Answer: I'm surprised they have survived, too! Magnolia is not something that is well suited to our climate. The stress of living here may mean they won't flower. When leaves brown around the edges, the problem is often salt burn. This is common in our area with low rainfall, alkaline soil and water high in salts. Browning usually occurs on the old leaves first. This excess salt accumulates in the leaf edges, where it kills the tissue and the leaf dries out and turns brown. It's important to water deeply, slowly and usually infrequently. With trees, make sure the water penetrates 2-3 feet deep. Use a pointed stick or piece of metal to push in the soil. It will move easily through moist soil and stop at dry soil. At least once a month, water deeply enough to "leach" or push salts well below the root zone. Frequent, light "sprinklings" or short time spans with drip irrigation, allow salts to accumulate in the top layers of soil, where the roots are, which is bad news.
Similar symptoms occur when too much fertilizer has been applied. It's generally not advisable to fertilize landscape plants in our summer heat. Plant roots often "burn."
As for the yellowing leaves, that could be lack of nitrogen (older leaves will yellow first), or lack of nitrogen (newer leaves will yellow, but the veins will remain green. Yellowing can also be caused by watering problems. I'd start by making sure the tree is watered properly and if problems continue, then apply nitogen or chelated iron, depending on the symptoms. Fertilize in late winter/early spring just before new growth starts. Fertilizer will sit in the cool soils during winter and the plant won't utilize it. If needed, fertilize again in late September. If the tree continues to struggle, you might consider replacing it with a desert-adapted tree, which will require little maintenance and no fertilizer! Hope this info helps!
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