Regular fertilizers, such as Miracle-Gro, contain too much ammonium and
urea and lack certain of the essential mineral nutrients, particularly
calcium. The ammonium causes the solution pH to drop too low. If your
water quality is poor, you may need to used distilled or deionized
water.
You might wish to try a hydroponic fertilizer at a reduced rate, say
10% of the recommended strength, to get good growth. If your school has
a well-equipped chemistry lab, you may be able to prepare your own
nutrient solutions. Once you have a solution that works well, modify
that for your other treatments.
One common hydroponic experiment is to obtain deficiencies of each
essential element, such as nitrogen, phosphorus and potassium. You can
also measure how plants change the nutrient solution pH. It usually
goes down with a little to a lot of the nitrogen as ammonium and up
with all nitrogen as nitrate.
Growing the plants under fluorescent light might be desirable for
reproducible conditions. I've not grown a lot of aquatic plants in
hydroponics but algae growth can be a problem.
As per the 1932 reference below, you might wish to count the increase
in humber of fronds as a measure of growth rate.
David R. Hershey
The following articles/websites should be useful:
Hershey, D.R. 1994. Solution culture hydroponics: History and
inexpensive equipment. American Biology Teacher 56:111-118.
http://www.angelfire.com/ab6/hershey/hydro.pdf
Hershey, D.R. 1993. Evaluation of irrigation water quality. American
Biology Teacher 55:228-232.
http://www.angelfire.com/ab6/hershey/water.pdf
Hershey, D.R. 1990. Pardon me, but your roots are showing. Science
Teacher 57(2):42-45.
http://www.angelfire.com/ab6/hershey/sthydro.pdf
Synthetic Media for Growing Duckweeds
http://www.mobot.org/jwcross/duckweed/media.htm
Essential Elements for Plant Growth
http://www.soils.wisc.edu/~barak/soilscience326/essentl.htm
Fluorescent lighting systems for plant growth
http://www.fastplants.org/instructions/lighting_systems.html
DUCKWEED AQUACULTURE
http://www.p2pays.org/ref/09/08875.htm
Peterson, H.G., M. Moody, 1997. Limited Report: Effluent toxicity
testing using Lemna minor. SRC Publication No: R-1640-20-E-97.
Contact author: http://www.safewater.org/aboutus/Resumes/hans.htm
McHargue J.S and Calfee, R.K. 1932. MANGANESE ESSENTIAL FOR THE GROWTH
OF LEMNA MAJOR.
Plant Physiol. 7(4): 697-703.
http://www.pubmedcentral.gov/picrender.fcgi?tool=pmcentrez&blobtype=pdf&artid=440182
