Abstract
Particle-size analyses are commonly run to standardize the description of field textures of most kinds of soils. Andisols are an exception, because they do not disperse completely. Failure to disperse completely, and the smeary feel of Andisols in contrast to the sticky feel of most other kinds of soils, cause some to question the reliability of Andisol texture (field clay) classes. We compare field clay classes, organic carbon (OC)-corrected 15 bar water percentages, and allophane contents to find if field clay descriptions are reliable. We selected over 390 Andisol samples from the conterminous USA and Alaska. All horizons with NaF pH ≥ 9.3 or some glass shard influence are included. We ran 15 bar water by pressure membrane extraction, OC by acid dichromate digestion and FeSO4 titration, Sio in an acid oxalate extract, and pH in 1N NaF. We accepted allophane as 8.3*Sio and used the midpoints of the USDA texture classes as proxy for field clay classes. Means for the pipette-measured clay contents of selected soils that disperse well closely match the texture-triangle-class clay midpoints. The means are only slightly different for two U.S. regions. Herein we use the texture-triangle mean-clay percentages for field clay classes. The OC-corrected 15 bar water percentage closely predicts (r2 = 0.97) the clay content of Ultisols. Except for loams and sandy clays, 95% of the measured clay contents are within the texture triangle clay ranges for the texture classes described. Thus, as expected, the model-field clay description approach is successful for these Ultisols. In Andisols with less than 16% OC, 15 bar water increased 2.01% for each 1.00 % increase in OC (%15 bar water = 5.99 + 2.01*OC, r2 = 0.61, SE = 3.8%, n = 370, variables are in %.). Because of the non-normal distribution of some of the data, we also examined the distribution of the class medians using box and whisker plots. Both the means and medians of the 15 bar water contents of the seven texture classes fall into four discrete groups. Differences between these groups are highly significant (P value = 0.0000). The field clay contents (estimated as 2.5* the OC-corrected 15 bar water contents), except for the loam class, increase progressively from the sands through the silty clay loams and clay loams. What is more important, 95% of the observations fall within class limits for all classes except the silty clay loams and clay loams. Few of the Andisols in the conterminous USA and Alaska are silty clay loams. Allophane content of the soils also shows a progressive increase from the sands through the silt loams. This study supports the view that field clay descriptions of Andisols are reliable. Also the OC-corrected 15 bar water content of air-dry samples provides a provisional standard for field clay. Copyright © 1999. . © Soil Science Society of America