Hi ajm83,
I think that the comment of ceg4048 is directed to he fact that Ca and Mg exchanges each other in soils, understanding from your original post that you were concerned about the availability of Mg in soil because of this exchange. He is right in that context, and in aquatic plants the proportion Mg/Ca in soil is not critical, as Mg is available anyway in the water column.
About your questions, you are right when you talk about limited number of positions available for ion exchange. In fact, there are more rules about it. It is not only the number of places but also the type of iones that can be exchanged. For instance, there are three factors ruling here, mainly:
-The radius (size) of the ions involved in the exchange.
-The charges of those ions.
-The electrostatic charge of the surface.
The exchange happens in two different way:
-By replacing ions within the crystaline structure of the different minerals.
-By exchanging ions adsorbed to the surface of the minerals due to the local electrical balance.
The first one is usually quite independent of the media conditions and related to the stability of the crystaline structures. Because of the crystals have a given entropy, they tend to look for structures with lower level of energy, more stable, which can produce replacement of some ions. However, this is only possible if the ions being exchanged are similar in terms of size and charges. This phenomena, for instance, is the one that controls the exchange of Mg and Ca in dolomite and calcareous rocks, but is quite limited in terms of the exchanges that are allowed, being usually Mg/Ca and Na/K. There are more, of course, but those are the most commons.
The second one is not really related to the crystaline structure rather than the electrical charge balance of those structures, which generates different densities of negative/positive charges around the surface of the minerals. Some soils like clay, for example, have a lot of surface due to the small size of the crystal compounding the clay, as well as lot of electrostatic charges due to the different minerals composing it. In fact, this electric charges are a big part of the mechanical properties of clay.
The exchange of ions in the surface of those minerals is much more extensive than the one I mentioned, and it is also controlled by pH at a some point. Low pHs will favour the H+ occupying the negative charge locations, which means that the soil will tend to release elements already attached to the surface of the minerals, like Mg, Na, Ca, Be, Fe, Al, etc. On the other hand, high pHs will favour the negative locations being occupied by the same elements. Of course, when ions are released to the water they are available, but the recovery of the conditions does not mean that the same ions will attach to the same positions, as that depends on the relative concentration of ions in the water vs the ones in the soil.
In normal conditions, there is always a certain degree of exchanges of cations and anions with the soil, which is depending mainly on the temperature of the soil, pH conditions, as well as the composition of the water column. Temperature favours the exchange of ions, reason why some soils in the past were requiring some heating (not only to favour bacterial activity). pH, as mentioned, has impact in the availability of negative charges in the electrostatic surface of the minerals, and water composition drives what is replaced by what.
Going back to your question, if your water column is very rich in Ca ions, then it is likely Ca will take over the vacancies in the soil within the time, as the proportion will grant him more chances to get the negative charged locations available in the soil, but the main victim of that will be probably Mg, as is the second most common cations with 2 positive charges.
However, this is not necessarily negative. The ions need to be released to the water to be available to the roots, so the exchange ion helps to provide them with the elements and molecules they need, as not only cations or simple elements participate of this exchange, as organic compounds like NH4+, NO3-, PO4(3-) also play in this exchange, so a soils rich in some specific elements/molecules acts as fertilizer for the plants.
Of course, the process depletes the soil after some time, as there are outputs in the system: water changes, trimming of the plants, precipitation of minerals in other forms, etc. This can partly being compensated by adding liquid fertilizers to the water column which helps the deficiencies created in the time for this phenomena in the soil. In fact, the famous ADA step system is thought, precisely, to progressively cover these deficiencies generated in the soil, but relying more in the soil to fertilize the water column.
I hope this help and/or answer your question.
fortunately it also sinks away rather quickly (Carib Sea also offers a "Hawaiian black" but I've not seen it locally) 
)
