NPK ratios?

J.E. Shields
Wed, 30 Jan 2008 14:53:12 PST
Diana and all,

I agree with Diana --- my understanding from 30 years ago is that ammonium 
nitrogen is absorbed under all conditions relatively poorly by plant roots, 
compared to nitrate.  On the other hand, ammonium is the preferred form of 
nitrogen for all fungi and many bacteria.  Len Doran drove this point home 
to those of us whom he undertook to coach:  Ammonium feeds the pathogens 
trying to kill your bulb, nitrate feeds the plant!

In Narcissus poeticus (Vickery et al, 1946, cited in Rees) they found that 
the plant tissue analyzed as follows:

Bulbs grown on nitrate as source of nitrogen, 15.9 g N per 50 bulbs
Bulbs grown on ammonium as source of nitrogen, 14.0 g per 50 bulbs.

My bibles, (post Doran), have been

"The Growth of Bulbs" by A. R. Rees, pub. by Academic Press, New York & 
London, 1972
"The Physiology of Flower Bubls" by A. de Hertogh and M. Le Nard, pub. by 
Elsevier, Amsterdam & New York, 1993

The point made by Doran was the same as that made by Rees:  Bulbs need 
nutrients in the approximate ratio the nutrients occur in the healthy bulb 
tissue.  Plants are also limited by any nutrient that is present in 
significantly less than needed levels.  Unfortunately, only Gladiolus, 
Lilium, and Tulipa have been studied in detail.  As a general thing, the 
ratio of N, P, and K in plant tissue is about 4 N to 0.3 P to 2 K.  The 
precise ratios can vary not only from genus to genus but from cultivar to 

The question of the effects of nutrient ratios on efficiency of absorption 
is not resolved so far as I can recall -- or my knowledge may be decades 
out of date.  I am not aware the N, P, and K affect each others absorption 

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)

At 01:12 PM 1/30/2008 -0800, you wrote:
>It is also my understanding that nitrate nitrogen, in the form of potassium
>nitrate or calcium nitrate, is better absorbed at lower temperatures, so
>this is important for those bulbs that grow during the winter.  The
>formulation I use has both ammonium nitrite and potassium nitrate, but the
>main ingredient is potassium nitrate.
>Telos Rare Bulbs

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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