plant roots need oxygen to absorb iron in the soil. To help prevent chlorosis, always water slowly, deeply and infrequently.
Soil with a high pH (alkalinity) also inhibits iron absorption. If you are using correct irrigation methods and symptoms are still present, apply iron chelates or ferrous sulphate to the soil. Both are readily absorbed by a plant?s roots. I hope this information helps!
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