Miracle grow has a substance within it which makes it remain in solution for long periods of time. It is called a chelate it helps to keep the Miracle grow from forming chemical bonds. Unfortuantely this chelate builds up in the soil. Ineffect this chelate becomes a toxin. Not to mention the salt nutrients in Miracle Grow are poor quality. The only thing you can do is flush your soil with water. Flush so that some water runs out the holes in the bottom. Then the next time you fertilizer use a higher quality fertilizer.

I hope this helps,

Herbus