Evaluation of the Reliability of In Situ, Real-Time Probes for Precise Determination of Nitrogen, Phosphorus and Potassium in Agricultural Crops

Reliable tools for assessing soil nutrient availability are essential for the accurate determination of fertilizer application rates. The aim of this study was to evaluate the reliability of commercially available electrochemical sensors for the real-time determination of water-soluble N, P, and K in soil as indicators of nutrient supply to crops. The experiment was conducted under both greenhouse and field conditions using seven probe models. Four probes (designated 1–4) were equipped with three sensors for the measurement of N, P, and K. Probes 5, 6, and 7 were equipped with sensors for N, P, K, electrical conductivity (EC), pH, soil temperature, and soil moisture, although probe 7 was manufactured by a different company. Each probe was inserted into 4 L pots filled first with a clay-loam soil and subsequently with a sandy soil. The soils were irrigated with nutrient solutions containing different concentrations of N, P, and K in order to modify nutrient concentrations in the soil solution and compare sensor readings with values obtained through standard chemical analyses. The results showed no significant correlation between probe readings and soil N, P, or K concentrations in either clay-loam or sandy soils. At the same time, all probes exhibited a constant relationship among N, P, and K readings regardless of soil texture or nutrient concentration. This finding suggests that the probe algorithms rely on a single measured variable that is subsequently converted into multiple output variables using fixed conversion coefficients. In conclusion, the NPK probes tested in this study were unable to provide reliable real-time measurements of soil N, P, and K concentrations.