Exploring a low-cost turbidimetric sensor for available potassium determination in soil

Potassium (K) is among the macronutrients required for crop production and it is absorbed through plant roots from the soil solution. Replenishment of soil K is usually done by fertilizer application; therefore, it is crucial to know the amount of this nutrient that's available in the soil. There's very little literature reporting a turbidimetric method for K determination, and even less in soil samples. The objective of this work is to evaluate a portable low-cost system that allows turbidimetric determination to assess the amount of available K in soil. The turbidimetric method consists of K precipitation with sodium tetraphenylborate (NaTPB). The time and Mehlich-1, Mehlich-1 diluted 1 : 1 and water solutions for the extraction procedure were evaluated. The effect of pH, along with NaTPB volume and sensor signal stability, was also evaluated. With the optimized conditions, calibration of the sensor showed a good linearity (r² = 0.9982), and the limits of detection and quantification were 0.1 and 0.3 mg L^-1 of K, respectively. No significant difference (t test with 95% confidence level) is observed between the results obtained with the sensor and the MP-AES and ICP OES determinations using Mehlich-1 solution for K extraction. The results presented here demonstrate that it is possible to use simple equipment to measure available K in soil on the field by using low amounts of reagents, which could make this analysis more accessible.