Low cost soil sensor based on impedance spectroscopy for in-situ measurement

Abstract

Soil moisture is a significant element in the water cycle, on an agricultural and in land interaction. In crop production, soil plays a key role as a physical support and a reservoir of water and nutrients. Decisions for optimized input rates of water are largely based on physical, chemical, and biological properties of soils. The aim of this work is to develop a low cost soil moisture sensor (SMS) based on impedance spectroscopy by means of magnitude ratio and phase difference detection method. Using impedance spectroscopy, more information can be delivered from real and imaginary part of the complex permittivity for several frequencies at the same moisture value. For this purpose, a probe has been designed which uses a simplified impedance measuring system to determine soil water content. The circuit sweeps at pre-programmed frequencies from 10 KHz to 10MHz with 10 mV AC amplitude. A local inceptisol soil of East Sumatra was especially selected for this investigation because measurements of soil moisture in peat swamp area were generally reported as challenging to analyze. Samples at defined soil moisture of 2%, 8%, 15% measured using commercial soil sensor Lutron PMS-714, was characterized. A model has been developed in order to correct the frequency influence upon the measurement. The results obtained by the sensor show good results with an overall mean error of 0.21% in impedance.