ON THE CALCULATION OF THE SURFACE AREA OF DIFFERENT SOIL SIZE FRACTIONS

Abstract

A model that accounts for contributions to the total surface area (SA) by different size fractions of the soil is considered from a theoretical point of view. Calculations, based on continuous particle sizes and forms, explain SA mainly as a geometric SA. It is common for coarse soils to have a discrepancy between the measured SA, say inferred from adsorption of gases, and the calculated geometric area of up to two orders of magnitude. This discrepancy is removed by the present method. Size distribution is the main factor influencing the SA; taking particle forms into account resulted in a 2–3 times increase of SA. The several orders of magnitude range of grain sizes leads to crucial variations in the contributions that soil fractions make to weight, SA and number of grains. The fundamental lower limit of variation of soil properties, originating from the discrete nature of soils, is introduced. Despite the deterministic physical origin of SA, high sensitivity to the finest fractions can be considered on the environmental scale as a cause of the dual—stochastic and deterministic—nature of SA. Small variations of weight within experimental error and the fundamental limit may result in significant variations of SA, close to the same order of magnitude for coarse soils. An empirical equation (Sverdrup, 1990) relates textural data to SA at landscape scale. It is applicable to a collection of samples, while individual samples must be characterised on a probabilistic basis.