Plant grow and hydroponics experiments – an article for the Classroom teacher

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A key function of plants is to "mine" soils for the essential mineral nutrients they require. Humans and other animals also depend on plants to provide most of the mineral nutrients that occur naturally in the food chain. Adding mineral nutrients to crops via fertilizers and liming is a major component of agriculture and home gardening, although the inefficient use of fertilizers is also a major cause of water pollution. The use of Knop's solution, described in Demchik's article, was developed in approximately 1865 as a nutrient solution for growing plants without soil in solution culture, today often called hydroponics. If you grow plants in potting soils with a full, mineral nutrition treatment and de-mineralized water control as Demchik advises, you may as well use a commercial houseplant fertilizer, such as Miracle-Gro. Houseplant fertilizers are designed for use with potting soils. Using them eliminates the time and expense of buying the five Knop's chemicals that Demchik suggests, as well as using a balance to weigh them out.

A problem with using Knop's solution, with its seven mineral nutrients, is that although it was considered an "ideal plant-growth solution" (as Demchik states), that was the case in 1865. Since that time, seven more mineral nutrients have been found to be essential for plants. Therefore, it is inaccurate to portray Knop's as an ideal solution. The solution can give decent or optimal plant growth in potting soil if there happen to be appropriate contaminants in the salts used in Knop's solution preparation, or 12 essential mineral nutrients are routinely added to potting soils during preparation (Bunt 1988). The only essential mineral nutrients not intentionally added are nickel and chlorine because they are always present in sufficient amounts as contaminants. It is quite possible that a modern nutrient solution, such as a modified Hoagland solution will outperform Knop's, even for soil-grown plants. It would be desirable to retire Knop's solution from active use in teaching and use a modern solution that better reflects the current knowledge of essential mineral nutrients.

In addition, potting soil is not used to perform successful mineral nutrient-deficiency experiments. Instead, teachers grow plants in a solution culture that contains a modern nutrient solution. The standard approach is to examine the deficiency of one mineral nutrient at a time to determine the specific deficiency symptoms for that one mineral nutrient. Recipes for Hoagland solutions, each deficient in a single mineral nutrient, are available for such experiments (Hershey 1994). To make a solution deficient in a single mineral nutrient, you usually cannot simply leave out one salt in a Knop's solution, as Demchik recommends. For example, a Knop's solution without potassium nitrate still contains substantial nitrogen and potassium from other salts. Also, leaving out magnesium sulfate will make the solution deficient in two mineral nutrients, not only one.

For teaching, solution culture has major advantages over soil culture because the plant's roots are visible. Also, nutrient solution pH (Hershey 1992a), electrical conductivity (Hershey and Sand 1993), and nutrient concentrations (Hershey and Stutte 1991) are easily measured. Students can easily weigh the entire plant, and at the end of the experiment they can weigh the roots separately from the shoots, to calculate a shoot-to-root ratio. In addition, solution-culture plants do not require as much watering, if any, on weekends compared to soil-grown plants. Hydroponics avoids soil-borne diseases, the dust and dirt associated with potting soils, and the overwatering and drainage water problems of potted plants. It clearly demonstrates that roots do not need to eat soil, a common student misconception. Hydroponics also has a fascinating history, and its study intrigues students. The hydroponics display in the Land Pavilion was the most popular exhibit at Walt Disney World's EPCOT Center in Orlando, Florida (Hershey 1994).

The following section discusses some current solution culture basics, which I have found helpful in a teaching situation. Hydroponics appears more difficult than it is so teachers should not be intimidated by it. The results are almost always worth the slight amount of preparation required, if only to clearly see a plant root system and unequivocally show that plants are truly autotrophic and do not need organic matter from the soil.

Solution Culture Basics

Nutrient Solutions

Today, many hydroponic solution products can be purchased from scientific supply houses and via the Internet. These products simplify nutrient solution preparation because they consist of a concentrated solution or salt mixture that is simply diluted or dissolved. However, if your students will be doing nutrient-deficiency experiments, they should prepare solutions from individual salts.

The easiest way to mix your own solutions is to prepare a series of stock solutions for each salt required, in addition to making a micronutrient stock solution that contains boron, manganese, zinc, copper, molybdenum, and chlorine. Rather than iron sulfate, use an iron chelate stock solution. Chelates are a major advance because they greatly increase the availability of iron in hydroponics. Stock solutions can be stored for years in sealed plastic bottles in a dark, room-temperature location. Dispense small volumes of stock solutions, using graduated cylinders, pipettes, or plastic syringes, and dilute them to make larger volumes of nutrient solution. For example, a modified Hoagland solution number 1 requires the following ingredients per liter: 5 ml of molar calcium nitrate; 5 ml of molar potassium nitrate; 1 ml of molar monopotassium phosphate; 2 ml of molar magnesium sulfate; 1 ml of micronutrient stock solution; and 1 ml of iron chelate stock solution.

You can easily mix nutrient solutions in a standard 5-gal. plastic bucket. For ease of nutrient solution preparation, first use a IL-graduated cylinder and mark the inside of the 5-gal. bucket with a China marker or another permanent marker at 5- and 10-L volumes. It is also convenient to mix solutions directly in hydroponic reservoirs, such as those made from 2-L soda bottles. One- and 5-ml plastic syringes, one for each stock solution, are ideal for this purpose. Less than full-strength nutrient solutions are often used, particularly in the early stages of an experiment. For deficient-nutrient solutions, I recommend distilled or de-ionized water, although tap water is satisfactory if it contains minimal concentrations of the nutrient under investigation. Tap water often contains significant levels of calcium, sulfate, potassium, and possibly other essential mineral nutrients, so ask your municipal water company for a tap water analysis.

For experiments lasting several weeks, you will need to replace or renew nutrient solutions. You can completely replace the nutrient solution every week or every few days. (The change period will depend on reservoir volume, solution strength, plant size, and rate of growth.) Between changes, add distilled or de-ionized water to replace that which is lost via transpiration. Another method is to refill the reservoir with fresh solution and never completely change the solution. An inexpensive, electrical conductivity meter is ideal for quickly estimating the extent to which the nutrient solution has been depleted or concentrated by plant uptake of water and mineral nutrients (Hershey and Sand 1993).

Because they are designed for soil-grown plants, houseplant fertilizers such as Miracle-Gro are not satisfactory nutrient solutions for hydroponics; they can injure or kill plants (Hershey 1990). Comparing hydroponics using a Miracle-Gro-solution versus a Hoagland solution makes an interesting school experiment. Miracle-Gro and most houseplant fertilizers contain their nitrogen mainly as ammonium, rather than nitrate, which causes the nutrient solution pH to drop too low or causes an ammonium toxicity. Soils are buffered against pH changes and contain nitrifying bacteria that convert ammonium to nitrate. Miracle-Gro and most houseplant fertilizers also lack calcium and possibly other essential mineral nutrients.

Containers and Aeration

Plastic containers are the preferred reservoirs, because glass containers are breakable and may leach certain nutrients such as potassium and boron. Clear plastic containers are also advantageous because roots can be viewed easily. Plastic food containers, for example, 2-L soda bottles, work well. You need to cover clear or translucent containers with an aluminum foil "skirt" to block out light that would cause algae growth in the nutrient solution, however. You can use plastic, 35-mm film cans when working with dwarf or small plants. The black film cans require no aluminum foil.

Aeration is usually provided by gently bubbling air through the solution using aquarium tubing, aquarium valves, and an aquarium air pump. Slipping a plastic soda straw over the immersed end of the aquarium tubing will keep the tubing from curling. An alternative aeration method is to keep the top third to half of the root system above the nutrient solution. You can use a Mariotte bottle to automatically maintain the solution at a set level (Hershey 1994).

Plants and Light

Traditionally, crops such as corn, beans, sunflowers, and tomatoes have been used in school hydroponic studies. These crops require high amounts of light to grow well and often get very tall and require staking, however. If you do not have a greenhouse or a roomy, sun-filled windowsill, they may not be the best choices for study. For a poorly lit classroom, an excellent solution is a fluorescent light bank that has six, four-foot, cool white tubes. Small or dwarf plants such as Wisconsin fast plants (Hershey 1992b), `Thumbelina' zinnia, and `Brownie Scout' marigolds grow well under such light banks (Hershey 1994). Wisconsin fast plants complete their life cycle in approximately 35 days under continuous fluorescent light. For an edible rosette crop, leaf lettuce is ideal. Houseplants do well under low light indoor conditions and also under fluorescent light banks. Houseplants have the additional advantages in that they are easy to vegetatively propagate, and many are vines or rosettes that require no staking. The piggyback plant (Tolmiea menziesii) and the mother of thousands, or devil's backbone, (Kalanchoe daigremontiana) are easily cloned, using their foliar plantlets. Wandering Jews (Zebrina pendula and Tradescantia species) are vines and root rapidly from cuttings. The piggyback plant, a rosette, is an excellent plant for iron deficiency studies, because iron deficiency is easily induced even when its roots are coated with iron oxide (Hershey 2000).

Germinate small seeds on wet paper towels stuck to the inside vertical walls of a wide-mouth jar or soda bottle. The seeds will stick to the wet paper towel, and the paper towel will remain constantly moist because it dips into the solution at the bottom of the container. Cover the top of the container with clear, plastic film, held fast with a rubber band; however, remove it soon after germination to acclimate the plants to normal humidity conditions. When the seedlings have well-developed roots and their cotyledons or first leaf are expanded, saturate the paper towel and gently remove and place the seedlings in the hydroponic reservoir.

Seeds, particularly ones that are too large to stick to wet paper towels, can also be germinated in perlite, which is easily removed from roots prior to transplanting into solution. Germinating seeds on wet paper towels or on filter paper in petri dishes is often not satisfactory; the seedling becomes distorted. Place houseplant cuttings or foliar plantlets directly into a hydroponic reservoir in tap water or diluted nutrient solution. Covering them with a plastic bag for a few days may prevent them from suffering water stress.

Plastic lids from coffee-can or plastic food containers make good covers for hydroponic reservoirs. Cut a hole in the center of the lid for the plant with a cork borer or scissors. It is useful to also make a cut from the outer edge of the lid to the hole, so you can twist the lid to insert or remove the plant. To secure plants, gently wrap cotton or polyester fiberfill around the stem where it is inserted in the lid, being careful not to let the cotton dip in the solution. I rarely use cotton; instead, I make small holes and allow the seedling cotyledons to rest on the lid to support the seedling. Generally, rooted cuttings will also support themselves without cotton. If the plants are in culture long enough, the hole in the lid may have to be enlarged later so as not to constrict the stem.

Types of Experiments

In addition to classic mineral nutrient-deficiency experiments and comparison of plant growth in different solutions, the use of hydroponics is ideal for other teaching experiments. The major advantage of using hydroponics is that it removes a host of variables associated with commercial potting soils. These can vary widely in composition and are proprietary, which makes their exact composition unknown. Plant root and nitrogen source effects on nutrient solution pH are easy to accomplish, because the pH of the nutrient solution is conveniently measured without damaging the roots (Hershey 1992a). Hydroponics is excellent for experiments on rootzone aeration, rooting of cuttings, rootzone salinity, nitrogen fixation, root pruning, gibberellin effects, and mineral nutrient toxicities. Hydroponics is also necessary for carbon dioxide deficiency experiments to eliminate carbon dioxide from soil microbe respiration (Hershey 1995a).

Aquaculture and hydroponics are a natural combination (Emberger 1991). Younger students can grow paperwhite narcissus, hyacinth, amaryllis, and other flowering bulbs in hydroponics or use Chia Pets, a novelty form of hydroponics (Hershey 1995b). There are dozens of potential hydroponic experiments that you can have your students perform (Hershey 1994).

Consequently, I recommend hydroponics as a more efficient technique for growing plants in a wide variety of school experiments, primarily because they can go longer periods without watering than soil-grown plants in the same size container. It also offers the advantages that roots are readily observed, and the root environment is better defined, more easily controlled, and more conveniently measured.

Can we Grow plants in frozen soil ?

THINK: What are some things that plants need to grow? What might happen to a plant if it couldn't receive these things?

PREDICT: How does frozen soil affect plant growth?

YOU'LL NEED:

* LARGE PLASTIC SANDWICH BAG

* 1 1/2 CUPS POTTING SOIL

* WATER

* PLASTIC SPOON

* 2 CLEAR PLASTIC CUPS

* 12 DRIED PINTO BEANS

* TOOTHPICK

* PAPER TOWELS

* FREEZER

PROCEDURE:

[] 1. Pour the soil into the sandwich bag.

[] 2. Add 5 tablespoons of water to the soil to moisten it. Use a spoon to stir the soil as you add water.

[] 3. Fill each cup halfway with the moistened potting soil. You should have some soil left over--keep it for Step 5.

[] 4. Scatter 6 beans on the soil surface of each cup. These are

[] 5. Pour half of the remaining soil into one cup, and half into the other cup. This layer of soil should cover the beans.

[] 6. Place one cup on a paper towel inside a freezer.

[] 7. Keep the other cup at room temperature, in a shady place.

[] 8. Leave the cups alone for 24 hours. Then, test to find out if the soil is frozen: Try to stick a toothpick in the soil of each cup. Remove the toothpick when you are finished.

[] 9. Keep one cup in the freezer and the other at room temperature for about 5 days. During this time, keep the cup at room temperature moist but not wet. When you water the room temperature cup, water the cup in the freezer with the same amount of water.

[] 10. Each day, check the cups for signs of plant growth.

CONCLUSIONS

1. After 24 hours, were you able to insert the toothpick into each cup? Why or why not?

2. After 5 days did both cups show signs of plant growth? Explain.

3. Based on your findings, what kind of conditions support plant growth?

SCIENCE CONTENT STANDARDS

For Grades K-4

* Abilities necessary to do scientific inquiry

* Properties of earth materials

For Grades 5-8

* Abilities necessary to do scientific inquiry

* Structure of the earth system

INTEGRATE YOUR CURRICULUM!

Life Skills--Following directions

BEFORE EXPERIMENTING

ESTIMATED TIME: 15 minutes (plus 5 days for results)

Discussion Questions

* How does plant growth change during winter? (Possible answer: Many plants don't grow.)

* What time of the year do more plants grow? (Possible answer: Most plants grow its the spring and summer when temperatures are warmer.)

AFTER EXPERIMENTING

Conclusions:

1. The toothpick could not be inserted into the cup that was in the freezer because its soil was frozen.

2. The cup left at room temperature showed signs of plant growth. The cup in the freezer did not.

3. Plants need light, water, and a moderate temperature in order to grow. Frozen soil does not allow seeds to sprout.

RESOURCE

http://mbgnet.mobot.org/sets/tundra/plants/ This Web site features cool pictures of the plants that have adapted to life on the tundra.

References

Bunt, A. C. 1988. Media and mixes for container-grown plants. Boston: Unwin Hyman.

Demchik, M. J. 2001. Experiencing experimentation and project design. Science Activities 38(1): 25-27.

Emberger, G. 1991. A simplified integrated fish culture hydroponics system. American Biology Teacher 53: 233-35.

Hershey, D. R. 1990. Pardon me, but your roots are showing. Science Teacher 57(2): 42-45.

--. 1992a. Plant nutrient solution pH changes. Journal of Biological Education 26:107-11.

--. 1992b. Culturing Brassica by hydroponics. Carolina Tips 55(1): 1-3.

--. 1994. Hydroponics for teaching: History and inexpensive equipment. American Biology Teacher 56: 111-18.

--. 1995a. Plant biology science projects. New York: Wiley.

--. 1995b. Don't just pet your chia. Science Activities 32 (2): 8-12.

--. 2000. Hydroponics: Iron deficiency of piggyback plants. In Exciting plant science activities for the secondary classroom, 147-55. Ed. Gerry M. Madrazo, Jr. and Steven E. Dyche. Chapel Hill, N. C.: University of North Carolina Press.

Hershey, D. R., and S. Sand. 1993. Electrical conductivity. Science Activities 30 (1): 32-35.

Hershey, D. R., and G. E. Stutte. 1991. A laboratory exercise on semi-quantitative analysis of ions in nutrient solutions. Journal of Agronomic Education 20: 7-10.