I have two 15 foot tall / 60 inch box fruitless wilson olive trees planted in my yard one year ago. Every few months all the leave's tips on the two trees start to brown. Then the leave starts to yellow and fall off. After new leaves grow in. I water each tree a few times a week for a total of about 50 gallons of water a week per tree. I use a moisture meter and make sure the moisture level never drops below 3 out of 10 on the moisture meter. I have driven a pole down 3 feet and did not find any soggy soil down deep. Not sure what is going on? Thanks in advance for the help.