Reflectance Multispectral Imaging for Soil Composition Estimation and USDA Texture Classification

Abstract

Soil texture is a foundational attribute that governs water availability and erosion in agriculture, as well as load bearing capacity, deformation response, and shrink-swell risk in geotechnical engineering. Yet texture is still typically determined by slow and labour intensive laboratory particle size tests, while many sensing alternatives are either costly or too coarse to support routine field scale deployment. This paper proposes a robust and field deployable multispectral imaging (MSI) system and machine learning framework for predicting soil composition and the United States Department of Agriculture (USDA) texture classes. The proposed system uses a cost effective in-house MSI device operating from 365 nm to 940 nm to capture thirteen spectral bands, which effectively capture the spectral properties of soil texture. Regression models use the captured spectral properties to estimate clay, silt, and sand percentages, while a direct classifier predicts one of the twelve USDA textural classes. Indirect classification is obtained by mapping the regressed compositions to texture classes via the USDA soil texture triangle. The framework is evaluated on mixture data by mixing clay, silt, and sand in varying proportions, using the USDA classification triangle as a basis. Experimental results show that the proposed approach achieves a coefficient of determination R^2 up to 0.99 for composition prediction and over 99% accuracy for texture classification. These findings indicate that MSI combined with data-driven modeling can provide accurate, non-destructive, and field deployable soil texture characterization suitable for geotechnical screening and precision agriculture.

Figures (15)

• Figure 1: Schematic of the multispectral imaging system.
• Figure 2: Map of Sri Lanka showing the locations of soil collection sites: clay rich soil from Menikhinna, silt rich soil from Gelioya, and sand rich soil from Chavakachcheri. Coordinates are provided in WGS84.
• Figure 3: USDA soil texture triangle showing the distribution of samples used in this study. Green dots indicate the three source soils (clay, silt, and sand rich endmembers), blue dots indicate mixtures used for model development (training/testing), and red dots indicate mixtures used for external validation.
• Figure 4: Representative monochrome spectral band images of a soil sample acquired using the proposed MSI system under thirteen narrow band LED illuminations (one image per wavelength).
• Figure 5: Workflow of the proposed MSI based pipeline. After preprocessing and LDA based dimensionality reduction, three studies are evaluated: (Strategy 1) direct USDA texture class classification, (Strategy 2) regression based clay, silt, and sand prediction, and (Strategy 3) indirect USDA texture classification via texture triangle mapping, using the designated test/validation splits.