Yellow and brown tips on palm tree fronds can be an indication of a deficiency of
magnesium and manganese in the soil.
Believe it or not, you can go a long way toward fixing the problem by spreading Epson Salt around the trees. Epson Salt is not really a salt, but rather a mineral compound of
magnesium and sulfate.
You can buy
manganese at various hardware stores.
Applying these minerals will not change the appearance of the fronds that have already turned brown, but should prevent future problems with new growth.
How to Apply Epsom Salts to Palm Trees | Home Guides | SF Gate