Re: Does water quality affect root growth rate?

Dear Amber,

one of the most important things for you to learn in a science fair project is how the scientific method works. This web site has a good, brief explanation of the process.

The first step in the scientific method is observation. It sounds like you, or your teacher, has already observed that plants grow at different rates depending on the nutrient levels they receive.

Right now, it sounds like you have reached the step called "formulating a hypothesis". This means that you come up with a possible explanation for what you have observed. For instance, you could hypothesize that plant roots grow faster when they have more nutrients. Many scientific studies have shown that other parts of plants, such as leaves, grow faster and get larger with higher nutrient levels (Larcher, 2001). It's important to also think of alternate hypotheses. In their natural environments, many plants actually produce longer roots in nutrient-poor soil (Fransen et al., 1999; Taub, 2007), presumably so they can forage more widely for the few nutrients available.

It sounds like you are also involved in the step of the scientific method called "experimental design".� This is where you set up a way to test your hypothesis. The experimental design that you present involves comparing plant growth in four different types of water with different nutrient levels. You have to be careful, though, when you set up your experiment. You need to know exactly what your hypothesis is, so you can make sure that your experiment is a direct test of your hypothesis. For instance, if your hypothesis is that plants will grow faster with higher nutrient levels, you should know what the nutrient levels are in the different types of water that you will be using. You don't know what the nutrient levels are in your bottled water vs. your well water, and this limits your ability to draw useful conclusions from your experiment!

If you want to test the effects of nutrient availability on plant growth rate, you need to design an experiment where you know the nutrient concentrations available in the water. One way to do this would be to use just one type of water (either well water or bottled water), and to add different, known concentrations of liquid plant food to each group of plants.

In any experiment, it is important to have replication: instead of just growing one spider plant in each nutrient level, you should have at least seven or eight plants. Then you can take an average growth rate or root length at the end of your experiment. If you just have one plant in each treatment, it won't be clear whether the effect you see is really due to the variation in nutrient levels, or whether some other factor may have caused it. (Perhaps one plant was closer to a window and had higher light levels, or another plant got damaged during handling and lost some of its roots as the experiment started.) You also need to have randomization: instead of grouping together all the plants that get a particular nutrient level, you should distribute them randomly along your bench or lab table. If all the plants in a treatment are grouped together, some random factor (like increased light from an overhead bulb in that area) could influence the growth of all those plants, and make it seem like that treatment has a strong effect, when in reality the effect was caused by something outside your experiment!

The next step in the scientific method is "analysis and conclusions."� Do your data (the facts that you collected in your experiment) support your hypothesis? In this case, do plants in higher nutrient levels have a faster root growth rate? If so, you have provided support for your hypothesis. Note that a hypothesis can never be "proved." Each successive experiment that turns out in favor of a hypothesis adds more support to the hypothesis. If a well-designed, well-conducted experiment does not support a hypothesis, then the hypothesis can be rejected. You may need to do some more experiments to examine alternate hypotheses. In the scientific world, hypotheses that have been tested many, many times and are always supported by the tests become known as "theories"� (e.g., the theory of gravitation, or the theory of evolution).

In order to do scientific research, it is essential to have an interesting hypothesis and an experimental design that actually tests the hypothesis. Then, you need to examine your data and see whether it supports your hypothesis. If not, you can go back and look for a better hypothesis, and new experiments to test it. The scientific method has brought some amazing discoveries into the world, and it continues to do so every day. It is one of the most powerful tools that we have for understanding the world around us. If you can learn to understand and apply the scientific method, it will help you in whatever field you choose to pursue.

I hope my answer is helpful, although it's not exactly an answer to the question you asked. Offhand, I would guess that the plants in well water with plant food would grow the fastest, since I assume that is the combination that would contain the most nutrients, and in my experience plants grow faster in high-nutrient environments. But science is not about guesses and assumptions; science deals in things that you can measure, compare, and test, and science provides the best framework we've found yet for making these comparisons.

Literature cited:

Fransen, B., H. de Kroon, C.G.F. de Kovel, and F. van den Bosch. 1999. Disentangling the effects of root foraging and inherent growth rate on plant biomass accumulation in heterogeneous environments: a modelling study. Annals of Botany 84: 305-311.

Larcher, W. 2001. Physiological Plant Ecology: Ecophysiology and Stress Physiology of Functional Groups, 4th Edition. E. Huber-Sannwald and J. Weiser, translators. Springer-Verlag, New York.

Taub, D.R. 2007. A meta-analysis of studies on plant growth rate and allocation to roots and shoots. Nature Precedings Pre-publication Research, available online at http://precedings.nature.com/documents/185/version/1.