A Combined Soil-Moisture and Light Probe for Field Agriculture

A design is presented for a push-in field probe that measures volumetric water content by capacitive sensing of soil dielectric permittivity at 70 MHz (Topp equation as the adopted calibration), soil temperature, and photosynthetically active radiation (PAR, 400-700 nm) at an above-ground head. The calculated performance budget, produced by the accompanying computation script, gives a volumetric water content resolution of 0.0023 m3/m3 at an apparent permittivity of 15, a combined mid-range accuracy of 0.0339 m3/m3 dominated by soil-specific calibration, a PAR resolution of 0.12 umol m-2 s-1 with a daily-integrated cosine error of -3.51%, and a season energy use of 0.615 Wh at a 900 s sampling interval over 180 days. SIMULATED, NOT MEASURED: no hardware was built or tested.