Science Buddies: "Ask an Expert"

Hello! I am currently working on a science fair project dealing with hydroponic growth with and without nutrients. To take my project a step further, I decided to investigate rooting pothos/philodendron plants with and without nutrients, when an excess of a nutrient is added. I want to add either, nitrogen, potassium, or phosphorous to my solution, and I was wondering:

Which of the 3 nutrients would best support my experiment?
I am going to use Peter's 20-20-20 Mix for my nutrients in water, but I'm unsure of how to add single elements to this mixture, and If I did, could I make this an environmental experiment and show the effects of an excess of nitrates in aqueous solutions? If I did, I could tie in DO and Ph and cool things like that.
Could I use ph paper to test for nitrates? I didn't know if there was a relationship between the two.
and finally, which part of the plant should I use? I leaning towards part of stem and leaf.

Thanks for any information provided!

Hello again! I have slightly altered my project. I am using the same basis, but I thought to add the Nitrogen, I would add a diluted mix of dog poop, and it would also have a connection to industry. People have found that an excess of nitrogen added to plants in the ground is positive, but when an excess of nitrogen is added to aqueous plants, it is bad, so how about an excess of nitrogen in a ground plant grown in water? So, this should be a new idea, and a great application to science.

Does anyone know what the exact coorelation to DO levels and Nitrogen levels? Is there a direct proportion, or does it vary?
Thanks,
scotty1345

Hi,

I am not sure I can answer all of your questions (hopefully another expert can chime in), but I can definitely help with a few things. This is definitely an interesting project!

Here's a paper I would suggest reading if you can. It's a scientific paper, so it may be a bit difficult to read, but you can ask us if you need help with any of it:

Elser, J. J., Bracken, M. E. S., Cleland, E. E., Gruner, D. S., Harpole, W. S., et al. 2007. Global analysis of nitrogen and phosphorus limitation of primary producers in freshwater, marine and terrestrial ecosystems. Ecology Letters 10: 1135-1142. doi: 10.1111/j.1461-0248.2007.01113.x

Abstract:
The cycles of the key nutrient elements nitrogen (N) and phosphorus (P) have been massively altered by anthropogenic activities. Thus, it is essential to understand how photosynthetic production across diverse ecosystems is, or is not, limited by N and P. Via a large-scale meta-analysis of experimental enrichments, we show that P limitation is equally strong across these major habitats and that N and P limitation are equivalent within both terrestrial and freshwater systems. Furthermore, simultaneous N and P enrichment produces strongly positive synergistic responses in all three environments. Thus, contrary to some prevailing paradigms, freshwater, marine and terrestrial ecosystems are surprisingly similar in terms of N and P limitation.

One thing to consider in your project is that it growth is often not just limited by N or P, but very quickly becomes limited by both. You might therefore consider adding N to one group of plants, P to another, and both to a third group (along with controls, of course).

As for how to add nutrients, I would not add dog poop simply because it can add bacteria and other things to your experiment that (a) you would need to clear with your teacher, because (b) it may actually be dangerous to grow these bacteria in large quantities. Instead, I suggest going to a garden supply store and asking if they have N-only and P-only fertilizers; I'm fairly sure they will (and at worst, they'll have fertilizers that are heavily skewed toward one or the other.

Here's a great site that will tell you more about measuring pH and nitrates in water: http://depts.alverno.edu/nsmt/archive/K ... Battle.htm. Definitely measure pH if you have the opportunity--it can't hurt to measure as many things as possible, even if there is not a 1:1 relationship between pH and nitrate concentration! I'm afraid I don't know how to measure DO, but maybe one of the other experts can chime in on that.

I'm not sure what you mean by 'what part of the plant to use'. What are you trying to measure, plant growth? Science Buddies has a great site about how to do that, here: http://sciencebuddies.com/science-fair- ... owth.shtml. Take a look and see if that helps!

Thanks very much! I am going to use a 95% Potassium Nitrate 5% Sodium Nitrate Mix to add my nitrates, seeing as this is the mix which is evident in the fertilizer.
Thanks again,
scotty1345