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Re: Watts, Lux, Lumens

"wayne jones" <waj at MNSi_Net> asks:
> >It turns out that plants don't really care that much where the spectra
> >are as long as they fall within the range 400 - 700 nM.
> Is this really true?

If you don't want to take my word for it checkout the following
http://www.uaf.edu/coop-ext/publications/freepubs/HGA-00432.html which
was posted at a University of Alaska website by folks who really ought
to know.  <:-)

For our purposes, it doesn't make _very_ _much_ difference to the plants
where the light spectra are because the growth limiting factors tend to
be nutrients (assuming you have already met the _basic_ light

For example, you can stimulate faster plant growth by providing nitrogen
and phosphates at the roots EVEN WHEN those nutrients are still present
in the water!! This is because the availability of those nutrients from
the water is limited by the plant's transpiration system and their
ability to extract nutrients in low concentration from the water. Plant
roots are able to transport much higher concentrations of nutrients
which might be found in the substrate interstitial water.

This is a simple experiment anybody can conduct in their own plant tank.
Just add a small piece of Jobe's Plant Stix under a small plant and
watch it grow.

> As I understand it PAR sensors are calibrated to be
> less sensitive to toward the blue end of the spectrum as plants
> photosynthesize on a per photon basis. Photons at the at the blue end of the
> spectrum contain higher levels of energy but more energy does not translate
> into more photosynthesis. Also I wonder why PAR sensors are not less
> sensitive to green radiation as most plants are green and obviously reflect
> green light.

It is true that plants do not react uniformly to all the light
frequencies. Grow lamps appear dull to us because they lack green wave
lengths which our human eyes are most sensitive to. I have to assume
they are more efficient at converting electricity to PAR than plain old
cool white lights despite the prevailing opinions that higher lumen
value lights are superior.

Other examples of different responses to light frequencies are
phototropism (turning toward the light) and flowering which is
controlled by red and far red light.

Another example is if you wanted redder plants, you can either increase
the overall light intensity or you can increase the intensity of the UV
light which is what stimulates the production of the red leaf pigments,

But I haven't answered your question about whether PAR meters are
weighted lower for green light. I think some one else can better answer
this question.