Predicting Soil pH and Buffer pH In Situ with a Real-Time Sensor

Received for publication September 6, 2006. Measuring soil pH is an important step in assessing the chemical status of a soil and in remediating high and low soil pH situations. Soil pH measurements on a spatial scale are limited by labor and lab analysis costs. A pH sensor mounted on a mobile sensing platform may be able to reduce these costs while providing a high-resolution soil pH map. The objective of this research was to evaluate a pH sensor mounted on a mobile platform on two fields in Kansas, one with a uniform soil and one with six different soil complexes. Real-time measurements were taken at a density of 17 points ha-1 and compared with soil samples collected at depths of 0 to 7.5 and 7.5 to 15 cm. The real-time sensors predicted soil pH most accurately at the 0- to 7.5-cm depth (R2 from 0.75 to 0.83) and less accurately for the other depth and combined depth (R2 from 0.53 to 0.79). The inclusion of soil electrical conductivity (EC) improved pH predictions in the field with six different soil types, but not the uniform field. Buffer pH predictions were less accurate than pH predictions (R2 from 0.04 to 0.43) across locations and depths. However, if a relationship between lab pH to buffer pH was developed and used to predict buffer pH from real-time pH, the accuracy improved (R2 from 0.75 to 0.95), suggesting that real-time pH measurements may be capable of predicting buffer pH and lime requirements in real time. These results suggest that real-time pH sensors on a mobile platform can be used to measure spatial pH and buffer pH and provide subsequent variable-rate lime recommendations.