Hi all,
I also have a vague recollection of soils preferentially took up cations rather than anions, and phosphate is the latter I believe,
Phosphate is an anion, PO4--- and Anion Exchange Capacity (AEC) does differ from Cation Exchange Capacity (CEC) in that it is usually a <"
greater effect in acidic conditions">, while <"
CEC is usually stronger in basic situations">.
In the same way you have a sequence of how strongly held cations are dependent upon their valency, you have <"
a series for anions">. Clay minerals will hold PO4--- ions and these may, or may not, become available again. The reason for this is that phosphate adsorption and availability is pH dependent, but unfortunately not in a linear manner.
.......... Among various anions, the exchangeable phosphate is of most importance since sulphates or nitrates are not retained in sufficient quantities. So the phenomenon of anion exchange is most important for the release of fixed phosphate in the soil and thereby increases its availability to plants. The anion exchange is carried out mainly by the replacement of OH- ions of the clay minerals.
The reaction is as follows:
Such type of phosphate adsorption renders phosphate unavailable to plants. Since the anion exchange is reversible phenomenon a part of the adsorbed phosphate ions again become readily available when soils are dominated with OH ions i.e. means under high pH conditions (may be induced with liming).
....The origin of OH- ions is from silicate clay minerals and hydrous oxides of iron and aluminium that are present in the soil.
In this reaction takes place under slightly acidic soil conditions, the exchange phenomenon is reversible, and soluble phosphate is further released when hydroxy phosphate compound (insoluble) comes in contact with OH- ions.
In this reaction occurs under moderate to strong acid conditions, the phosphate ions are irreversibly fixed and not available to plants.
I tend not to worry too much about phosphate deficiency, because plants are very good at shuffling it around and, even though it is a macro-nutrient, <"
plants need to run out all together"> before you get real growth problems. As soon as it becomes available again? Growth resumes.
Because PO4--- forms a lot of <"
insoluble compounds"> you can always remove it from the water column, but it will take a while before deficiencies show. The vendors of <"
Rowaphos">,"Phosban", "Phosguard" etc know that it takes a while for deficiencies to show, so can carry on peddling their wares to the, initially delighted, freshwater fish keeper.
On a separate note. My tap water is very soft and over the past few months I’ve been dosing with AV carbonate/JBL Aquadur to raise hardiness which seems to have been fine. I think my tap water is pretty much okay except it has exceptionally high levels of silicates and phosphates and my LFS advised the two products phosguard and silicatex in order to try and reduce them to manageable levels. They’ve now advised I use RO water as it’s a cheaper way to manage those two levels rather than repeatedly buying silicatex every 2 weeks at £20.
It also allows them to sell a phosphate test kit etc. and you then have the repeat sales of both PO4--- remover and test kit, add in a "pH buffer" and a fertiliser containing PO4--- as well? You can sell punter the phosphate remover, the buffer, the test kit and the fertiliser and they will work together to extract <"
an ever increasing amount of money"> to solve a "problem" that <"
never existed in the first place">.
cheers Darrel
