Soil Texture Estimates: A Tool to Compare Texture-by-Feel and Lab Data

Abstract

Soil texture is a fundamental soil property that impacts agricultural and engineering land-use. Comparing texture estimates-by-feel to laboratory-known values to calibrate fingers is a common practice. As educators, it is difficult to assess this field skill consistently and fairly. The instructor may give full credit for the correct texture class and partial credit for adjoining classes. This works well if the sample is near the center of a class, but not so well if it is near a class boundary. This article describes a computer program methodology using simple trigonometric principles to accurately assess student performance for estimating particle-size distribution (PSD) and soil textural classes. The concept is to plot the estimated PSD and the actual PSD on a texture triangle, calculate the distance between the two points using the scale along any side of the triangle (100 units), and subtract that distance from 100 to obtain a score. If the estimate coincides exactly with laboratory results, the score is 100%. If the estimate and laboratory results are as far apart as possible, at opposite corners of the texture triangle, the score is zero. Other scores are based on the distance between points representing the estimate and the actual on a texture triangle in relation to the length of one side of the triangle. The texture estimator provides a quantitative, consistent, and easy-to-use method of assessing students' performance. Additionally, this program allows students to observe the errors in estimates, which provides a further educational benefit.