Soil aggregate stability testing

Soil aggregation is a fundamental property of soils and is a primary control of soil aeration. It determines the hydrological properties of soil such as water-holding capacity and the storage of organic carbon.

The stability of soil aggregates is also important because it influences how these properties change with time, and the susceptibility of soils to erosion by both wind and water, which can lead to a loss of soil nutrients.

Existing methods

A wide range of methods for measuring water stable aggregates (WSA) has been developed and applied.

In most of these methods, aggregates are manually passed through a set of sieves of a particular mesh size.

The limitations of sieve-based methods to measure the stability of soil macro-aggregates include:

  1. the mass of stable aggregates is measured for only a few, discrete, sieve-size fractions
  2. no account is taken of the particle size distribution of the sub-sampled material
  3. they are labour intensive

These limitations could be overcome by undertaking aggregate stability measurements with a laser granulometer instrument, but this technology has not been widely applied to the quantification of aggregate stability.

New BGS method

We have developed a novel method to quantify the stability of soil aggregates (1–2mm) in circulating water in a laser granulometer (Figure 1).

Figure 1. Arrangement of laboratory instrumentation for aggregate stability measurement using a laser granulometer

Figure 2. The particle size distributions of water-stable and disaggregated material.

We measure the difference between the mean weight diameter (MWD; µm) of aggregates that are stable in circulating water of low ionic strength, and the MWD (µm) of the fundamental particles of the soil to which these aggregates are reduced by applying a disrupting force through sonication.

We refer to the difference between these two measurements as disaggregation reduction (DR) — the reduction in MWD on disaggregation by applying sonication (Figure 2).

Soil types with more stable aggregates have larger values of DR since the stable aggregates are resistant to both slaking and mechanical breakdown by the hydrodynamic forces during circulation, and are disrupted only by applying sonication.

Advantages

One of the advantages of the laser granulometer-based aggregate stability method is the speed with which analyses can be undertaken; there is no requirement to dry and weigh aggregates trapped on sieves of differing mesh size, or for cleaning sieves between measurements.

With just two sub-samples, we estimate that a single operator could apply the test to 10 independent samples per working day; or 20 samples per day if only a single sub-sample is analysed, with a small associated increase in the measurement error of DR.

We have used the technique to understand the processes controlling aggregate stability in the Wensum Demonstration Test Catchment (DTC).

Given the speed of the analysis combined with an accurate, quantitative measure of aggregate stability, we consider that this procedure could be used to measure aggregate stability as a soil physical indicator in future soil monitoring programmes which will likely require analyses of many hundreds of samples.

This test will be used in the newly established environmental monitoring programme funded by the Welsh Government under its Axis II project.

Bibliography

Rawlins, B G, Wragg, J, and Lark, R M.  2013.   Application of a novel method for soil aggregate stability measurement by laser granulometry with sonication.  European Journal of Soil Science, Vol. 64, Issue 1, 92-103.

Contact

Contact Dr Barry Rawlins for further information.