And yet, in my experience, over 90% of the time the first reading tells you the story of the soil in that level over a very wide area, unless some new soil has been brought in or if there are varying levels of drainage.
Where I consistently get different readings is from the top few inches and a foot down here. It tends to be more acid in the humid regions as you get lower. During drought, the availability of adequate calcium can depend on the lower soil, but it is difficult getting lime down there after planting. Commercial guidelines therefore recommend separate readings for upper and lower depths before planting to allow tilling in lime.
If you are growing fruit without irrigation this can be important, according to the literature. Probably more for the commercial grower than others. For home growers, a lower pH lower in the soil can be helpful when growing blueberries. I’ve found that blueberries can be very healthy in soil where the upper 9" is close to neutral but lower soil is as high as 5.8.
I discovered this quite by accident while trying to prove that blueberries can flourish in a pH in the mid 6.s. I actually proved myself probably wrong, but learned something about the ability of roots to get what they need from a limited soil profile. Blueberries can also thrive with a thin layer of acid soil on the surface, as long as it doesn’t dry out during the growing season.
Somehow my son found out that Michael Phillips gave me credit for this observation in one of his recent books. Phillips never told me but read my comments in a NAFEX article. Of course, I’m probably the only one who would find that interesting.