Development of an express method for measuring soil nitrate, phosphate, potassium, and pH for future in‐field application

Background In practical farming, there is often a need for short‐term availability of information on the soil nutrient status. Aims To develop a new express method for the extraction of major plant‐available nutrients and measurement of soil nutrients. In future, this method shall serve for in‐field measurements of soil samples with an ion‐sensitive field‐effect transistor (ISFET). Methods Various extraction conditions such as type of extractant, soil‐to‐solution ratio, time, and intensity were investigated on a broad selection of dried soil samples in the laboratory. Based on 83 field‐moist soil samples with varying clay contents, these conditions were compared to standard laboratory methods. Results With increasing extraction time, the nutrient concentrations increased. When the soil‐to‐solution ratio was reduced, a greater share of nutrients was extracted, independent of soil type. H2O and 0.01 M CaCl2 and standard calcium‐acetate‐lactate (CAL) solution proved to be too weak in the short period to reach the ISFET sensor measurement range. Higher concentrated CAL solutions performed much better. Finally, a 5‐min CaCl2 extraction followed by the removal of an aliquot for the determination of soil pH and NO3− was found to be effective. The remaining solution was then mixed with 0.20 M CAL solution for the analysis of H2PO4− and K+ at 10 min of extra extraction time. This extraction method showed very good correlations with the values based on the German laboratory reference methods for pH (R2 = 0.91) and for nitrate (R2 = 0.95). For phosphorus and potassium, we obtained an R2 of 0.70 and 0.81, respectively, for all soils. When soils were grouped according to clay content higher correlations were found. Conclusions A new express method based on a wet‐chemical approach with a soil preparation procedure was successfully developed and validated. This seems to be a valuable basis for future in‐field measurements via ISFET.