Re: What type of fertilizer, organic or inorganic, promotes better plant growth?

You are correct that most commercial potting soils have some fertilizer in them 
initially so you may not have to fertilize immediately, however, it probably 
will not hurt to apply fertilizer in the irrigation water soon after seedlings 
sprout. It is almost impossible to do an exactly fair comparison of organic 
versus inorganic fertilizers because they will not have the same analysis. Cow 
manure is about 3.3 - 0.35 - 2 analysis so you would expect a 20-20-20 
fertilizer would provide a lot more nutrients. 

However, you can still have a valid experiment even if the nutrient amounts are 
not the same for both kinds of fertilizers. It would be very useful to be able 
to calculate how much nitrogen was actually added to each treatment. You would 
use the weight of fertilizer used and the analysis to calculate that. For 
example, if you used a total of 10 grams of 20-20-20 fertilizer per pot, that 
would be 0.2 x 10 = 2 grams of nitrogen. To be scientific, you can also 
calculate the mg/liter of nitrogen in the fertilizer solution you apply. 
Fertilizer solutions applied to greenhouse crops with every irrigation 
typically contain about 200 mg/liter of nitrogen.

Your use of cow manure might be problematic given that it is a solid, which 
should be mixed with the soil. You obviously cannot do that every week so you 
would have to add cow manure once before planting or instead of cow manure use 
a liquid organic fertilizer such as fish emulsion or liquid seaweed that could 
be added weekly.  

Height is not the best measure of plant growth because plants that receive 
inadequate light often grow taller than plants under high light but are not as 
healthy. It would be better to also measure the fresh weight or dry weight of 
the leaves and stems at the end of the experiment. It is also fairly easy to 
measure the leaf area by tracing leaves onto graph paper and counting the 
squares. The more data you have the better.

Try to be sure that you have replication or several plants of each species for 
each fertilizer treatment. Also, the goal in any experiment like yours is to 
provide the same amount of light, same temperature, etc. for all plants so only 
the fertilizer type is varied.


References

Hershey, D.R. 1995. Plant Biology Science Projects. New York: Wiley.

Hershey, D.R. 1990. Sleuthing the nutrients that make your houseplant grow. 27
(4):17-20.