NPK ratios?

Thu, 31 Jan 2008 03:06:33 PST

this is an important point you mentioned with Ca, Mg and K competing for 
free ions to combine.  They do, but, as you wrote, the reaction is 
largely governed by pH.  Applying these elements at different times 
would be the wisest solution, otherwise they may not be effective.

The same can be said of phosphates, but they will precipitate with Ca+ 
at higher pH, say around 7.2 or higher.  I do not know at which pH it 
combines with other elements.

Again, with phosphor compounds, they have a wide range of 'users' in 
bio-systems.  All plants require them, which may mean that in a pot 
there are more takers to consider than the main plant.  Algaes are huge 
absorbers of PO4.  Perhaps we have a situation similar to nitrogen in 
orchid bark?

Just some more thoughts,

> Mg (magnesium), Ca (calcium) and K (potassium) are similar in ionic charge 
> (Mg and Ca) and in size (Ca and K) and are said to be able to interfere 
> with each other's absorption.  This alone might indicate that applying 
> calcium at a different time than magnesium would increase the effectiveness 
> of the applied nutrients.
> Loading the fertilizer up with phosphate is relatively harmless in soilless 
> growth medium.  In soil, excess phosphate can tie up iron, calcium, and 
> probably a couple of the trace elements, depending on the ambient soil 
> pH.  Phosphate is of course essential for plant growth, but in 
> significantly smaller amounts that N and K.  I've never understood where 
> people are coming from when they recommend high phosphate fertilizers; this 
> just does not make physiological sense to me.
> Jim Shields
> in cold central Indiana (USA)
Jamie V.


Köln (Cologne)
Zone 8

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