Cheap NPK sensors need a lab

Most low-cost probes estimate nutrients from conductivity. In acidic West African soils, that guess goes wrong.

Many affordable RS485 “NPK” probes do not measure nitrogen, phosphorus or potassium directly. They read electrical conductivity and moisture, then apply factory multipliers.

Those multipliers ignore the weathered, acidic soils common in humid West Africa, where aluminium locks up phosphorus. Uncalibrated, the readings can push a farmer toward the wrong fertiliser.