Vis-NIR spectroscopy for multivariate classification of soil samples
throughout Denmark into Danish Soil Classification System
Abstract
Different spectroscopy methods such as visible near-infrared (Vis-NIR)
spectroscopy have proven to provide useful information on soil physical
and chemical properties. The majority of previous studies have focused
on multivariate regression methods such as partial least squares
regression (PLSR) to predict soil characteristic features from soil
spectra. The objective of these efforts was to provide precision data
for agricultural and land management practices where the knowledge about
the soil differences in each part of the field can be used for
variable-rate irrigation, seeding, liming, fertilising and pesticide
application. Since the currently available machines can only apply few
(less than 10) variable classes of seed, fertiliser, etc., using soil
classification seems to be a more appropriate option compared to
regression. As a part of Danish National Soil Survey in 1980s, 2460 soil
samples were collected from 789 soil profiles in 4 depths (0-30, 30-60,
60-100 and 100-200cm) throughout Denmark (some profiles did not have
samples from all 4 depths) and tested for several soil characteristics
including complete soil texture, organic carbon content and calcium
carbonate in the lab. Based on these soil characteristics, all soil
samples were classified into 8 soil types or 12 soil classes in the
Danish Soil Classification System (JB system). Later the Vis-NIR spectra
of samples were measured using a FOSS DS2500 spectrometer in the range
of 350-2500nm. Partial least squares and support vector machines
discriminant analyses (PLS-DA and SVM-DA, respectively) were used to
calibrate and cross-validate classification models where soil Vis-NIR
spectra were used to classify each sample from each depths into its
corresponding JB soil type and soil class. The results show excellent
classification accuracy and specificity (>80% and
>90%, respectively) for samples from the same depths and
when samples from all depths were combined. We found that the high false
negative rate (low sensitivity) was mainly due to the models classifying
samples in the neighbouring classes of the actual class (e.g.
classifying a sample in JBC 2 as belonging to JBC 1 or JBC 3). This
clearly indicates that calibrating the classification model on the
uncertain hydrometer data (with 1.4-2% reproducibility error) was the
main reason for classification of samples in the neighbourhood of the
actual class. In conclusion, given the highly uncertain reference
methods for soil classification, using Vis-NIR spectroscopy with PLS-DA
provides a very rapid, inexpensive, and highly reliable method for soil
texture classification on a national scale.