Also, you can see my response to JM's post, but in regard to your 4th answer: Yes, you could definitley change up some variables, but I also think you can make your goals/claims more specific and easy to measure. Things like plant height, time of germination, and leaf size are more narrow goals and much easier to measure. Another scientist could argue with you about what "fastest growth" actually means. With more specific claims, it's also easier to see if you have success in the experiment or not. 

I like to see you thinking more deeply about your results, and I think you can get even more specific with your answers. You say the amount of dirt and rock is a possible explanation, but never actually state if you think it was too much or too little. Obviously I can get what you're saying with context of your next two sentences, but people need to understand the take-aways from your entire experiment by *only* reading the conclusions. 
Also, if you so happen to design another experiment in the future, make your claims more attainable! Instead of a broad statement like "grow the fastest" (which puts a lot of pressure on the experiment and makes the experiment seem competitive), alternative claims could be "increased height", or "earlier germination", or "larger leaves". All of those characteristics I would qualify as faster growth, but it makes your goals much more specific and more easily measurable. Another scientist could argue with you about what "fastest growth" actually means. Making your goals/claims more attainable may almost sound like cheating the system (and it kind of is), but it also makes the *point* of your experiment much more clear and easier to measure success against. Again, great work, and I hope that's helpful feedback. 

If any of you were interested in looking how traits are affected by the environment, I suggest you look research the term "epigenetics". It's quite fascinating! I would agree with you that the environment can have affects on particular behaviors or traits a plant may show. Large changes, like a cactus's ability to store water as Andrew mentioned below, is a huge shift in phenotype and are traits that evolved over the entire evolutionary course of that species of cactus. These huge changes would be in the plant genome (deletions, insertions, rearrangements, duplications, etc.!). However, tons of little changes happen in the environment, and plants need to adapt to these too. So the genetic alternative to huge changes to genes is what's called epigenetics. Epigenetics is *how* a gene is expressed, and can be regulated during the life of plant. This is how genes (and/or behaviors) get "turned on" or "turned off". Sometimes this may be more beneficial than hugely altering the plant genome, because the individual can better respond to a variety of small changes in conditions. Obviously I'm ranting a bit here, but I think it's pretty neat stuff!