Do different methods for measuring the hydrophobicity of soil aggregates give the same trends in soil amended with residue?

Most soils have a low level of water repellency and several studies have shown this to be a major property underlying soil structural stability. Various approaches have been developed to measure low levels of repellency, but selecting the one most appropriate is hampered by the following: (i) different methods have been used in separate studies and have not been compared to date; (ii) they rely on different theoretical backgrounds; and (iii) they are technically different. The objective of this paper was to compare three methods that allow measurements of low levels of water repellency on soil aggregates. The chosen techniques include water drop penetration time (WDPT), capillary rise method (CRM) and the water repellency index (R) tests. Soil macroaggregates (3–5 mm) from silty soil were used in the study after addition of several rates of maize residue and different incubation times in order to achieve a wide range of organic matter contents, microbial abundances and water repellency levels. The water repellency results from all used techniques were in general agreement. Using the well established concepts we developed the theoretical relationships existing between WDPT, R index and CRM, which are in agreement with the experimental data. The methods do not determine the same soil property, however, as WDPT estimates the breakdown of repellency with time, whereas CRM and R index estimate initial soil hydrophobicity upon wetting. We propose mechanistic and practical arguments to select the appropriate method to measure soil repellency.