Digital transformation accelerator. Technical report of the probe: Low-cost portable soil fertility sensors: Myth or reality?

This probe evaluated the performance and accuracy of low-cost portable sensors for measuring Nitrogen (N), Phosphorus (P), and Potassium (K) in soil as a rapid alternative to conventional laboratory analyses. The research comprised two stages: an initial evaluation in Guatemala and a subsequent, more comprehensive validation in Colombia.
In Guatemala, three commercially available NPK sensor models were tested using ten soil samples collected from different regions. Sensor readings were compared with laboratory reference analyses through linear regression models, which yielded low determination coefficients (R² ranging from 6×10⁻⁵ to 0.2458) and revealed a strong dependence of the results on soil moisture content. These findings emphasized the importance of controlling soil moisture and developing calibration procedures prior to field applications.
Building on these results, the Colombia experiment assessed the same sensors using 63 soil samples from various regions, standardized at 20% gravimetric moisture. Polynomial regression models (linear, quadratic, and cubic) were applied to evaluate accuracy using R², adjusted R² (R²_adj), the coefficient of variation (CV), and RMSE as performance metrics. Results indicated limited predictive capacity, particularly for Phosphorus and Nitrogen (the latter compared against Organic Matter as a proxy), with adjusted R² values generally ranging between 0.052 and 0.479. Potassium (K) showed moderate performance, reaching a maximum adjusted R² of 0.539. A complementary experiment examining moisture effects (0–40%) confirmed that NPK and EC readings are highly dependent on soil water content.
Overall, both experiments demonstrated that although these sensors can detect general nutrient trends, their use for precision applications such as fertilizer management is not recommended due to low accuracy and strong sensitivity to soil moisture.