Spectroscopy Could Simplify Soil Texture Analysis

In this study, researchers prepared soil samples for placement into a laboratory spectrometer for visible near-infrared spectroscopy measurement. The measurement takes approximately one minute. Portable vis-NIRS units potentially could collect soil-texture data in the field.

Some seasoned farmers can take a pinch of soil, rub it between their fingers, smell it, maybe even taste it, and provide an accurate description of its texture and other characteristics. For the rest of us, laboratory analysis offers a more reliably accurate analysis of soil type, for use in decisions regarding irrigation and application rates for nutrients and crop-protection products.

Rapid, in-field analysis of soil texture, using visible near-infrared diffuse reflectance spectroscopy (vis-NIRS), could allow more precise measurements at multiple locations within a field, according to research recently reported in the Soil Science Society of America Journal. NIRS technology already sees use in agriculture for other types of soil and plant-tissue analysis.

Soil texture reflects the ratios of different particle sizes of the mineral components in the soil, along with organic-matter content. The smallest clay particles provide the most reactive surface area for binding water molecules and nutrients. Larger sand particles provide the least reactive surface area, with silt particles intermediate in size and binding ability. Organic material (OM) retains moisture while also providing nutrients. In addition to water retention, and nutrient availability, soil texture also affects compaction and aeration, influencing tillage decisions.

Researchers from Aarhus University in Denmark conducted the study, using 431 soil samples from seven agricultural fields in Denmark and Greenland. They used two different analytical models to analyze the samples for soil texture and organic-matter content. Their analysis categorized soil particles on the particle-size curve (PSC), which defines the continuous size distribution of mineral particles smaller than 2 millimeters. The PSC, the researchers say, is used for soil classification and to derive functional soil parameters such as the soil–water characteristic (SWC) curve, soil hydraulic properties, and gas transport properties.

Conventional methods for measuring texture, the researchers say, are time-consuming and most methods only provide discrete particle-size intervals – clay, silt and sand. The analytical models they used, known as the Rosin–Rammler and Fredlund functions, enable a continuous description of the size distribution of mineral particles along the PSC. The researchers analyzed their samples in a laboratory, but portable vis-NIRS devices, adapted for this use, potentially could measure soil texture in the field.

Among their findings, the researchers report that both of the tested models performed well in describing the PSC of a broad sample of soils, within- and between-field variations in PSC and OM are well predicted by vis-NIRS, and the new vis-NIRS concept enables soil type classification in any texture system worldwide. “From one vis-NIRS scanning the complete texture comprising the PSC and the OM content was successfully characterized,” the researchers conclude.