Since the data on what's below the testing horizon is an unknown, I default to assuming that it's exactly the same as the slice that was tested. We know that's untrue, but it's the only assumption I can make.

What actually changes is anything expressed in pounds per acre. The average soil weight per acre differs by how deep you go, so the standard 4" results in just about 2 million pounds per acre for most soils (there's surprisingly little difference between sand and clay in this instance).

Consequently, a soil that's measured at 3" only weighs about 1.5 million pounds. Anything expressed in PPM results in a different weight. Anything expressed in PPA results in a different weight when adjusted to the different depth.

That influences how much stuff I add to correct things. So 100 pounds short on a 3" test would result in 200 pounds short on a 6" test (all else being equal, again, the only assumption we can really make).

Typically, grass' feeder roots live in the top four inches (there are exceptions, but that's a good average), and that's the only depth we really need to worry that much about. Structural and water-tapping roots go deeper, but don't absorb much in the way of resources and care far less about pH, as long as it isn't completely ridiculous (it almost never is).

Good stuff. It never dawned on my PPA or PPM were in reference to total weight from the root structure up. Makes sense though...

Thanks for taking the time to explain...