MadSci Network: Agricultural Sciences |
The main reason is that there generally are more soluble Ca-ions present in the soil. Roughly there is about ten times more Ca than Mg. The leachability of ions in soil depends mainly on two things: The electric charge and the hydration. Both Mg and Ca have two valence electrons and consequently the ions have a two-fold positive charge (2+). This make them both relatively strongly bound to the negative charges on mainly clay-particles (or colloids) and humus. However, Mg2+ is slightly more hydrated than Ca2+. This make the ion larger and slightly looser bound to soil particles. Subsequently the Mg-ion is more prone to leaching then the Ca-ion. This difference, though, is not large enough to compensate for the richer occurrence of calcium in the soil. Acidification of the soil will increase the risk of leaching as hydrogen ions will occupy some of the negative charges instead of Ca, Mg and other positive ions. Not being a specialist on the nutrition of fish I do not think that the fish directly takes up this calcium (or magnesium). It probably has to go through the food chains first, starting with plants and microbes.
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