plants and milk

Ross Koning Koning at ECSUC.CTSTATEU.EDU
Tue Feb 27 11:44:39 EST 1996


At 10:19 PM 2/26/96 +0000, Kathleen Greco wrote:
>My fifth grade daughter is doing her science project on the differences
>between growing plants with tap water, sugar water and milk.  We've done
>our best to control all other variables - light, temperature, potting
>medium, timing, plant variety.  The experiment is going fine - the
>problem we're having is finding suitable resources for her background
>research.  The books we've found that were written for the average
>gardner aren't specific enough about what a plant needs (or doesn't need)
>to grow.  On the other hand, a few articles we've found on the 'net were
>so detailed we couldn't make heads or tails out of them.  Can anyone help
>supply the kind of information we're looking for?

Kathleen,

The information you need is probably "hidden" in
other terms.  The sugar part of the project has at least
two possible impacts.  If the concentration of sugar
is low enough (below 0.3 M) then you are examining
the possibility of sugar uptake by the plant through
its roots.  This process might be sought in literature
on pinocytosis (cell drinking) as it is thought that
sucrose in flooding quantity and concentrations close
to isotonic (0.3 M approx) are taken into cells by
this mechanism.  If your concentration is 0.3 M or
greater, then you are probably in a hypertonic
situation.  The cells of the root will lose water,
lose turgor pressure, and no longer supply the
shoot with needed water.  The plants will likely
wilt rather abruptly.  Even at concentrations below
0.3 M this could happen (if more slowly); as the
plant removes water from the sugar solution it will
become more concentrated.  This would especially be
true if pinocytosis and sugar utilization were slower
than water intake. You won't find much literature on
fertilization with sugar.  It just isn't done (see
why below).

The milk part of the project has similar possibilities.
Transpiration (removal of water) could make the concentration
of milk solutes become hypertonic and the plants could
wilt.  On the other hand, milk contains some protein
that could be used by the plant as a source of nitrogen
for improving growth.  Additional calcium from milk might
improve plant growth.  The milk sugar (lactose) may not
be utilizable by the plant...this could cause a build-up
in the soil which could be detrimental.  You should be
able to find a reference for the composition of cow's milk.
It is likely in a nutrition handbook somewhere.  If your
library is light on applied science, perhaps a local
physician can help, or the local hospital's dietician.
With that information in hand, you can then look at a
book on mineral nutrition (soil science and fertilizers)
to find out what nutrients a plant needs to help you
understand what might be going on.  The local university
or college library might have an agriculture section or
a horticulture section to assist.

Both the sugar and milk part of the project have a
serious concern.  Putting sugars and protein solutions
in a soil environment will encourage bacteria and
fungi to grow.  These could, in turn, begin to attack
plant roots and stems in the soil.  If it seems as
everything is dying except your water controls, then
I would suggest diluting your sugar and milk solutions
by a factor of about 10, or try a range of concentrations.
Also, a one-time treatment followed by normal watering
with water for the remainder of the project could be
a good approach.  The initial "jump-start" of nutrients
(especially the milk) could be noticeable.  The lack of
repeated application of sugar or milk would help cut back
on bacterial and fungal growth.

To help you understand how to assess your plants, most plant
physiology text books have a section on mineral nutrition
of plants.  The symptoms described there will be for mineral
deficiencies.  I would expect to see those symptoms in a
control plant receiving only water.  The milk-treated plant
should show a reduction in calcium and nitrogen deficiency
symptoms.  The sugar-treated plant should be unaffected in
this regard.  Sugar is only an energy source...not a fertilizer.
One book that I like to refer people to is Brian Capon's book
"Botany for Gardeners" (Timber Press).  It is rather inexpensive
and has color photos of plants with mineral deficiency symptoms.
Your local bookstore should be able to get it for you.

"Plant food" is a badly-used term in our language.  Plants
use sugars and proteins, but make their own.  They do not
"eat" it from the soil.  "Plant food" usually refers to minerals
that plants do take from the soil.  These are mostly found
as small ions in the soil water: nitrogen, phosphorus, potassium
calcium, iron, magnesium, sulfur, copper, zinc, manganese,
boron, cobalt, and molybdenum.  These are supplied to our
gardens as fertilizer...the three numbers on the package
refer to the percent nitrogen, phosphorus, and potassium
in the mixture.  A project comparing fertilizer concentrations
might have been easier to interpret.

The reason that I have predicted milk will provide nutrients
is that milk contains calcium ions that can be absorbed
rather directly.  The nitrogen in milk is bound-up in protein,
but bacteria and fungi in the soil will break this down and
some of the nitrogen will be released as nitrate or ammonia.
These nitrogen-containing ions can then be taken into the plant
roots for growth.

I hope some of this helps.  Good luck to you and your daughter.
I'm glad you are getting her started on some study of plants.
It can lead to a lifetime of pleasure.  My parents got me
started with a vegetable garden when I showed a spark of
interest.  Now, years later, I'm teaching botany at a university
and loving it.  It can be a rewarding career.  You might point
out that a woman working with plants, Barbara McClintock, has
won a Nobel prize.  She is one of only three people who might
be called botanists to win such a prize, and one of the other
two was a prize for peace rather than science.  Women can make, and
already have made, their marks on the science of plant biology.

ross

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 \ Ross Koning                                  \
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