Yesterday afternoon I dug a planting hole per the instructions given to me at the nursery. The hole is about two feet deep (for starters). To test the drainage, I filled the hole about half way with water. This morning the water is pretty much in exactly the same spot! In other words, it hasn't soaked in or absorbed at all! I'm thinking this is indicative of poor drainage. My question, how much deeper do I need to dig? If there is a caliche or layer of clay, how deep might I need to go to bust through it? Can mesquite roots bust through it? Some of my neighbors have really tall trees (originally planted by the home builder) and I'm assuming their backyards have the same drainage issues that mine does. Again, Thanks for all your help.

It sounds like you're dealing with very compacted soil or a restrictive layer of some kind. The most common remedy is to dig one or two drainage 'chimneys' in the hole with the hope that it will break through the compaction and allow water to drain away from your trees roots.

The chimneys only need to be 3 or 4 inches in diameter. Don't place them under the rootball but rather off to one side.

Here's a link to a publication by the University of Arizona with a diagram.

My personal thinking is you could dig a 10' wide hole and amend the soil but you'd still have a hole that didn't drain, nothing would really be solved. I'm a "it is what it is" kind of person. A mesquite has the best chance of finding its own way, they do this for a living. So I never add anything for natives, and a mesquite, hybrid or not, is close enough for me.

If it helps at all I grew thornless mesquite from seed this summer in a bucket with no drainage.

Good luck.