Normal beneficial soil bacteria which promote necessary soil enrichment are unable to survive in very acid or very alkaline soil. Most soils drift to the acid condition and need regular evaluation so that they can be controlled and productivity increased. A direct reading tester goes with you. It is portable, rugged, convenient, operates without a power source and is not overly sensitive to temperature variances. You can't carry a laboratory with you, and you can't rely on an inaccurate substitute.
Many thousands are in everyday professional use.