Question
i am doing a science fair project on plant growth.  I am comparing how plants grow using different fertilizers.  I am using plain water, miracle gro, epsom salt, cow manure and dish soap on 5 different plants.  Can you predict which of these things i added to the soil will help the plant the most?  Also, do you have any type of growth charts I could use to monitor the height and width of my plants?  Thank you!

Answer
Plants need a certain amount of fertilizers to grow.
To make a completely fair science project you would need to start the plants in inert (e.g contain no nutrients) growing media such as rockwool, vermiculite, or builders (rough) sand. If you pot up in potting soil or garden soil, nutrients may be available which makes testing difficult to conclude within a short period of time.

However, I can say that miracle grow contain all nutrients required for plant growth, including mirco nutrients. This will bode well for these plants. This is a "concentrated" fertilizer so it should have plenty for plant growth.

cow maure is also a good source of nutrients, especially nitrogen. HOwever, it can hurt some plants due to high salt content. Therefore, if your plants die it may be due to this rather than due to lack of fertilizer.

Epsom salt only contain two (relatively minor) nutrients: sulfur and magensium (in fact epsom salt is magnesium sulfate). Plants also need many more nutrients to grow. Therefore, I dont expect the plants to live as well as Miracle grow / Manure, but it depends on the type of soil in which they were planted.

Dish soap has no nutrients needed for plant growth. Dish soap works as a "surfactant" which allows water (which may not penetrate hard compact soils) to seep into the soil. If you use potting soil (which likely is well aerated) a surfactant is not needed. Note that this is not fertilizer. It is a soil conditioner. Without fertilizer, these plants will die (but depends on type of soil in which the plants were planted)

Note that some plants, such as beans for example will have a natural supply of foods in the seed (or egg) which will sustain the plant for a considerable time.

However, if you grow the plants in "inert" material and use plants which are rooted from cuttings you would have a good test start

(you can pinch off tips of plants such as the annual impatiens and root them in water. Place in vase and change water daily. Eventually roots will form). Cuttings will take 2-3 weeks to create so I am not sure if you have time.

I am just mentioning this because your test result may give false readings (or false positives) if the soil already contained fertilizer and/or the plants/seeds you used have a natural supply of nutrients stored.

Even if you can not redo your experiment (due to time) with rough builders sand and impatiens cuttings, you may want to mention in your report the factors which may have affected / interferred with the outcome:
- existing nutrients in soil
- plants natural supply of stored nutrients

This is usually the kinds of remarks which show that you are aware of the limitations of your project, which is often much more valuable than the outcome of the experiment itself. As teachers we look for this.

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Thank you.

Kenneth