I'm reposting this from my earlier post from the "Miracle Gro works" thread, with a few minor tweaks.
Do I love it or hate it? Well . . .see below. . .there is more than one kind of "Miracle Gro" and you can't lump them all together. In my experience, the soluble fertilizer does do what its supposed to do, and bang-for-buck there is actually pretty high. The Miracle Gro potting soil can certainly work, but I think there are much better mixes out there, and given the choice, I'd usually pick something else.
Cannabis is a common flowering (ie "weed") plant that thrives in temperate climates all across the planet. Its not some exotic tropical plant requiring special nutrients or conditions to grow. All it needs to thrive are nitrogen, potassium, and phosphorus, plus trace elements, the same as just about every other common flowering plant on the planet. Synthetic commercial fertilizers containing nitrates and trace elements (yes. . .like Miracle Gro) are used to feed all sorts commercially grown plant products, including tobacco, which, last time I checked, is also smoked.
Yes, some formulations are going to work better than others for given applications, and I don't think anyone is claiming that Miracle-Gro is hands-down the best product out there for cannabis growth.
This idea that marijuana requires some super-duper special/secret fertilizer to thrive is just nonsense. It defies basic plant biology, common sense, and most people's experience in growing, I think. The plant doesn't care or know what the source is of the elements it takes in, it just has needs that have to be met for optimum growth. They can be met in different ways, and everyone has their own personal formula that they like for different reasons, including personal style and superstition. What Miracle Gro has going for it is that its readily available and inexpensive, and those are pretty useful features!
Can Miracle-Gro work? Not only can it work, it works GREAT. . IF you use it correctly and understand its limitations. I've seen it used many times with considerable increase in both plant vigor and size.
To use it, first of all you have to pick the right product designed for your particular application. Right now, there are more than a dozen different products branded as "Miracle-Gro", including soluble fertilizers you add to water, slow-release fertilizers, and Miracle-Gro branded soil, and obviously they don't all work the same. Just saying "Miracle Gro is great" (or terrible, etc) isn't really all that informative; you have to be a little more specific.
For the casual cannabis grower, who simply wants a fertilizer to add in regular waterings, that could be "all purpose" Miracle-Gro for vegetative growth, and high phosphorus "Bloom buster" for flowering. Other Miracle-gro branded products could work too, again, depending on what exactly you're trying to accomplish (eg outdoor vs indoor, etc). As mentioned above, Miracle-Gro soil mixes already contain added nutrients, so if you use one, you probably shouldn't add more until they're depleted.
Next, as with any chemical fertilizer, you have to use it as directed. With any soluble fertilizer (not just MG) its very easy to mix up fertilizer that's too strong, or to use it too often, and these are common mistakes of new growers. A teaspoon isn't a tablespoon, and "a little good" does NOT equal "a lot better!". As a general rule of thumb, seedlings don't need any fertilizer, and young plants usually need less than normal strength. Once your plant is in good growth mode, you can typically get away with fertilizing only once per week at normal strength. If you want to fertilize more often (say every time you water) then you have to use less than normal strength, or you can damage the plants. If you are fertilizing heavily, then every once in a while (say once per month) a flush with lots of water is good to remove any possible built up fertilizer salts from your soil. And of course you may wish to flush with water 1-2 weeks before harvest, to ensure that there are no residual traces of fertilizer lingering to shade the taste of your harvested plants.
If your leaves are turning way dark green, you've got plenty of nitrogen and can probably back off the fertilizer. If your leaf tips are turning brown, now you've got WAY too much fertilizer and have leaf burn. Flush with water and back off the fertilizer.
Last caveat is that while Miracle Gro actually does have good trace element content, depending on your strain, soil, and other growing conditions, you *might* benefit from some additional mineral supplementation like added magnesium (eg epsom salts) or calcium (eg lime).
Do these things, and I think you can have as good results with Miracle Gro, as with any other commonly available fertilizer.