The QuantiSlakeTest, measuring soil structural stability by dynamic weighing of undisturbed samples immersed in water

Abstract

We evaluated the performance of a new, simple test to evaluate soil structural stability. The QuantiSlakeTest (QST) consists in a quantitative approach of the slake test, a dynamic weighing of a dried structured soil sample once immersed in water. The objective of this work was threefold: we aimed to (i) derive indicators from QST curves to evaluate soil structural stability, (ii) establish the relationship between soil properties and QST indicators, and (iii) assess how QST indicators respond to contrasting soil management practices. To reach these goals, we sampled the soil of 35 plots from three long-term field trials in the silt loam region of Belgium dealing respectively with contrasting organic matter inputs, tillage and P–K fertilisation. For each plot, indicators calculated from QST curves (e.g. total relative mass loss, disaggregation speed and time to meet a threshold values of mass loss) were compared to the results of the three tests of Le Bissonnais (1996), used as a reference method for the measurement of soil aggregate stability. Shortly after immersion in water, soil mass increases due to the rapid replacement of air by water in soil porosity. Then soil mass reaches a maximum before decreasing, once mass loss by disaggregation exceeds mass gain by air loss. Our results confirmed that the early mass loss under water is mainly related to slaking, whereas after a longer time period, clay dispersion and differential swelling become the dominant processes of soil disaggregation. The overall soil structural stability was positively correlated to the soil organic carbon (SOC) content and negatively correlated to the clay content of soil. Consequently, the SOC : clay ratio was closely related to QST indicators. Nevertheless, for a similar mean annual carbon (C) input, green manure and crop residues were more efficient in decreasing clay dispersivity and differential swelling, whereas farmyard manure promoted SOC storage and was more efficient against slaking. QST curves had a strong discriminating power between reduced tillage and ploughing regardless of the indicator, as reduced tillage increases both total SOC content and root biomass in the topsoil. The QST has several advantages. It (i) is rapid to run, (ii) does not require expensive equipment or consumables, and (iii) provides a high density of information on both specific mechanisms of soil disaggregation and the overall soil structural stability. As an open-access programme for QST data management is currently under development, the test has a strong potential for adoption by a widespread community of end users.