Abstract
Four proximal soil sensors were tested at four smallholder farms in Embu County, Kenya: a portable X-ray fluorescence sensor (PXRF), a mobile phone application for soil color determination by photography, a dual-depth electromagnetic induction (EMI) sensor, and a LED-based soil optical reflectance sensor. Measurements were made at 32–43 locations at each site. Topsoil samples were analyzed for plant-available nutrients (N, P, K, Mg, Ca, S, B, Mn, Zn, Cu, and Fe), pH, total nitrogen (TN) and total carbon (TC), soil texture, cation exchange capacity (CEC), and exchangeable aluminum (Al). Multivariate prediction models of each of the lab-analyzed soil properties were parameterized for 576 sensor-variable combinations. Prediction models for K, N, Ca and S, B, Zn, Mn, Fe, TC, Al, and CEC met the setup criteria for functional, robust, and accurate models. The PXRF sensor was the sensor most often included in successful models. We concluded that the combination of a PXRF and a portable soil reflectance sensor is a promising combination of handheld soil sensors for the development of in situ soil assessments as a field-based alternative or complement to laboratory measurements.
Highlights
Cation exchange capacity varied from 18.0 cmolc ·kg−1 at Embu University Campus (EUC) to 2.4 cmolc ·kg−1 at Irangi
The following elements measured with the portable X-ray fluorescence sensor (PXRF) were omitted from the predictive modelling because they were frequently below the detection limit: aluminum (Al), antimony (Sb), arsenic (As), barium (Ba), bismuth (Bi), cadmium (Cd), calcium (Ca), cesium (Cs), chlorine (Cl), cobalt (Co), gold (Au), hafnium (Hf), lead (Pd), magnesium, (Mg), mercury (Hg), niobium (Nb), phosphorous (P), rhenium (Re), scandium (Sc), selenium (Se), silica (Si), silver (Ag), sulfur (S), tallium, (Ta), thorium (Th), tin (Sn), uranium (U), and tungsten (W)
Our overall conclusion is that proximal sensors can be used for assessment of some, but not all, soil properties, and a well-designed validation is necessary before bringing any method out for use in practice: 1
Summary
Agriculture is the backbone and livelihood of a majority of the population in East Africa. Economic and efficient farm production is highly determined by soil conditions. Soil testing is an important procedure to undertake, since it determines the fertility status, i.e., the physical and chemical properties that affect a soil’s suitability for growing plants. Soil health determines how well the soil can function [1]. Different methods of soil assessment have been adopted over the years. An alternative or complement to laboratory soil analyses could be to test the soil directly in the field, e.g., by using handheld instruments. Knowledge of the local soil properties is crucial to farmers as it enables them to determine best management practices. The main bottlenecks for soil testing among smallholders in Sensors 2016, 16, 1950; doi:10.3390/s16111950 www.mdpi.com/journal/sensors
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