I prefer to put some soil in a glass, pour enough distilled water to cover the soil, stir, let sit a few minutes, then stick in the probe.
My sandy yard was about 6.8 pH. After sheet composting with fresh prunings and leaves brought from my job, and after adding one ton of phosphatic clay spead over my 85' x 110' lot, it went down to 6.4 pH.
Phosphatic clay is also called colloidal phosphate or soft phosphate or soft rock phosphate. It settles out in the wash ponds of rock phosphate mines. The main ingredient is Tricalcium phosphate. As there is so much calcium in it I had supposed that it might raise my soil's pH too much--- not so--- the pH went down.
Both organic matter and phosphatic clay, when added to sandy soils, help to suppress harmful nematodes.