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balocco, Mi trovo completamente d'accordo con il tuo ragionamento sull'assorbimento di ferro e altri sali, per completezza riporto questo esratto molto interessante...
"Technically, the Soilmaster should not affect the kH. Not in a direct way at least. Soilmaster, like most soils or clays have a negative (-) charge to the individual soil particles. There are only a few ancient and tropical forest soils with a positive charge. They are rare and unlikely to find their way in to most aquariums.
So for our general purposes, how could Soilmaster affect the water parameters. From the basics, Soilmaster is a highly fired (like pottery) montmorillite clay. Unfired, it has intense shrink/swell abilities. Often in the Southern US, Miss, Louisiana, etc places with high montmorillite clay contents will swell a foot or more between wet and dry seasons. This clay can be black, red, or whitish in color. Its shrink/swell tendencies make it good for absorbing spills (product called OilDri) and it is often used as a packing material around drilling bits to prevent contamination at groundwater/oil interfaces, etc.
Moving on from the factoids: This clay tends to have a high CEC ability. This means that it will readilly "absorb" or bind/attract any positively charged particles, called cations. Most metal elements in their elemental form, Ca++, Mg++, K+, Na+, Fe++/+++, etc will all be attracted to the negatively charged clay/soil particles, called anions. Technically, clay's are found in sheets. Regardless of particle size, if you look at the clay microscopically, you could see individual sheet like the pages of a book, with air or water and metals separating and binding each layer. Each clay sheet is a mostly repeating pattern of Al, O, Si, etc and if memory serves, a singe sheet of this stuff is made of a crystalyne structure sheets with an octahedral shaded sheet structure bound and sandwiched between two tetrahedral structural sheets. Each 3 layer sandwich is what you would see looking at it with a scanning electron microscope. The type of structure and the age of the particles determines the space and type of specific clay. It also determines the amount of space between each individual sheet. This space, in turn, decides what elements, wet or dry, can fit between the layers. For example, K+ is very small yet very strong. It can actually bind the sheets of some clay together so tightly that all the water in solution is forced out from between the sheets. This won't happen with our stuff but its a neat concept.
Regardless, in our water, many of these elements are free and many are not. For example, KNO3, when dissolved into the aquarium will immediately separate into the individual components of K+ and NO3-. NO3- is an ion and cannot be broken down by dissolving into solution. It requires a chemical/biological process to be converted into the individual elements. That's why you see NO3-, PO4-- and CO3-- always written together. They are the smallest forms of those ions. Ca++, Mg++, Fe++/+++ etc can be free, at least until they find some more attractive floating anion (-) adn bind to for insoluable compounds. Fe++ and PO4-- will do this. There are all kinds of rules and guidelines saying what will be soluable and insoluable with what. I won't carry on more with that.
So, what does all this mean? Long story short, Soilmaster cannot directly effect the alkalinity (kH) of the water. Free CO3-- has a 2- charge, bicarbonate, HCO3- has a 1- charge (H has a 1+ charge). So the negative soil and the negative carbonates simply see each other and try to stay as far away from each other as possible. Remember in chemistry, like with refridgerator magnets, like repels like.
Alk (CO3 adn HCO3 forms) is a buffer. In other words, it prevents water from becoming acidic by attracting and binding the free H+ cations that make the water acidic in the first place. Remember that pH is a measure of the free, or lack of, H+. Acid water (<7) has more H+ the OH-, basic (>7) has more OH- the H+ and neutral water has equal amounts. Carbonates and bicarbonates in the water will collect and bind as many free H+ ions are it has bi/carbonate ions. That is why water with higher Alk (kH) or bi/carbonates is considered more buffered. It can absorb more acid before the pH shifts.
So, here is where I am not sure of what happens. If Soilmaster is effect/reducing the kH of the water initially, I can only assume that it first attracts and initial cations attached to the bi/carbonates then later it attracts the H+ ions that would bind with the bi/carbonates. The free bi/carbonates are then more available to the plants as a carbon source or are more likely to form insoluable compounds with other things. My assumption also is that people who do not experience a noticable change in kH using this stuff are canceling out the absorbtion of cations by adding to the tank forms of K+ and CO2, creating more H+ ions that will combine with the the bi/carbonates and allow the buffer to stay in solution.
Where exactly does the kH go? It is obviously combining with some element or being consumed by some biological activity.
An interesting experiment for someone who adds NaHCO3 to their water and finds the carbonates gone by the next day would be to add a lot of K+, say from KHCO3 or K2SO4 and then try adding in a carbonate source and see if it stays. Also, as I have said before, I speculate that this is why ADA ferts are so high and focus so much on adding K+. I think the substrates suck up the K+ so fast that adding more is necessary. Don't know.
Anyway, the only way to know is to experiment and see. I realize this is in may ways a repeat of earlier posts, but I got carried away.
Lastly, if your having issues with controllers and injecting CO2 due to the kH changes, try a consistant bubble count, no controller and adrop checker style way of monitering your CO2 levels. It may take you a few days to really get the CO2 dialed in but adding x amount of CO2 will add x amount of CO2 regardless of the pH or kH. For example, if 2 bubbles a second of CO2 was good for the tank with 3kH and end pH of 6.0, then 2 bubbles a second will be good for water with kH 0 and pH 5.5. Using a controller, which relies on the pH and indirectly on the kH will not always give you the same amount of CO2. This is one of those situations where a controller will get you into trouble and why people don't use them with ADA substrates. Also, remember that as the substrate loads up and the kH starts to creep up, if you don't catch it, the controller will still keep forcing the pH down, adding way to much CO2 and killing everyone"
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