The impact of soil water content and water temperature on wet aggregate stability: What answer do you want?

Wet aggregate stability determined by immersion wet sieving is commonly used as a measure of the stability of soil aggregates. Over the years different researchers have used a variety of methods with a wide range of sieve sizes, amplitudes, cycles, sieving times and pre-drying methods. The most common drying measurement prior to sieving is air drying but the soil water content after drying is generally not stated and often not determined. This water content can vary depending on soil texture, clay mineralogy and the air temperature at drying. This experiment studied the impact of soil water content following drying at different temperatures prior to wet sieving, and on the temperature of the sieving water, on the mean weight diameter (MWD) results obtained for three different soil types. The drying temperature affected the resulting soil water content particularly for the high clay soils and this also influenced the MWD results, with the lower drying temperature and higher soil water content resulting in a greater MWD in the soils with higher clay contents. The soil with the highest clay content and predominantly montmorillinite clay mineralogy had a MWD following drying at 15°C more than five times greater than its MWD after drying at 25°C. The results for the sandy loam soil were not significantly different as there was little difference in the soil water content across the range of drying temperatures. The temperature of the sieving water also significantly affected the MWD results for all soils with different soil types being affected differently. This is likely to be because of the different solubilities of binding agents within the soils. MWD results can vary significantly unless drying temperatures and the temperature of the water during sieving are standardized. The choice of conditions can significantly bias results.