Lesson Five: Let it Grow and See What Happens

Step One: Maintaining the Experiment

During the course of the experiment, the nutrient solution will have to be replenished frequently due to water evaporation. When the level of solution is low, add deionized water until the level of solution is up to where it should be. This may seem like you are diluting the solution, but it actually maintains the concentration of the solution at about where it should be. The reason for this is that when water evaporates, it leaves behind any chemicals that are dissolved in it, so the solution becomes more concentrated after evaporation. Be sure to refill all the nutrient solution containers at the same time.

We Protist Against Algae Although evaporation is the primary reason the level of nutrient solution decreases in the container, the plants use up nutrients too. Also, algae tends to grow in a nutrient solution that is left out for a while. Because of this, you should replace the nutrient solution in each hydroponic apparatus about once every two weeks or so. When you replace the nutrient solution, first remove the pots from the hydroponic apparatus. If the felt strips are covered with green algae, rub off as much as possible with your fingers. We are not sure if the algae is harmful to the experiment, but it may use up nutrients, making them unavailable to your plants, and it makes the experiment look ugly. Place the pots in bowls of water so that the roots and the felt do not dry out.

Remove the support boards and clean them thoroughly with soap and water. Then pour the nutrient solution out of its container, wash the container with soap and water, and disinfect it with rubbing alcohol or a chemical cleaner to discourage algae growth. Make new nutrient solution in the mixing bottle according to the recipe you calculated in Lesson Three, and pour it into the nutrient solution container. When you do this, you should also pour some through the felt and the Perlite in the pots to feed the plants directly and encourage capillary action in the felt. When you have made enough nutrient solution to fill the container, replace the boards and the pots the way they were before. Your hydroponic apparatus is now ready to go again. Be sure to replace all the nutrient solutions in the entire experiment at the same time. Note in your observations each time you do this.

You should also make sure the controlled variables are staying as consistent as possible. The most important thing is that the controlled variables are the same for all the plants at any given time, but it is also important that they stay within reasonable limits to keep the plants healthy. Make sure the plants are getting about twelve hours of light (give or take a few) from the grow light or the sun. Make sure the temperature is between 55-85° F. Make sure the relative humidity is at least 45% and ideally between 60-75%.

Source: Casana, Maritza. maritza_casana@hotmail.com. "Práctica de producción de hortalizas bajo la técnica de hidroponía en agua y perlita." 7 Jan 2001. Personal e-mail. (14 Jan 2001).

Step Two: Changes in the Experiment

During your experiment, the plants may grow large enough that they are overcrowding their pots and competing with each other. When this happens, you may need to remove some plants to alleviate the overcrowding. If you do this, be sure to remove the same number of plants from all the pots in the experiment at the same time. Select for removal the plants that look the least healthy. Note in your observations when you do this.

You also may need to switch to different nutrient solutions if you begin your experiment with a starter solution for all the plants. Do this in the same way that you would replace the nutrient solutions as in Step One, but make different nutrient solutions for each hydroponic apparatus. Be sure to mix the correct volumes of each concentrated part as you calculated in Lesson Three. Note in your observations when you do this.

Step Three: Handling Emergencies

If the plants are not looking healthy, here are some things to check:

Are the strips of felt moist? If not, the nutrient solution is not coming into the pots by capillary action. Make sure the level of nutrient solution in the container is high, and make sure the felt is as far immersed in the solution as it will go. Also, if you are refilling the nutrient solution tub, pour some on the felt strips to "prime" them for capillary action. If you are not making more nutrient solution, you should do the same with deionized water.
Are the light conditions appropriate? If the plants are getting significantly more or less than twelve hours of light per day, you may need to adjust the lighting conditions.
Is the air humid enough? If not, use a hoochie-coochie machine to raise the humidity or spray water all over with a spray bottle or hose.
Is the temperature appropriate? If things are too cold, you may need to get a heater. If things are too hot, you may need to shade the plants, put ice in the nutrient solution, or use an air conditioner. The temperature of the water in the hoochie-coochie machine can also influence temperature conditions.

Spraying water with a spray bottle, though not a substitute for fixing what is wrong with the experiment, is generally good as a temporary remedy to alleviate some of these common problems.

Step Four: Making Observations

During the experiment, you may want to measure and record data on the following dependent variables:

Length of plant leaves
Color of plant leaves
Anything unusual you observe

You also may want to take pictures of each hydroponic apparatus to document the growth of the plants subject to different treatments.

At the end of the experiment, you may want to measure and record data on the following dependent variables:

Length of plant leaves
Color of plant leaves
Length of plant roots
Color of plant roots
Weight of entire plant
Weight of edible parts (if there are edible parts)
Color of edible parts
Taste of edible parts
Anything unusual you observe

Note that you cannot make any observations on the roots of the plants during the experiment. If you pull the plants out of the pots to observe the roots, you may break off the roots, which is bad for the plants. This is especially true if your plant has "super roots" that grow out of the bottom of the pot and down the strips of felt.

You should end your experiment after a predetermined number of days or after your plants are full-grown or have developed edible parts.

Step Five: Organizing Data and Drawing Conclusions

The goal of data tables and graphs is to present data in a way that is as easy to understand as possible. Because of this, you may not want to include all the measurements you made of every plant; rather, you may only want to include the average measurements of the plants as well as perhaps the highest and lowest measurements to indicate the range.

Graphs are helpful to show visually what happened in your experiment. To show progress throughout the experiment, you may want to use a line graph of, for example, average leaf length vs. time. You might make multiple lines on the graph to indicate the average leaf lengths for plants in different nutrient solutions. You could show the same thing in a data table as well. Be sure to give titles to all your data tables and graphs, and make sure the titles clearly summarize the purpose of the data table or graph.

To show the results at the end of the experiment, a bar graph is a good choice. For example, you might make a bar graph of average plant weight for plants growing in different nutrient solutions. The independent variable (in this case, which nutrient solution) should be graphed on the x-axis, and the dependent variable (in this case, average plant weight) should be graphed on the y-axis. You could show the same thing in a data table as well.

You can then use these graphs and data tables to draw conclusions. Which nutrient solution produced the healthiest plants in your experiment? You could conclude that the nutrient solution that produced the healthiest plants had the closest-to-ideal concentration of the chemical that was your independent variable. Note any sources of error in your experiment that could cause you to draw mistaken conclusions.

Source: Campbell, Neil A. "Lab Topic 1: Scientific Investigation." Lab Manual for Campbell, Fifth Edition. Ed. Dan Wivagg. Menlo Park, California: Benjamin/Cummings, 2000, pp. 1-27.

Focus Questions

Why is it okay to add deionized water to your nutrient solution when it is running low?
Why might algae be a problem in a hydroponic experiment?
Why should you pour nutrient solution through the felt when you replenish the nutrient solution?
What conditions should you check if your plants do not look healthy? What can you do to change each of these conditions?
What dependent variables can you measure during the experiment?
What dependent variables can you measure after the experiment is over?
Why can't you measure the plants' roots during the experiment?
How do you know when to end your experiment?
How do you summarize your data to make it easier to understand?
What is a bar graph good for showing? What is a line graph good for showing?
How can you draw conclusions based on your data tables and graphs?