Rapid, quantitative and spatial field measurements of soil pH using an Ion Sensitive Field Effect Transistor

The objectives of this work are threefold. Firstly, to determine adequate times for rapid and accurate field-based measurements of soil pH (field pH). Secondly, to develop a simple protocol for quantitative spatial field measurements of soil pH using an Ion Sensitive Field Effect Transistor (ISFET) and thirdly, to demonstrate and validate its implementation. In order to determine adequate times, the kinetics of soil pH reactions in 1:5 soil/0.01 M CaCl2 (pHCa) and soil/H2O (pHw) suspensions were quantified for 17 representative agricultural soil samples from Brittany, France. The mean and variance of pH deviations at various times, from the long-term equilibrium pH as well as the accuracy of the measurements, were calculated for both pHCa and pHw. These data together with the required accuracy of the measurements may be used to determine an adequate time for field pH measurements. The mean and variance of the deviations decreased and the accuracy of measurements increased with longer reaction times. For example, the expected accuracy of 10 s pHCa and pHw measurements was 0.15 and 0.33 pH units, respectively. The expected accuracy of 20 s pHCa and pHw measurements was 0.11 and 0.15 units, respectively. Based on these data, a simple protocol for rapid field pH measurements was developed. This protocol was implemented by making 10 s pHw measurements in a 4-ha agricultural field near Rennes, France. A total of 476 pHw data were collected in 6 h. At 57 validation sites, measurements were obtained using conventional grid sampling and laboratory analysis (laboratory pHw). The observed accuracy of the 10 s field pHw measurements was 0.34 pH units. The efficiency of the field and laboratory pHw measurements was also compared. Field measurements were more efficient in terms of both the time and cost involved in obtaining the measurements. Semivariograms and kriged maps of both laboratory and field pHw measurements were compared, the latter appearing to more truthfully depict the spatial structure of soil pHw in the field. A cross-semivariogram of the laboratory and the 10 s field pHw measurements was also derived and co-kriging performed. Where time and/or economic restrictions prevent the use of sampling and laboratory analysis or the collection of more accurate field pH measurements, a co-kriging of a smaller set of these data as the primary variable along with a more exhaustive data set of more rapid but less accurate field pH measurements as the secondary variable may be practical and advantageous. Temporal measurements of field pHw at the 57 validation sites over 2 years showed good agreement.