35+ Avocado Tree Leaves Brown. Avocado trees need iron, zinc, magnesium, phosphorus, potassium. Avocado trees, even ones in the ground, have trouble drawing up enough water and browning at the end of the leaves is very common.
Brown leaves are dead leaves, so they aren't contributing anything to the tree, and it will drop them on its own soon if you don't.
Sometimes avocado leaves turn brown because of the accumulation of chloride and sodium salts. If there are a great many (you say they're mostly brown), then they're indicative of a problem. I grew an avocado tree from seed this summer and now the edges of the leaves are turning brown. There is one primary culprit of the typical leaf burn that develops in the fall, and that's chloride in our irrigation water.