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Re: Potassium dosing levels
Frank Reiter <FIR at istar_ca> wrote:
> I became concerned about other elements of the
> PMDD mix, notably the K2SO4 and MgSO4. Should I be adding those in
> proportion to the nitrate or in proportion to the iron?
In a word, the proportion of potassium to nitrate is more important.
Let's assume that your tap water and other sources do not contribute
much potassium inputs to your system (a pretty fair assumption) If you
have a source of nitrogen (nitrate/ammonium/nitrite), then you could
decrease the amount of nitrate relative to potassium and magnesium.
Heavy fish loads/feeding and certain kinds of fertile soils can produce
a fair amount of nitrogenous compounds however these types of systems
can be unstable and a little tricky to control. Moderate nitrogen
sources in the substrate can be very productive especially for certain
kinds of plants. The PMDD recipe calls for more potassium than nitrate
because there tends to be more sources of nitrogen in aquariums and
potassium levels can be a little higher without any complications. You
should probably aim to keep potassium between 3-10 ppm and nitrates in
the same range. It is pretty simple to calculate your potassium levels
by the weight of chemical that you use. K2SO4 is 45% potassium by
weight. I base my dosing upon the amount of fresh water which I
introduce into the tank each week. I dose this water at 10ppm so this
means if all the potassium is consumed each week, I will still have
2.5ppm. I dose magnesium at half this level and I test for nitrates to
see what the concentration is doing. I also need to add fairly
significant amounts of calcium carbonate each week because of the lack
of minerals in my tap water; 1 tsp added with a 10 gal water change. Not
all of this calcium is being used by the plants; some of it gets
absorbed into the peat substrate and perhaps by fish and snails.
In summary, dose K, Mg, and Ca relative to the Nitrogen not the Fe. Your
Fe levels can be very misleading and you should only use enough to
ensure consistent growth. With experience you will be able to see which
nutrient is limiting growth by ensuring that all the others are in
sufficiency and observing growth rate changes by changing that variable.
You need to understand how to calculate your concentrations based upon
your doses and it really helps to have an analysis of your tap water. I
don't know if I've explained the process in a clear and concise way. It
would be easier to write a detailed recipe that I've evolved based on my
Vancouver water however, using this will not teach you how to observe
changes and determine the cause and effect relationships. Remember that
water sources vary a lot; local soils have several parameters which can
all vary independently; and there are specific environments which favour
certain types of plants (nutrients, organics, calcium+magnesium, pH etc)
For example some plants prefer high levels of calcium in the substrate,
others do not; some tolerate low redox environments well, others do not.
Hope this gives some insight!