In the Northern Hemisphere, the winter season is back. Our rhododendrons still hold their
leaves, but most of the other trees and shrubs, including deciduous azaleas, are bare.
Things are going on in the cells, though. The collective term for those activities in all
organisms is metabolism, the word for this issue of the Journal. The word derives from the
Greek meta, beyond and ballein, to throw. A good definition for metabolism is the sum total
of all chemical processes occurring within a living cell. It's appropriate at this season
because we have done or soon will do our winter fertilizing to induce outstanding flowers
in our rhodies come spring. Why do we need to fertilize when plants are autotrophic (Greek
autos, self, and trophos, feeder)? As we know, they trap light energy in carbohydrate
molecules by the process of photosynthesis (Greek photos, light, syn, together and
titheni,
to place). Animals could not exist without the molecules plants produce. We fertilize plants
because not all of the elements necessary for cells are trapped by photosynthesis. All
organisms use the same basic elements. The most central ones can be remembered by the mnemonic
device CHNOPS. It's easy to remember because it sounds like the German word for strong liquor.
Carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur are found in all cells, along with
smaller quantities of several other elements, including potassium, iron, magnesium, calcium and
a number of others. Plants extract carbon dioxide from the air and water from the soil and air
to produce carbohydrates and give off oxygen as a waste product. To do so they use a molecule
called chlorophyll (Greek, kloros, green and phyllos, leaf). Chlorophyll has an atom of
magnesium at its center that acts to assist photosynthesis. Adding Epsom salts, magnesium
sulfate, to our gardens often helps plants take up sufficient quantities of magnesium.
Both plants and animals use the byproducts of photosynthesis and other molecules to carry
out respiration, the central metabolic process in all cells. Respiration yields the energy
needed for all other processes. In the broadest sense, respiration results in the production
of proteins. Proteins are important because all major molecules in cells are proteins or, as
with carbohydrates and fats, are assembled by enzymes which themselves are proteins. Plants
often cannot get enough of some elements to produce leaves, growth and flowers that are
satisfactory to us, so, as with people, we feed them supplements. We call these supplements
fertilizers and we look at the contents to determine what fertilizer to use. The contents
are listed on the container as something like 10-10-10 or some other set of three numbers.
The first number tells us what percentage of usable nitrogen is in the mix. All proteins
contain nitrogen and here's where we run into the first reason for fertilizing our garden
plants. Nearly 80 percent of the gas in the atmosphere is nitrogen. You'd think that it
would be easy for plants to extract it, but it's not. Nitrogen by itself is inert; it doesn't
combine easily with other atoms. That's why nitrogen floats around alone in the atmosphere.
With some exceptions, plants can't use it unless it is attached to oxygen as a nitrate. The
process of combining nitrogen with oxygen takes place mainly in the soil and in the roots
of some plants and is carried out mostly by certain bacteria called nitrogen-fixing bacteria.
Higher plants depend on the nitrates produced this way to make their proteins. When we want
"better" plants we often add fertilizers containing significant amounts of fixed nitrogen.
The second number on the container indicates how much phosphorus is in the mix. Energy is
required to keep all of the cell processes going; it is held in molecules containing phosphorus.
Plants require a lot of energy to make flowers; thus we use fertilizers containing high
phosphorus numbers when we want to encourage flower production. The third number on the
container tells how much potassium the mix contains. Potassium is extremely important in
small quantities in plant cells because it helps a number of other elements get into cells.
Sulfur is an essential part of many proteins including some enzymes. Because of that we often
find by reading the fine print that the potassium is represented as potassium sulfate or some
of the nitrogen is ammonium sulfate.
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| Rhododendron 'Fire Rim'
showing chlorosis in the leaves. Photo by Bruce Palmer |
It's not terribly difficult to discover if a plant needs fertilizer or has had too much. Just
like people, if a plant looks pale and unhealthy it probably needs some help. Plants with an
overall yellowish look to the leaves are likely to be deficient in nitrogen. That's the most
common nutrient deficiency. A general fertilizer with a high first number (nitrogen) is
usually just the ticket; the plant will respond by greening up nicely. Sometimes after we
have "fed" a plant some or most of the leaves, especially new ones, will develop brown tips
and edges a few weeks later. We have given the plant so much fertilizer that it has been
transported to the tips of the leaves in excessive quantities and the cells there have been
killed with kindness. It may be that a plant needs fertilizer, but discovering what is
deficient in a plant is not always straightforward. In the photograph accompanying this
article an older leaf of Rhododendron 'Fire Rim' is shown with yellow areas called
chlorosis (Greek, kloros, green and - sis, disease or condition) between the
veins but with dark green with dark green veins. This pattern, sometimes called a
"Christmas tree" effect, is probably magnesium deficiency. The cells closest to the veins
are gobbling up all the available magnesium before it can get out to the other cells. It
could be iron deficiency except that the younger leaves, not shown in this photo, don't show
the same problem. It's difficult to be certain which problem it is, but giving the plant
some trace minerals that include chelated iron (iron attached to an organic molecule)
and/or Epsom salts should help.
To carry out the processes of metabolism our rhododendrons and all other organisms need
to take in quantities of specific nutrients. The overall requirements and processes are
essentially the same, although some of the specific atomic and quantitative requirements
vary slightly from one organism to another. In the case of our rhodies, to get the best
performance we need to give them an extra boost from time to time. Except in the areas
where there is little to no frost we avoid nitrogen-rich fertilizers in fall. In winter
we give them a boost of phosphorus (the second number) to encourage great blooming in
spring. After blooming they need a general fertilizer (all three numbers in roughly equal
quantities) to help their growth spurt in spring and early summer. In the temperate, mostly
frost-free U. S. West Coastal region where I live we give them a fertilizer boost in early
fall or late summer, often with slightly higher nitrogen (the first number). It has to be
early fall, though, or we may lose nitrogen induced new growth during the winter, as
happens in colder climates. We remember the times by fertilizing near Valentine's
Day, Mother's or Father's Day and Labor Day. If you haven't done your winter fertilizing
yet, it's probably time. Put on your heavy clothes and get out there. Your efforts will be
well-rewarded when your plants bloom.
Bruce Palmer is a member of the Eureka Chapter. He was a teacher of biology at Maui Community College in the University of Hawaii System for twenty-five years.