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

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 < 20 mu 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.

Automated summary

Aggregate stability in soil is influenced by the strength of binding substances, such as calcium oxide-based compounds. This study investigated the effects of calcium ion concentration and counter-anions (CO32-, OH-) on the binding strength of newly-formed macroaggregates that resist abrasive force. The results showed that increasing Ca2+ concentration led to larger stable aggregate diameter, and the type of counter-anions had similar effects. The presence of montmorillonite clay minerals in the soil enhanced the formation of larger stable aggregates. Calcium oxide binding played a crucial role in stabilizing macroaggregates in arid desert soils.