In a experiment what are variables and controls ?

Question

my project is growing two plants with fifferent types of soils

Answer 1

A soil experiment example: peat is the usual potting media so peat is the standard to measure against. The new concept is coir made from coconut hulls.
Because peat is a known potting media it is the control to compare the coir based media against. Coir is the variable to be tested.
First state the question you are asking about the plants growth.
Simplistic example; Will begonias grow better in peat or coir? Compare coir against the standard peat as soil-less media components for plant growth.

Possible plant variables:
Ambient temperature day and night.
Light intensity and duration.
Humidity
Nutrients in the soil.
Water and air in the soil.
Soil texture.
Soil pH
What impacts a plants growth? Check this list against the environment where the plant evolved to be sure you have the correct values for your plant of interest. What ever the normal conditions are for the plant should be used.


Research to show why coir and peat represent the same soil variable. Show they have similar soil qualities.
http://www.krgprosoil.com/library.htm
www.usu.edu/cpl/PDF/CoconutCoirPaper.pdf
www.actahort.org/members/showpdf?booknrarnr=747_35
http://www.ag.auburn.edu/hort/landscape/STGOfeb00.html

http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2007/09/05/HOHTRTMAL.DTL
http://forums2.gardenweb.com/forums/load/soil/msg0715241330846.html
http://flrec.ifas.ufl.edu/Hort/Environmental/Media_Nutrition/COIR%20potential.htm


Select a single soil mix like Peat-lite Mix A that consists of 50% sphagnum peat and 50% vermiculite by volume. A non-ionic wetting agent may be incorporated.

Organic recipe
1/2 cu. yd. Sphagnum peat or coir
1/2 cu. yd vermiculite

5 lbs. ground limestone if peat has low pH

6 lbs. bonemeal
4 lbs. bloodmeal
7 lbs. greensand
for N/P/K = 7/10/5
Adjust pH of peat to match coir first. Thoroughly blend all other ingredients. Moisten with water; store in plastic garbage bags.
http://www.aces.edu/counties/Baldwin/pages/PlantMedia.tmpl
Mix enough material for the entire experiment, but leave out the material you wish to test, like the peat moss & coir. This way all the plants will grow in the same mix that you made all at once. This will eliminate possible mixing & measuring variables in making the basic soil.
Now measure the basic mix into 2 portions by weight.
Add the peat moss to one portion at the correct level and add coir to the other at the recommended concentration. completely mix each portion. Moisten both and measure with water meter.
http://www.mountainvalleygrowers.com/rapitestwatermeter.htm
Plants may vary so to control for that grow three to five separate plants in each soil type. Ensure each plant is grown in the same amount of soil by weighing the soil that goes into each pot. Use matched pots so this is not a variable because the material and shape of a pot may effect the growth.

Food is included in the soil mix so should be the same for each plant.
Water should always be given at the same temperature. Control this variable by letting the water sit out overnight to equalize to the ambient room temperature. Record the water /room temperature each day. Record the ambient humidity each day. This can be found on a local weather site.
Give each plant the same measured amount of water. Use a moisture meter to measure the soil saturation.
Give each plant the same amount of light for the same amount of time. Control for the amount of light with a set of grow lights rather than daylight. Use a timer so the plants get the same amount each day. Adjusting the initial distance from the source so each plant gets the same exposure. Measure with a light meter each day and record the data.
What you are trying to do is reduce the possible differences in the plants environment so you are left with the one variable you are testing.

Peat needs to be pH adjusted for most plants to grow so note if the peat is labeled as pre-adjusted or test yourself. Test the coir also before use. Test the pH of each mix after the peat or coir is added to ensure they are in the plants preferred pH range.
Other parameters to consider testing-
Real soil with sand, silt and clay has a greater ability to maintain pH over time. Does the pH of the soil-less mix stay the same after the plants have grown? Test the pH of each pot at the end of the entire experiment.

The wetting agent in the mix is necessary for peat to allow it to absorb water readily. To have 4 soil conditions do 2 groups with this as the variable.
peat + wetting agent
peat without wetting agent
coir + wetting agent
coir without wetting agent
grow three plants in each group for a total of 12 plants.
How will you measure plant growth? Mass or height x width?
Another choice would be vary organic vs manufactured fertilizers and use only one soil mix.
Instead of peat or coir vary perlite, vermiculite or sand.