I have four two year old trees in my yard, and about the first of July they started dropping the bottom leaves. Just turn brown, and fall off.
I know that is a general description, but can anyone give me a guess,
or a range of guesses which might be the causative agent.??
thanks
Bill Kimbrough

If the soil is moist, then it's a natural occurence in our area, Bill.

Bill, got enough water? is the soil allowing the water to penetrate to the root zone? What symptoms do the leaves show before the get brown and drop off? Most times when I see something like this, it takes place over days or a week. This is a situation where being a good CSI and observer makes finding a solution much easier.

Sounds like it's on the dry side to me too.  If a tree gets too dry to the point that it wilts severely the damage is usually already done. Then after it's watered and the leaves perk back up they seem ok but later which could take anywhere from a few days to a couple weeks the damaged leaves start to fall.

You don't mention the most critical detail:  potted or in-ground?  Left to assume "in my yard" means actually IN the ground in the yard, but can you clarify?

Mine do this if they get excessively dry.  Whether too dry or too wet, sounds like a watering issue to me.  Important that you're not just watering the top inch or two.

Thanks for the replys.
I suspect that the problem may be too much water. Seems like that just below that area of the yard became too wet for a while due to the sprinkler system,
and then a period of lots of rain.
I have adjusted the sprinkler system to less moisture, and we have had a
lot less rain, and the dropping has not gotten worse. Still, I was hoping that
you experts would have answers, and all of your advice seems to be related
to either too much or too little moisture.
Thanks
Bill

 Growing in containers though and probably same in ground that yellowing of leaves starting on the canopy is not enough water and yellowing starting from bottom of plant is too much water .
This has been my experience in the past.