Question
I am doing a project on fertilization and i would like to know which fertilizers would be best for experimenting on tomato plants.

Answer
This is very interesting.  Let me tell you how I would have you proceed if you were my son.

I would tell you to grow the tomato plants in pure peatmoss.  Peatmoss is a sterile medium.  It does not have any nutrients by itself.  It has no Nitrogen, no Phosphorus, no Potassium, no Magnesium, etc.

You would have 5 kinds of Fertilizer.  I think a local Home Depot or Lowe's might donate a small amount for your experiment.  Garden centers will have a lot on hand right now because it is the end of the season.  And many people have this stuff in the garage.  This is the hardest part, getting your hands on the fertilizer.  Don't let that stop you and don't be afraid to ask.  You can also go on Craigslist and ask for some help getting these things.  If you were in my area, I would do that.  Are you on Long Island?  If you are, write me back, and check the box that says 'Private'.  Have your mother or father help you with that, and I can give you my personal email without showing it to the entire planet.

Back to your fertilizers.  One would be high in ONLY Nitrogen.  It would not have any other nutrient in it.  Only Nitrogen.  UREA is a kind of Fertilizer with ONLY Nitrogen.  Urea is popular for Lawns.  Grass loves high Nitrogen fertilizer.

The next fertilizer would be high in Phosphorous.  The fertilizer that I know that is high in ONLY Phosphorous is called Triple Superphosphate.

Next, a fertilizer high in Potassium.  Potassium Chloride and Potassium Sulfate are loaded with it.  Potassium Chloride is used in winter to melt ice.  Its analysis is actually 0-0-60, in case your teacher wants this information.  Potassium Sulfate is a commonly used fertilizer; it also contains Sulfur, a trace mineral for plants.

The next tomato plant would grow in a combination of all 3 of the fertilizers.

And the last tomato plant would grow with NO fertilizer.  This is known as the 'control' plant.

Let me give you a little background.

All fertilizer by law must have an 'analysis' on the container of what its nutrients are -- this is called sometimes the N-P-K analysis.  N is the Chemical symbol for Nitrogen.  P is the Chemical symbol for Phosphorous.  And K is the symbol for Potassium (also sometimes called Potash).  If you have a Periodic Table of the Elements, you will be able to find these on the chart as N, P and K.

And every container of fertilizer, somewhere on the container, will show the percentage of N, P and K.  If it says 20-20-20, it has equal parts of Nitrogen, Phosphorous, and Potassium.  if it has an analysis of 15-0-0, which is what breaks down for Sodium Nitrate, it has 15 percent Nitrogen, and Zero Phosphorous and Potassium.  If you look on a container of Miracle Gro, you will see an N-P-K number somewhere on the box that tells you the amount of each of those 3 nutrients.  All plants need those, and they need more of them than any other nutrient.  They use them up in the soil, so we add more.

Good luck with your project.  Thanks for writing.