There have also been similar experiments that have been conducted in the past. For example, Reinders (1942) and Commoner and coworkers (1943) have collected data showing that potassium ions promoted water uptake by potato slices, thus also increasing their growth rate. They mainly focused on the effect of auxin of the uptake of water from a hyper-tonic solution and found that the effects of auxin on growth was promoted by potassium ions (Parpart 1949).
Strengths
Since the 25 samples of Rotala indica were grown in separate cups, if one of the samples was contaminated, it would not affect the growth of the other samples. On the other hand, if I had grown 5 samples in the same environment, one contaminated sample would have detrimental effects on all other samples as the samples would come into direct contact with the contaminated substances. These foreign substances could potentially cause fungal growth among other things. Therefore, this method allowed for the prevention of all samples in a group from being impacted by only one sample.
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Weaknesses
The apparatus that was used had a relatively large error margin compared to the masses of the plant samples and potassium chloride measured. For example, the centigram balance measured values to two decimal places, but the masses of the plant samples were also very small (~0.10 g) meaning that there is a larger uncertainty. Throughout the experiment, the plant samples were not sterilized. This may mean that other unknown substances were present on the samples during the entirety of the investigation. This may result in the growth of substances that take the nutrients and carbon dioxide needed for the Rotala indica samples to grow, thus limiting its growth.
A source of error may have occurred while creating solutions. After measuring the appropriate amount of potassium chloride needed to create the solutions, it was dissolved in the distilled water. The 250 ppm and 1000 ppm solutions were left to dissolve overnight whereas the rest of the solutions were mixed with a stir rod and immediately transferred into the plastic cups. Since these solutions were not left to dissolve overnight, there may have been an unequal distribution of potassium ions in each sample. The potassium ions may have gathered at the bottom of the beaker, leaving the last cup with a greater quantity of potassium ions compared to the other cups. In order to improve the procedure, there should be time left for the potassium chloride to completely dissolve before transferring it into cups.
Another error may have resulted from the use of forceps to plant the samples into the aquarium substrate. As the forceps pinch the stem of the plant, this may have caused damage to the plant sample, resulting in no further growth. This may account for the reason why no growth was seen in some of the plant samples even though other samples grown in the same concentration had increased in mass. In order to avoid this issue, sterile gloves could be used to gently plant the samples in the substrate since the strength with which we hold the stem can be better controlled.
An uncontrolled variable was the starting mass of the plant samples. I tried to keep the starting mass around 10 g but nevertheless, some samples were greater in mass than others. Samples with a lower starting mass increased in mass more quickly as they would require less potassium and other nutrients to grow. I could minimize the impact of this by having larger samples because a small difference in the starting masses would not have as much of effect on the end mass.
A variable that I tried to control was the temperature. I did this by growing the plant samples in the same room at room temperature. But, I did not have a sure method of controlling the temperature. This may have resulted in the solutions being slightly different temperatures. This would have a large impact on the results of the investigation as higher temperatures increase the rate of growth of plants (Hatfield 2015). This may account for why some samples grew at faster rates compared to other samples grown in the same concentration of potassium ions. The use of a device that would regulate the temperature of the solutions, such as an aquarium heater, could be used to control the temperature.
Extensions
Since the presence of other essential nutrients such as nitrogen, phosphorus, magnesium, sulfur and calcium is vital to plant growth, the effect of varying concentrations of these nutrients on the growth rate of Rotala indica could be investigated. Specifically, nitrogen is present in many fertilizers on top of being an essential nutrient for plants. Quick release nitrogen fertilizers provide nitrogen immediately for the plant (Wise, n.d.). Since the presence of both potassium and nitrogen aid in the growth of plants, the differences in the ways that nitrogen impacts plant growth compared to potassium could be investigated. Furthermore, since an excess of any nutrient could be potentially detrimental to the growth of the plant, an experiment could be carried out to determine the concentration of nitrogen at which negative effects could be observed.
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