Feeding Odontoglossums and Other Cool Growing Orchids

Plants, like all living things,  possess the ability to self-assemble.
The instructions for such self-assembly are buried in a plants genetic
code. In addition to self-assembly, with time, living things can modify
these instructions to best suite the environment (adaptation &
evolution). I grow plants, specifically odontoglossums, because of their
great beauty and their challenge. I try and grow my plants well. My
opinions on culture follow, with an emphasis on feeding odontoglossums
and cool growing orchids. Most growers, using simple feeding procedures
should have good results. I have written this article with some detail
for those interested. To summarize my recommendations I have bolded the
most germane text.

Fundamentals
Plant growth requires the input of both matter and energy. Matter comes
in several forms:
      * Water which constitutes about 90% of most plants mass
      * Minerals and chemical compounds, supplied in solution with water
      * Carbon, which is derived from the atmospheric gas carbon dioxide.
Energy comes in two forms:
      * Light
      * Heat

Light energy is utilized for the well-known process,  photosynthesis,
which is the process of building chemical compounds through the
interaction of light with small molecules. Heat drives transpiration
which is the upward movement of water and minerals within a plant as
water evaporates  from the leaves. Without light and transpiration plants
cannot grow.

Andy Easton, New Zealand's famous orchid grower and breeder offers
simple advice for growing orchids. "Give them the correct amount of
light, the correct amount of feed, the correct amount of water, and the
correct temperature and orchids will grow like weeds". Invariably, the
question is, "how much of each of these variables is correct"?

Insufficient light deprives a plant of energy needed for growth,
resulting in slow and weak plants. It is best to provide odontoglossums
with the correct amount of light all year long. This is not easy to do.
Classic odontoglossum species grow close to the equator where day and
night periods are equal all year-round. Most of us live far away from the
equator and our day and night periods vary with the season. For those of
us who grow in a greenhouse, we must vary the shading during the year to
achieve optimum light levels. There is not much we can do to compensate
for the short day length during the winter months. Most growers agree
that light levels around 1800 - 2200 foot candles produce good growth in
odontoglossums. The choice of light levels will depend on how the
temperature of the greenhouse can be controlled during the hot summer
months. It is better to sacrifice some light than allow odontoglossums to
grow too warm.

Temperature is an easy parameter to define. Day temperatures around 21 -
26C (70F through 80 F) grow excellent plants. While night temperatures
around 10-14C (50F - 55F.) produce the best growth. Robert Dugger points
out good results can be obtained during the summer months if a sufficient
temperature drop occurs from day to night, even if  the optimum
temperature cannot be achieved. Much of the chemical activity of a plant
occurs at night. It is important that the temperature and humidity are
correct in the evening. Odontoglossums evolved in the cool, buoyant
neo-tropics.  Their physiology, roots, vascular system and leaf structure
cannot replace water lost through transpiration if the surrounding
atmosphere is not correct.

The primary purpose of this article is the discussion of feeding orchids.
Next to water, fertilizer is the source for most of the building blocks
for a  plant (an exception is carbon which is derived from CO2  through
respiration. Fertilizer is typically purchased as a powder in a bag or in
liquid form, which is added to water in Mr. Easton's recommended amount f
"the correct amount". This nutrient solution, composed of both fertilizer
and water, has mineral concentrations which are the sum of the fertilizer
plus the minerals already dissolved in your water. Some of us have water
low in minerals while others of us have water with moderate or high
amounts of minerals dissolved in it. Unless you know the quality of the
water you start with you cannot know what you are feeding your orchids.
This is one reason it is hard to compare grower A with grower B.


N-P-K
Fertilizer is labeled with the quantity of macro elements, nitrogen,
phosphorus and potassium sold in the package. In the US, fertilizer
labeling must conform to guidelines imposed by each state and all states
have relatively similar regulations. Labeling provides the N-P-K values,
nitrogen, phosphorus and potassium. Why K for potassium? The K comes
from the German word for potassium, kalium. K is the internationally
accepted symbol for the element potassium. To confuse matters, the N in
N-P-K is the percentage of nitrogen and this nitrogen can occur in
several forms such a nitrate, ammoniacal nitrogen and urea. P is the
percentage of phosphorus as expressed by the molecule P2O5  even when the
phosphorus source is not P2O5 and similarly K is the potassium (kalium)
percentage as expressed by the molecule K2O. (For this discussion these
packaging quarks are not particularly important; however, they point to a
lack of cogency of regulations f there is better nomenclature available).

In addition to N-P-K, fertilizer contains micronutrients, nutrients
needed by plants in very small quantities. Regulation requires that
micronutrients exist in certain measurable amounts before they can be
listed. Regrettably, these measurable quantities can approach or exceed
phytoxic levels for plants. Micronutrients already exist in our water
supplies and as impurities in the constituents of fertilizer, even when
they are not listed. A confusing aspect of nomenclature is ratio. A
30-10-10 fertilizer has the same ratio as 15-5-5 fertilizer; the former
twice is twice as strong per unit of weight but the ratio is the same.

Feed Ratio
Back to plants. Plants are about 90% water, 10% solids. The ratio of
nitrogen, phosphorus and potassium in plant tissue once the water is
removed is about 3 - 4.5% N, .3 - .6% P and 3 - 4.5% K  (1 - 2% Ca and .2
- .5% Mg. So what do plants need to be supplied for good growth? Plants
are adaptable and are not damaged by moderate amounts of N, P, or K,
regardless of the ratio. Experimentally, a ratio was determined some
decades ago which supported excellent growth in orchids. This ratio is
3-1-2 ratio.  My own experience shows this ratio works well. I have
reviewed the fertilizing schedules of two superb commercial odontoglossum
nurseries. One uses a ratio of  4-1-2, the other 4-1-4. In other words,
nitrogen is added in the largest percentage, with phosphorus
significantly lower than nitrogen and potassium somewhere in between. My
recommendation: go for a feed close to the 3-1-2 ratio, long accepted as
ideal. Ironically, articles on orchid culture often recommend a
"balanced" fertilizer such as 20-20-20. If I recommend a "balanced" diet
of a pound of meat, a pound of butter and a pound of carbohydrate I would
rightly be called nuts! 20-20-20 is NOT a "balanced"  fertilizer
according to the needs of plants; however, the numbers are even and this
sells the product. Why do fertilizer companies make 20-20-20? High
phosphorus levels in fertilizers are intended to compensates for
phosphorus lost in reactions with constituents in some soils,  making the
phosphorus unavailable to plants. These reaction does not occur in
consequence with soiless orchid mixes therefor high P ratios are is not
needed. Providing an element in excess of a plants needs means you cannot
feed other elements at an optimum rate.
 
The ratio of a fertilizer does not express its strength. One can add a
small amount or a large amount of fertilizer of a given ratio to water,
thus varying strength. What strength should we feed? Orchids are
relatively light feeders compared to other plants. Some orchids, like
disas are exceptionally sensitive to feed and are injured at modest feed
strengths. I have not found odontoglossums particularly sensitive to
moderate feed strengths. How strong you make your feed is partly
dependent on the quantity of salts (the word "salts"  used here means
dissolved compounds, not sodium chloride or table salt) you have in your
water before you add fertilizer. If your water is relatively pure, you
can add more fertilizer than if your water starts high in salts. The
important factor is not to exceed a certain total value of dissolved
salts. The total quantity of salts in a nutrient solution, added to the
mix, (more correctly, substrate) effect a value called the "salt index"
or osmolarity. As  the osmolarity of the substrate solution increases,
less water transport occurs through the roots. Typically, orchids are
watered and allowed to partially dry between each watering, thus a
further increase in osmolarity occurs as water evaporates from the
substrate. With excessive substrate osmolarity, a plants water loss from
transpiration will exceed water replaced by the roots. This results in
several problems. One common  problem is leaf tip die-back, a condition
where the leaf tips dies because of lack of water. Warm days, a poor
root system and excessive drying out of the substrate in the presence of
high feed rates will exacerbate leaf tip die-back. What is a good, safe
level for nutrient solutions? In simple terms, about _ the application
rate found on most fertilizer bags is a foolproof rate.

Is there a better way to express fertilizer strength? On the side of most
bulk fertilizer bags are the calculations for achieving a certain number
of parts per million (ppm) of nitrogen. This is usually in pounds of feed
per 100 gallons of water. If you can do the arithmetic and extrapolate
the rate for your feed program, I recommend feeding at about 80 - 100 ppm
nitrogen.


Conductivity Meters
There is even a more sophisticated methods of determining feed strength.
Pure, distilled water has exceptionally low conductivity and virtually
all the compounds relating to feed programs, dissolved in water,
significantly increase the conductivity of the solution (urea is an
exception). Until recently, the conductivity of a solution was expressed
in conductivity units, mhos. (Mho is ohm spelled backwards, and the
reciprocal of the resistance unit, the ohm, i.e., 1/ohm's = mho's. For
the solutions we work with in plant culture conductivity numbers are
easier to interpret than resistivity values. To confuse matters, the
International Committee on Scientific Units has changed the mho to the
Siemen honoring the famous German scientist of that name. In keeping with
convention, all international units now honor a scientist and are
capitalized. Scaling units, such a milli or micro are written lower case.

The conductivity of a solution is referred to as its EC (electrical
conductivity). Feed stocks vary in EC depending on crop needs. The EC for
most horticulture crops falls within a range of 0-2000 mSiemens (0-2
millimhos). EC meters are available for around $50 and these make
determining the conductivity of a feed simple. These meters are available
in units of parts per million (ppm) or mSiemens. I recommend the uSiemens
meter and use a 0-1990 mSiemens range meter every time I mix feed. Why
not a ppm meter? For most purposes ppm and uSiemens have a relatively
simple conversion, typically something like uSiemens x .64 = ppm. The
hitch is, 1 ppm of ammonium nitrate does not have the same conductivity
of say 1 ppm of ammonium sulfate. Thus, when measuring a mixture one does
not measure true ppm's, only an approximate equivalent. The errors are
not large but I prefer conductivity units in uSiemens.

Using a conductivity meter allows one to set the level of feed (water +
fertilizer) to a particular concentration of total salts. Since it is
most productive to run feed at a level high enough for good growth but
not excessively high, with the accompanying risk of leaf-tip die-back and
root damage, a simple EC meter makes adjusting feed strength easy. What
is a good EC value? I recommend an EC of 800 mSiemens in the summer
months and about 2/3 this rate in the winter. These are conservative values.

Those of us who have water supplies which are relatively pure can run
more of the nutrients plants need for growth than those who start with
water high in the salts which plants do not need. Supplying the N-P-K
levels to a plant at optimum ratio and strength provides the correct
quantities needed for good growth. Add the correct amounts of energy in
the form of light and heat and one is growing "spot on".  The grower who
does this for the most months out of the year ends up a winner.


Urea
Urea is a cheap form of nitrogen often used in fertilizer. It has low
conductivity in solution and in its better grades makes a good nitrogen
source for plants which are grown at temperatures that are above 16C 
(60F). It is not a good source of nitrogen for cool growing crops,
particularly during the winter months because bacteria populations needed
for nitrifying urea are in low population. Urea requires bacterial action
before its nitrogen is available to plants. Periodically, growers
recommend using urea as a foliar application of urea to "green up" plants-
works like magic. Skeptically, I ran tests on an excellent and vigorous
cross made by Philip Altmann of Australia. Fifty plants of Odm (nobile x
Augres) were selected and divided into two groups of about equal size
plants. For two months I fertilized both with my standard, 3-1-2,
urea-free feed. I then followed up with a foliar spray at 100 ppm urea on
a group of 25 plants. I have not seen any evidence that the treated
plants "greened up". To quote a plant physiologist friend, Dr. Shiv
Reddy, "there is no magic in the bag".

Substrate (Mix)
Substrates vary considerably from grower to grower. Ingredients, such as
fir bark and peat are not consistent products. With age, increasing
amounts of bacteria, responsible for substrate breakdown, progressively
consume larger and larger amounts of nitrogen thus reducing nitrogen
available to the plant. It is important to change the substrate before it
excessively decomposes. In addition, orchids are plants which require
good amounts of air at the roots. The air capacity (open space) of a
substrate decreases with decomposition. Repotting maintains good air
capacity and maintains a consistent level of nitrogen available to the
plant. I repot yearly.

Be careful when combining ingredients to make a substrate. The goal is a
substrate with sufficient water and air. Some mixtures enhance air
capacity while others mixtures can reduce it. Avoid making combinations
where the sum of the ingredients decrease the air capacity of a substrate.


Spray & Pray
Various elixirs are sold purporting super results. These invariably have
some superlative in their name. These will appeal to growers with a
pension for mysticism or who grow in pyramid shaped greenhouses. While
you "spray and pray", the sellers of these brews simply prey.



Bloom Promoters
Some feed programs purport to induce flowering when a switch is made from
a high N (nitrogen) feed to a low N (nitrogen), high P (phosphorus)
program once growth has been "made-up". I suspect these formulas, often
with superlative trademark names, are not much use. Similar results can
be obtained by simply reducing or eliminating feed once growth has been
made up. Some orchid species grow vegetatively, failing to bloom when
continuously fed high amounts of nitrogen. I have not found this true of
odontoglossums. Use the same feed ratio all year long.


Summary      
In summary, a good fertilizer for odontoglossums and other cool growing
orchids has an N-P-K ratio of about 3-1-2. Avoid fertilizers which use
urea as the nitrogen source because urea is not well utilized in cool
growing crops and therefor nitrogen, which is the nutrient needed in the
largest percentage, will not be available in sufficient quantity. Feed at
about 80-100 ppm nitrogen level or about _ strength of most manufacturers
recommendation every watering. If you have a moderate collection you may
find a liquid feed convenient and cost effective. If your water is high
in salts, fertilize at a lower rate. You may want to buy a simple EC
meter in the 0-1990 msiemens range to aid in mixing and measuring feed
strength. Avoid elixirs unless they have mental health benefits for you.
Strive for a good root system and you will be growing well.

During my piano studies (never was any good), I learned a quote of Wanda
Landowska's, famous harpsichordist. Her critical comment to a colleague,
"You play Bach your way and I'll play Bach his way". I think my way is a
pretty good way!

                                    Bob Hamilton 8/97 
                                    bob@eecs.berkeley.edu