Citation: Adamczuk, A.; Gryta, A.; Skic, K.; Boguta, P.; Jozefaciuk, G. Effect of Different Minerals on Water Stability and Wettability of Soil Silt Aggregates. Materials 2022, 15, 5569. https://doi.org/10.3390/ma15165569 Academic Editor: Antonio Riveiro Received: 11 July 2022 Accepted: 11 August 2022 Published: 13 August 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article Effect of Different Minerals on Water Stability and Wettability of Soil Silt Aggregates Agnieszka Adamczuk * , Angelika Gryta, Kamil Skic , Patrycja Boguta and Grzegorz Jozefaciuk Department of Physical Chemistry of Porous Materials, Institute of Agrophysics, Polish Academy of Sciences, Doswiadczalna 4 Str., 20-290 Lublin, Poland * Correspondence: a.adamczuk@ipan.lublin.pl Abstract: Knowledge on the effects of minerals on soil water stability and wettability is mostly gained from experiments on natural soils of different mineral composition. To gain a “clearer” picture, the water stability and wettability of artificial aggregates composed of soil silt and various proportions of pure minerals: kaolinite, montmorillonite, illite, zeolite and goethite, were examined. The wettability was attributed to contact angles measured goniometrically and to the water drop penetration time (WDPT). The water stability was measured by monitoring of air bubbling after aggregate immersion in water and the shrinking sphere model was used to analyse aggregates’ destruction kinetics. The rate of aggregate destruction in water increased with increasing mineral content and it slightly decreased for aggregates composed of all pure minerals except goethite. An apparent hydrophobicity period (a period where the bubbling stopped for some time), resulted most probably from the wavy shape of pores, was observed mainly for aggregates with low mineral proportions. Among all studied minerals, kaolinite increased the water contact angle and water repellency to the greatest extent. With increasing the mineral content in the aggregates up to 8%, contact angles decreased and then increased. Contact angles did not correlate with aggregates’ stability. Aggregates more rapidly penetrated by water (shorter WDPT) were destroyed faster. Water stability of aggregates containing all minerals except illite appeared to be higher for the more mechanically resistant aggregates. Keywords: contact angle; kinetics; aggregate stability; apparent hydrophobicity; soil minerals; wettability; water drop penetration time 1. Introduction Correct soil structure is a key factor for managing natural resources in sustainable agriculture to maintain high productivity while taking care of the natural environment [1,2]. Soil structure consists of solids of different size and composition, and spaces between them that form clusters of various sizes, i.e., aggregates [3]. The bonding forces inside them are stronger than those between the adjacent aggregates [4]. Soil aggregates affect the content and location of elements, interactions between solid and liquid phases, heat capacity and flows of mass and energy in soil [3,5,6]. Water stability of the aggregates is one of the most important factors determining soil fragments’ release and transportation by runoff and splash, and their re-deposition, surface sealing, pore plugging and the movement (hydraulic conductivity) and storage of soil water. The above processes influence soil health and functionality, biological activity, root penetration, crusting, erodibility, compaction, aeration and carbon sequestration [7,8]. Therefore, modification of soil water stability is used to reactivate the proper functioning of soil [9–11]. The decomposition mechanisms of aggregates in water include: slaking, breakdown by differential swelling (montmorillonite swells when water is absorbed, which increases its volume, illite and kaolinite are stronger and nonexpendable), mechanical breakdown by raindrops and physiochemical dispersion, all of which lead to the formation of smaller Materials 2022, 15, 5569. https://doi.org/10.3390/ma15165569 https://www.mdpi.com/journal/materials