Photometric Detection of NPK Nutrient Transport in Polyacrylamide Hydrogel: A Model Medium for Soil‐Less Agriculture

ABSTRACT This study aims to develop a cost‐effective, digital, and portable device to measure the diffusivity of nitrogen (N), phosphorus (P), and potassium (K) nutrients within a polyacrylamide (PAAm) hydrogel platform for soil‐less cultivation. Nutrient concentration is determined via spectroscopic absorption principles to correlate optical absorbance with concentration within the polymeric network. The device employs a multi‐wavelength system consisting of three light‐emitting diodes (LEDs) and a Light‐Dependent Resistor (LDR) detector. Spatio‐temporal concentration profiles are used to estimate diffusivity accurately. The portable photometer enables the measurement of N, P, and K nutrient concentrations without any significant interference over the wide range of concentrations (0.25 to 10 mM). XPS and EDXS analyses confirm the hydrogel's integrity and nutrient distribution. It is demonstrated that N, P, and K nutrients spread over a circular area with a diameter of ~4 mm within the PAAm hydrogel in ~20 min at ambient conditions. The estimated diffusivity for a model nutrient, KCl (10 mM), aligns with a model combining free volume and obstruction theories. This portable system offers a method for optimizing fertilizer application, potentially enhancing crop quality and reducing environmental impact.