MadSci Network: Agricultural Sciences
Query:

Re: We need a growing medium that has little or no nutriens.

Date: Fri Jan 5 04:56:58 2001
Posted By: Thiam Hock, Tan, Secondary School Teacher, Science, Dunman High School
Area of science: Agricultural Sciences
ID: 975785699.Ag
Message:

Dear Ms. Donelan's 3rd Grade Class:

Hello, nice to hear from you all! I read through your list of items you wanted to plant your seeds in. Some of them would probably count as having 'no nutrients', namely the rocks and styrofoam. The rest, however, probably would contain substances which might help plants grow, e.g. clay is a type of soil, and some plants flourish in it, a fruits and vegetables may also contain some of the vitamins and hormones which would also helpe the plant grow.

However, I think some of the media are quite unusual, i.e. the shirt and crickets. I have no idea what would turn out with them, though, so perhaps you could try. Other stuff with virtually no nutrients and that could support a growing plant would be powdered glass. You might want to ask Ms Donelan about that :).

Let me elaborate a bit on what nutrients plants need. Nutrients are the 'food' of the plant, which help them grow by being the raw material of the parts of the plant. As you know, plants are made up of cells, which consist of water, proteins (which have many, many functions in the plant, e.g. as enzymes and other 'building blocks'), fats, salts, carbohydrates (like sugar), chlorophyll (the green stuff in leaves which converts carbon dioxide and water with light into oxygen and sugar), and other stuff like DNA (which gives 'instructions' on how the plant will grow).

But, for all of these to form, the plant must obtain nutrients (usually from the soil and atmosphere) like oxygen, carbon, and hydrogen from the air and water. These four components alone make up 96% of the plant's mass. Imagine! So much of a plant used to be air and water. Other important nutrients are nitrogen, which comes from salts called nitrates in the soil; potassium, calcium, magnesium, phosphorus, and sulphur from salts also found in the soil; and other so-called 'trace elements' or 'micronutrients' like iron and boron, which are only needed in very small amounts, but still important to the plant. Nutrients are found in the environment and have to be taken in by them through their roots or leaves.

Plants also need chemicals called 'hormones' which act like 'reminders' to the plant, affecting how it grows. For example, ethene (C2H4) is produced by most plants, especially the fruits, which ripen faster when gassed with ethene. It can also cause the leaves of a plant to fall off (abscission), among other effects. Likewise, other hormones which are more complicated can cause a plant to grow faster, to flower, and so on. Hormones are produced by the plant itself.

Now on to germination. Before a seed can germinate, it may require special 'stimuli' for it to begin to sprout. These stimuli (an event which causes something else to happen) are different from plant to plant, so one might not work for the other. If a seed is to grow, it must be 'viable', this usually means that the embryo inside must be alive. This is why scientists doing experiments like yours will use many seeds and take the results on the whole, rather than just relying on one seed alone since it may not be viable, giving a false result.

Some seeds are 'dormant', that is, they may have a seed coat that is thick and tough, and need to be scratched or rubbed away before it can take up nutrients and grow. Nature does this for the seed when it is scratched by rocks, soil, animals, rain, and other such things. Some fleshy fruits, like apples and tomatoes, contain chemicals in the fruit (don't worry, they are safe for humans, at least) which prevent the seed from germinating while inside the fruit, so it must be outside the fruit before a new plant can grow. Imagine what would happen if it did not!

Other seeds, especially those living in cold climates, need a wet period followed by a cold period before they can germinate. Why do you think this is so? Think about the place such seeds grow in.

Another very important factor to take into account is the temperature. Most plants will germinate well when the seeds are kept at about 20-30 degrees centigrade. Other substances needed when the seeds germinate are air and water. These are also 'nutrients', if you use a strict definition, but of course, you cannot do your experiment without them or the seeds will surely not germinate!

So, to sum up, there are many things involved, all which must be just right, before a plant can germinate and grow. This is why gardening is so fun, and also so difficult :). Radish seeds are quite easy to work with though. I hope you didn't find my explanation too long or complex, I hope Ms Donelan won't complain about it :. Well, I hope this would be informative for you all, and good luck with your project! I'm sure it'll turn out well if you all are careful.

Keep up your interest in science!
Thiam Hock "Radish" Tan

References:
Stern, Kingsley R., Introductory Plant Biology. Boston, McGraw- Hill. 8th ed., 2000.
Hessayon, D.G., The Garden Expert. London, Expert Books. 2nd ed., 1993.
Leow, Atomic, A Guide to Hydroponics. Singapore, S'pore Science Centre. 2nd ed., 1995.


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