Comparison of Three Approaches to Measuring Individual Soil Aggregate Density.

Density is a fundamental quantity that characterizes internal structure of soil aggregates. Measurements of aggregate density provide key information for assessing the role of aggregates in various soil processes, including water flow, solute transport, microbial activity, etc. One established technique to measure densities of relatively big soil agglomerates is a clod method. The clods are covered with water repellent substance, such as paraffin or saran resin, and difference of their weights in air and in water is used to calculate the clod’s volume. Often, after determining the aggregate density researchers are interested in further using the aggregates for measurements and experiments, which is virtually impossible if the aggregate is already covered with saran resin. On the other hand, the paraffin cover can be relatively easily removed leaving the aggregate intact for further analyses. In this study we compared performance of paraffin and saran resin methods in terms of accuracy in aggregate volume determination and convenience of usage. Volumes measured using paraffin and saran methods were compared with volumes obtained from examining 3D aggregate images obtained via X-ray micro-topography. We studied aggregates 4-6.3 mm and 2-4 mm in size. Comparisons showed that saran resin trapped less air under the covering shell than paraffin, thus yielding more precise aggregate volume estimations. Paraffin, while trapping more air, allowed for uncovering and further processing of the aggregate. Densities of aggregates 4-6.3 mm in size were consistent with those computed using image analysis (r² = 0.97), while smaller aggregates exhibited high measurement errors (r² = 0.09).