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Question

suppose u wanted to design an experiment to find out what kind of soil is best for growing cactus plants. what would be your independent and dependent variable and constants in the experiment?

Answer 1

Your dependent variable would be something that tells you how healthy or fast-growing the cactus is, for example "height after 4 weeks". The dependent variable is something that describes the soil: It could just be a list of the brands you have bought ("brand a, brand b, brand c"), or it could be a physical property, like density of the soil, or soil pH (soil alkalinity/acidity), or "fertilizer content".

As you can see, there are several independent variables (as well as different dependent variables, in addition to "height after 4 weeks", you could have "height after 8 weeks", or "number of seeds after 1 years"). To figure out what combination of density, soil pH and fertilizer content is the best, you should probably first run them independently -- that means one trial with different densities, a completely separate trial with different pH levels, and a third separate trial with different amounts of fertilizer.

When you see which values of the independent variables in those three trials give you good results -- high values for the dependent variable -- try combining those: Make soil with density x (where x is the best density found in the first trial), pH level y (where y is the best pH level from the second trial), and fertilizer content z (where z is the best fertilizer content from the third trial). Let it grow, and measure your dependent variables again.

Answer 2

To be able to say that your results were applicable to "cactus plants" you would want to have several varieties of plants to use.  But you would want to set the variables for "constant" examples of each type of cactus.  In other words, say that there are six different varieties of cactus plants.  you would want several similar (i.e., same age and approximate size) plants of EACH variety to use in setting up the experiment.  You would also want to keep the amount of water, soil, sunlight and fertilizer that you gave to each experiment "subject" plant.  You would then set up containers of each kind of soil (sandy, clay-y, loamy, mixed, whatever "type" of soil - the independent variable - you want to examine) for each subject plant, placed in identical environments - preferable the SAME environment, though that might be hard to recreate exactly.  You would then measure the outcomes - probably in terms of volume growth (the dependent variable) over a certain timeframe.