I have planted my avocados in soil a month ago and since then they started getting brown edges on the leaf. What should I do?
I agree, high salts most likely. You've not had enough time to build up the salts from tap water to include the worst; fluoride. Hydrofluorosalicic acid. Heavy metals. Chlorine actually dissipates. So the culprit seems to be whatever was added to your soil such as compost, fertilizer, sand/rocks/gravel at the bottom of the soil which would cause the soil to stay too wet too long.
Your leaves look thin as if they've not been in much light for a good month or something is wrong with the chemistry (fertilizer) or the roots are rotting from too much water not enough air and drainage. Planting avocado seeds with water roots into soil is a huge shock to a plant. Water roots are not meant for soil. But seems to work more often than not...did you start your avocado in water and then recently plant in soil?
I am worried that you've got too much fertilizer in your soil. What have you used for soil, amendments, fertilizer and when do you know to water your plant? Is there gravel or rocks at the bottom of the soil in the bottom of the pot? Is there a drain hole? Check out the bag of soil you used to see if fertilizer was added to the soil.
Please get back to us with even more details...and soon, okay?