N vs Mg in leaves (was Leaf Color in Autumn)

John R. Porter porter at SHRSYS.HSLC.ORG
Mon Oct 21 13:56:02 EST 1996

Char A Bezanson wrote:
 > Thanks for the summary.  Just one question- I've always been under the
 > impression that it was the Mg ion in the center of each chorophyll
 > molecule that is the primary object of recycling by trees before
 > leaf-drop.  Since Mg++ occupies essentially the same position as Fe++ in
 > hemoglobin, its lack would cause the plant equivalent of "iron-poor
 > blood"...although I'm sure N is worth retrieving as well.  Do any of the
 > plant physiologists out there have any comment on the relative scarcity of
 > Mg and N, and of their fate and storage over winter?
 > Char A. Bezanson (bezanson at stolaf.edu)
 > School Nature Area Project
 > St. Olaf College
 > Northfield,  MN  55057

Both Mg and N are available from the soil, but the amount of readily available, i.e.,
mineralized, N, is usually very low, especially in somewhat acidic forest soils.  Think
about what we spend the most in supplying the plants we want to grow, NPK, with N being
the most expensive part of fertilizers.  Rock phosphate or limestone provides some Mg,
but it is low in amount and both of these are very cheap.
Why is N so low?  Every soil organism wants some, so there is high demand, and forms
like NH4+ and NO3- are rather soluble, meaning there are losses after each rain (less of
a problem for NH4+).  Mg, being divalent, gets held fairly tightly on soil particle
exchange sites (works for NH4+ also, although less tight, which is why it is lost less),
but the anion exchange capacity of soils, especially forest soils, is _very_ low.

Recycling N has to be a very high priority for the plants.  Yellow leaves in the fall
from long chain hydrocarbons (carotenoids) is much better than yellow leaves in the
growing season (N deficiency - can't make proteins or chlorophyll adequately).

Black (Soil Plant Relationships, 1968) gives the exchangeable Mg content of 20 New
Jersey soils as 1.6 milliequivalents per 100 g.  The comparable numbers for N are
0.027-0.23% depending on soil texture; however, the largest fraction of that is bound in
insoluble, and unavailable on the short term, organic forms (humus, soil organisms).  Of
the total nitrogen in virgin (forest) soils, 22% is ammonium, while 30% is
nonexchangeable or nonhydrolyzable.  53% is in various, relatively simple, organic
forms, amino acids, amino sugars, etc.  The chapter on nitrogen says "It is probably no
exaggeration to state that growth of agricultural plants is limited more often by a
deficiency of nitrogen than of any other nutrient."  Other factors, such as pH,
rainfall, temperature, and microorganism activity affect the availability of N much more
than is true for Mg.

Stating the absolute requirements for Mg and N is more difficult, because it is very
variable dependent on species, except that the needs for N always outweigh the need for
Mg by a considerable margin.  One set of data (Stark, 1977, U of MT) shows that Douglas
fir needles have 0.6-2.3% N (with <1%) being in chlorotic needles and 0.05-0.15% Mg; in
Scots pine, the N amounts are similar while Mg ranges from 0.02% (deficient) to 0.19% in
the leaves (needles), 0.01-0.21% in the stems, and 0.01-0.17% in the roots when grown on
defined nutrient solutions.  For fertilized tree growth (same source), 50-200 lbs N/acre
are recommended when N deficiency is limiting growth, while the recommended rates for Mg
are 20-30 lbs/acre.  For the growth of Jeffrey pine, soils are considered low in N at
1000 ppm and high at 12,000 ppm with low leaf level being 6,600 ppm.  Mg is low in the
soil at 22 ppm and high at 2450 ppm; low leaf level is 460 ppm.  More generally, plants
need Mg for chlorophyll and around two dozen or so enzymes (many of these ATPases).  N
is needed for every protein (multiple (read dozens to hundreds) per molecule), every
alkaloid, some lipids, some carbohydrates, every nucleic acid (DNA,RNA, thousands to
millions per molecule), and a variety of other compounds, including many pigments.  The
disparate numbers speak strongly for a massive requirement for N when this same element
is limited in availability in the typical soil.

Finally, and realizing this is probably already way beyond what you wanted, Walsh and
Beaton (1973, Soil Testing and Plant Analysis), state that "Crops all over the world are
probably more often deficient in N than in any other element....  This is a reflection
in part, on the fact that 97 to 99% of the N in soil is present in very complex organic
compounds that are not available to plants."  Two chapters later, "Uptake of Mg by crops
is relatively low, from less than 10 to about 25 kg/ha.  Occasional Mg deficiences have
been noted, especially on sandy soils or soils with low levels of Mg in relation to K
and possibly Ca."

I would venture a rough estimate that the order of quantitative importance for the major
nutrient elements is C>O>H>N>P>K=Ca>S>Mg>Fe, although the real order of any two adjacent
elements could be arguable based on circumstances and plant species.  By the way,
another element of major importance to photosynthesis is Mn (usually confused with Mg by
many undergraduate students).
John R. Porter
porter at shrsys.hslc.org

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