NPK ratios?

J.E. Shields
Thu, 31 Jan 2008 06:09:04 PST
Hi Jamie and all,

We can rely on Jamie to bring up some thought-provoking ideas!

The point at which Ca, K, and Mg compete, regardless of soil pH, is in the 
absorption receptors on the surface of the root hairs.  These receptors are 
large proteins that extend through the cell membrane and have a pore in the 
center that leads from outside the cell to inside the cell.  Here is where 
these three ions (ion = atom with electric charge) can interfere with each 
other, again because of similarities in size or electric charge.  They have 
to pass through that pore to get from outside the plant to inside the plant.

A point that is often made in regard to hydroponic culture -- and growing 
in soilless medium with continuous liquid feeding is much like hydroponics 
-- plants only absorb the nutrients they need and can use.  The rest they 
ignore.  That is why I said excess phosphate (PO4 with negative charge) is 
relatively harmless.

Natural soils can contain a variety of constituents that interact 
chemically with plant nutrients.  Clays can adsorb ions on their surfaces 
and can absorb ions internally.  Add calcium to clay and the fine structure 
of the clay changes, leading to a gross change in the texture of the clay 
-- from gooey when loaded with K (potassium) to grainy and crumbly when 
loaded with Ca (calcium).

Phosphate can, if I recall this correctly, adsorb onto the surface of 
clays.  These adsorbed phosphate ions can then bind Fe (iron) and Ca 
(calcium) ionically (that is, as salts).  Calcium and iron phosphates are 
quite insoluble at certain pH values.  This in fact is the mechanism by 
which phosphate can deplete the available iron in a soil.  An element must 
be soluble, as an ion in solution, for a plant to be able to take it up.

Soils also contain lignins and lignic acids that can bind cations (positive 
ions like Ca, Mg, and K).  These lignins are ion exchange agents, the 
cationic bindings are readily reversible, and lignic acid bound K 
(potassium) is a dynamic reservoir for K in soils.  Excess rain can indeed 
leach the nutrients like Ca, K , and NO3 out of the topsoil and carry them 
away in the groundwater.

In soilless media, as in true hydroponics, there is little in the way of 
commensal flora to assist the plants by processing nutrients from 
unavailable forms (e.g., ammonium, NH4 ions) to forms that the plants can 
physically absorb (i.e., nitrate, NO3 ions).  We use soilless media because 
it also provides little support for pathogenic flora.

My potting medium for bulbs is generally a mixture of Promix BX 
biofungicide (i.e., with commensal bacteria that eat fungi) + sand + 
granite chick starter grit in a ratio of about 2 : 1 : 1 by volume.  This 
is my "gritty mix."

I fertilize continuously from February through September (in the 
greenhouse) with Jack's Professional Peat Lite, a 20-10-20 with 2/3 N as 
nitrate (I'd prefer 100% as nitrate!) and most micronutrients, including 
some Fe.  I inject it into the irrigation water to a final concentration of 
100 ppm nitrogen (100 milligrams of N per liter).  I'd also drop the P (as 
phosphate) to 5% if I had my 'druthers.

Jack's Pro does contain a little iron, but not enough.  I have to interrupt 
feeding once or twice a year and inject chelated iron for one cycle, to 
keep the plants from becoming a bit chlorotic.

We grow arid land bulbs in greenhouses in the American Midwest.  We are 
fighting climate, seasons, and latitude.  It's about as unnatural a 
situation as you can get and want exotic bulbs to survive.

Jim Shields
in central Indiana (USA) where we are waiting for another winter storm to 
make up its mind

At 12:06 PM 1/31/2008 +0100, you wrote:
>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,
>Jamie V.

Jim Shields             USDA Zone 5             Shields Gardens, Ltd.
P.O. Box 92              WWW:
Westfield, Indiana 46074, USA
Tel. ++1-317-867-3344     or      toll-free 1-866-449-3344 in USA

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