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
and
"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
cultivar.

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out of date.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
rates.

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:

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