Mechanical stability of newly-formed soil macroaggregates influenced by calcium concentration and the calcium counter-anion

Abstract

Aggregate stability, an indicator of soil structural resistance to mechanical forces, depends upon the strength of binding substances within the aggregate. Adhesion of calcium (Ca2+) oxide-based compounds to soil primary particles is expected to create stable aggregates. This occurs because Ca2+ will flocculate clay and silicate minerals, and precipitate with carbonate or as Ca2+ hydrates (calcium-silica-hydrate, calcium-aluminium-silica hydrate), filling the air-filled pore space with solids that resist mechanical rupture. The objective of this work is to determine how the Ca2+ concentration, as well as the type of Ca2+ counter-anion (CO32−, OH−), affects the binding strength of newly formed macroaggregates (0.84–3 mm in diameter) that resist abrasive force. Soil from six arid desert locations was ground (<0.125 mm) and mixed with solutions containing 40 to 2560 mg Ca L−1 made with CaCO3 or Ca(OH)2. The stable aggregate diameter of newly-formed macroaggregates was based on their resistance to abrasion in an erosive chamber. Stable aggregate diameter tended to increase as the log of Ca2+ concentration increased and was similar in the presence of the CO32− and OH− counter-anions. Larger stable aggregate diameter formed as the Ca2+ solution concentration increased from 0.04 to 2.6 g Ca2+ L−1, using CaCO3 or Ca(OH)2 solution, and was enhanced when the soil contained 48–170 g clay kg−1 dominated by montmorillonite clay minerals. We observed Ca2+ accumulation on Al- and Si-rich surfaces, which may indicate formation of flocculants < 20μ m, as well as soil particle adhesion within newly-formed macroaggregates by calcium carbonate or calcium silicate hydrate precipitates. This work highlights the importance of calcium oxide binding for macroaggregate stabilization in arid desert soils.