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The Phosphate Experiment: Don't Try This At Home (Or Anywhere Else)
Objective:How do the proportions of phosphate and nitrate in a radish plant's nutrient supply affect its growth?
Hypothesis:Phosphate promotes root growth and nitrate detracts from root growth.
Prediction:If we feed radish plants a nutrient solution with more phosphate and less nitrate, then they will grow bigger roots.
This experiment was kind of an afterthought; it came into being during Experiment One when we were frustrated because the radish roots weren't growing fast enough. Consequently, this experiment was carried out on a very small scale and was kind of unscientific. However, we thought the results were interesting enough that the experiment was worth publishing.
We mixed nutrient solutions according to Dr. Alan Cooper's formula in the manner outlined in S.H.A.R.P. Lesson Three. We used the normal nutrient solution from Experiment Two, and we made a second nutrient solution using four times the amount of potassium hydrogen phosphate, half the amount of calcium nitrate, and no potassium nitrate. This solution was called Formula R because we thought it would produce Real Red Radish Roots. We also used the calcium nitrate and EDTA iron that we had made for Experiment Two.
Our control was the "Normal" plants in Experiment Two. For this weird little experiment, we built a mini hydroponic apparatus with one support board sitting atop a beaker of Formula R nutrient solution. On this support board sat a pot with felt and Perlite similar to the ones in the larger hydroponic apparatus described in S.H.A.R.P. Lesson Four, Step One.
Into this pot we transplanted several plants that had been crowded out of their pots. They spanned both generations of plants; we had some from Experiment One and others from Experiment Two. Because of this, we ended up with two separate harvests from this hydroponic apparatus.
We refilled the nutrient solution many times during the lab. Almost every day we added more water to replace what had evaporated. We turned off the fan in the greenhouse to reduce evaporation, but then the greenhouse got so hot that we had to turn it back on.
This experiment was subject to the same problems that faced Experiment Two. There was one week when the greenhouse got really hot; we tried to help the plants beat the heat with a sun shade and ice in the nutrient solution. The hoochie-coochie machine helped maintain an adequate level of humidity for the plants to grow. And the algae would not go away. We tried to clean it out of the system every time we changed the nutrient solution, but it just kept coming back! We weren't sure if it was harmful to the plants, but we thought it might use up nutrients from the system, and it looked pretty disgusting too.
We made a mistake at first when we fed the plants in this experiment a nutrient solution that was improperly mixed and thus was way too concentrated. Not surprisingly, the plants didn't grow too well with that. When we realized the mistake and changed the nutrient solution, the plants started to look a lot better.
One of our dependent variables in this experiment was how the radishes tasted. When the older generation of plants was full-grown, we asked Ms. V and Ms. Nick to taste the radishes and see what they thought. We gave them samples of the normal radishes from Experiment One and samples of the radishes from this experiment without telling them which was which.
We also asked Ms. Nick and Mrs. Cottingham to taste the second generation of radishes from this experiment and compare them to the ones from Experiment Two:
From these strange results, we concluded:
We did not make any quantitative observations during the experiment (we only took pictures), but we did measure the weight of the second generation of radish plants and the weight of their roots at the end of the experiment in order to compare how the high-phosphate nutrient solution had affected them. The results turned out very interesting:
Compared to the radishes in the Normal nutrient solution, the radishes in the high-phosphate solution seemed to grow better. The average weight of their roots at the end of the experiment was much greater than that of the radishes in the Normal solution, and their average total weight was higher too. Additionally, when one compares the weights of the edible roots to the total weights of the radish plants, the plants grown in the high-phosphate solution had larger roots compared to the rest of the plant. Since we changed two independent variables (a dumb thing to do), we could not draw any conclusions about the separate effects of phosphate and nitrate on root growth, but we did conclude that a solution with more phosphate and less nitrate does indeed promote root growth.
However, we would not recommend growing radishes in this solution, and it was a good thing that Ms. V stopped us from changing all the plants to high-phosphate solutions in the middle of Experiment Two. The reason for this is that the radishes from this experiment didn't taste too good compared to the Normal radishes in Experiment Two. We could not agree on how these radishes tasted, but our taste-testers concluded they were either too bland or too powerfully spicy. Despite the advantage in root growth, we concluded that radishes should not be grown in a nutrient solution with higher phosphate and lower nitrate than normal.
© 2001 S.H.A.R.P.: The Super Hydroponic Awesome Radish Project. All rights reserved. Photographs from this page may not be used without permission.