Effects of the fabric on the relationship between aggregate stability and color in a Regosol–Umbrisol soilscape

Differences in fabric may explain why relationships between aggregate stability and soil color have never been reported for a relatively narrow-ranging population of soils. Therefore, three size fractions (1–2, 0.25–1, and 0.05–0.25 mm) of 25 topsoils (75 samples) from a Regosol–Umbrisol soilscape (Sierra Nevada, SE Spain) were characterized for fabric by scanning electron microscopy, aggregate stability by wet sieving, and CIELAB color (hab, L*, C*ab) by diffuse spectral reflectance. A stability-index ranking from 0.17 to 0.98 paralleled a ranking of hab (88.5 to 66.1), L* (48.3 to 30.2), and C*ab (7.6 to 18.9): that is, the progressive increase in stability was related to increased soil redness, darkness, and chromaticity. The reason was an increase in binding agents such as clay, organic C, and free forms coating the surfaces of the aggregates. The values of the correlation coefficients (0.55 to 0.82), factor-analysis variances (73 to 88%), and multiple-regression coefficients (0.40 to 0.65) indicated, however, that much of the variability in stability cannot be statistically explained by color changes. Size fractions of soils with a similar binding-agent content did not significantly (P < 0.05) differ in color but they did in aggregate stability. They also showed fabric differences, indicating that the coatings are similarly effective as coloring, while their stabilizing capacity is regulated by size, shape, and arrangement of particles. A close packing of mixed silt sizes stacked like roof tiles resulted in a stable fabric itself. Color can indicate aggregate stability, depending on the binding-agent content, but, being insensitive to the inner particle arrangement, it cannot provide a complete prediction.