A technique to assess small-scale heterogeneity of chemical properties in soil aggregates

Abstract

A new method is presented which allows the separation of the soil aggregate exterior from the aggregate core. The method employs a combination of aggregate freezing with rapid separation of aggregate exteriors using ultrasonic energy. The factors influencing the thickness of the removed aggregate surface layer include water content of the aggregate prior to freezing, temperature difference between that of the frozen aggregate and that of the liquid it is submerged in during sonification, sonification time and energy, and the type of the immersion liquid.

The success of the method and the thickness of the removed aggregate surface were examined using barium (Ba²⁺) as a tracer. Barium (as BaCl₂) is rapidly absorbed by soil and is present at only very low levels in natural soils. Surface layers of 0.2–0.4 cm thickness were successfully removed from aggregates of 1–4 cm diameter.

Two examples are given from soils in northern NSW to demonstrate the occurrence of small-scale hetero-geneity in soil chemical properties. Compared with the surface fraction, a 4–7% higher calcium concentration was found in the core fraction of a clay loam soil (Dermosol). Conversely, on a cracking clay soil (Vertosol), atrazine concentration was around 15 times greater in the aggregate surface fractions compared with core fractions. Compared with the traditional estimation of soil chemical properties on homogenised bulk soil samples, it is suggested that separate analysis of aggregate surface and core fractions could provide useful additional information on the relationships between soil properties and environmental responses.