Avocado trees (Persea americana), which grow in U.S. Department of Agriculture plant hardiness zones 9 through 11, don't handle salt in the soil well. When the salinity gets too high, the leaves start looking brown and burned around the edges, sometimes curling up slightly. This can keep the healthy fruit off your family's table, but catching the problem early can save the tree -- and your homemade guacamole.
Testing the Soil
Start off with the right soil before you plant your avocado tree. Test the soil in several areas of your yard to see which might be the best location; if one already has a high salt content, it might have a chronic problem with salt and should be avoided. Although you can use a salinity pen kit to test the soil at home, these are often complicated and expensive. Sending soil samples to a cooperative extension service, however, usually means you can get a salinity test for only a small charge. Note that the cooperative extensions in some states do not do testing, in which case you might have to send the sample to a private lab.
Signs of Salt Burn
When the soil has too much salt for your avocado tree, you'll see the tips of the leaves start to turn brown, followed by all the leaf edges -- they look almost like someone burned them around the rims. The scorched look also appears on leaf interiors, often as brown or yellow circles. If the soil isn't modified, the leaves continue to turn yellow and curl, eventually falling off. The yellowing and falling leaves mean the tree can't photosynthesize well, which can keep it from growing and producing fruit.
Problems From Salt
Leaf burn is one of the first signs of salt damage, but that's not all that's happening in your tree when the soil salinity is high. You can't see the roots, but what's happening to the leaves is also happening down there. The root tips begin to look brown and burned, and they stop growing. Stunted root growth means the tree won't be able to grow, and it could keep the proper amount of nutrients from reaching other areas of the tree. Avocados need well-draining soil, and too much salt can make the soil dense -- this can cause water to stand around the tree's roots, eventually killing it.
Managing Salt Levels
Salt takes a long time to naturally leach out of the soil, but you can help it along with deep waterings. When soil salinity is too high, water the area for 24 hours straight to help dilute the salt. In soils with chronic salt problems, you might need to leach it about once a month; leaching too often can waterlog the soil, which is also dangerous to avocados. If you don't think there's a chronic salt problem, wait for a soil test to say there's a problem before leaching. To help prevent too much salt, stick with low-salt fertilizers and avoid those known to have high salt contents, such as manure.