A paper-based device for on-farm colorimetric detection of soil macronutrients

Testing the quality of soil is essential for appropriate fertilizer use. However, soil testing is currently conducted in dedicated centralized facilities equipped with expensive analytical instruments, not easily accessible to many farmers in lower-income agrarian economies like India. In an attempt to address this gap, we present a prototype for the colorimetric detection and quantification of soil macronutrients – nitrogen (N) and phosphorous (P). For this, we employ a technology previously developed in our laboratory called the barrier-free microfluidic paper analytical device. BF-PADs enable the simultaneous colorimetric detection of multiple analytes without having to pattern paper with hydrophobic barriers for the creation of flow channels. We demonstrate multiplex colorimetric detection of ammonium and nitrate ions for the detection of N, and phosphate ions for the detection of P. The linear quantifiable ranges for ammonium, nitrate, and phosphate ions dissolved in solution were 20-100 mg/L, 20-90 mg/L, and 4-80 mg/L, respectively, with limits of detection of 18 mg/L, 12 mg/L, and 3 mg/L, respectively, which are within the relevant ranges for soil testing. These single-use low-cost paper-based devices promise to increase access to soil testing and reduce dependency on laboratory-based soil testing.