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On-the-go Measurements of pH in Tropical Soil
M. T. Eitelwein, R. G. Trevisan, A. F. Colaço, M. R. Vargas, J. P. Molin
University of Sao Paulo

The objective of this study was to assess the performance of a mobile sensor platform with ion-selective antimony electrodes (ISE) to determine pH on-the-go in a Brazilian tropical soil. The field experiments were carried out in a Cambisol in Piracicaba-SP, Brazil. To create pH variability, increasing doses (0, 1, 3, 5, 7 and 9 Mg ha-1) of lime were added on the experimental plots (25 x 10 m) one year before the data acquisitions. To estimate soil pH levels we used a Mobile Sensor Platform (MSP - Veris Technologies, Salina, USA) with two ISE. In each plot, six on-the-go acquisitions were performed. Soil samples were collected at each of this acquisition points and sent to a laboratory for soil pH determination using two methods: H2O solution and CaCl2 0,01 M solution. The analysis of pH data showed that treatments were effective to create variability in soil pH, with a minimum pH of 5.6 and maximum of 7.4 determined in H2O solution. The average pH was higher in ISE determinations (6.87) and decreased using H2O (6.53) and CaCl2 (5.92) methods. The correlation between the readings taken on-the-go and in the laboratory were 0.90 and 0.91 using H2O and CaCl2 methods, respectively. These results revealed a great potential of using a MSP to estimate soil pH in Brazilian soils. Despite the good correlation coefficients, it is necessary to account for the difference between on-the-go and lab results before using the data to recommend variable rate lime applications.

Keyword: ion-selective electrode, high resolution soil mapping, proximal soil sensing