In-field soil spectroscopy in Vis–NIR range for fast and reliable soil analysis: A review

In-field soil spectroscopy represents a promising opportunity for fast soil analysis,
allowing the prediction of several soil properties from one spectral reading representing
one soil sample. This facilitates data acquisition from large amounts of
samples through its rapidity and the absence of required chemical processing.
This is of particular interest in agriculture, where the chance to retrieve information
from soils directly in the field is very appealing. This review is focused on infield
visible to near infrared (Vis–NIR) spectroscopy (350–2500 nm), aimed at analysing
soils directly in the field through proximal sensing. The main scope was to
explore the available knowledge to identify existing gaps limiting the reliability
and robustness of in-field measurement, to foster future research and help transition
towards the practical application of this technology. For this purpose, a literature
review was performed, and surveyed information encompassed sensor range,
carrier platforms in use, sensor type, distance to the soil sample, measurement
methodology, measured soil properties and soil management, among many
others. From this, we derived a list of tools in use with their spectral measurement
properties, including the potential cross-calibration with soil spectral libraries
from laboratory spectroscopy of soil samples and potential measured target soil
properties. Different instruments and sensors used to measure at varying wavelength
ranges and with different spectral qualities are available for a large range
of prices. The most frequently analysed soil properties included soil carbon contents
(soil organic carbon, soil organic matter, total carbon), texture (clay, silt,
sand), total nitrogen, pH and cation exchange capacity. Future perspectives comprise
the implementation of larger databases, including different instruments and
cropping systems as well as methodologies combining existing knowledge regarding
laboratory spectroscopy with in-field methods. The authors highlight the need
for a broadly accepted measurement protocol for in-field soil spectroscopy, fostering
harmonization and standardization and consequently a more robust application
in practice.