Answer: It's really difficult to get grass to grow under a blue spruce for any length of time. That's because of the shade, the acidity of the soil and the fact that the tree roots suck up all the moisture in the soil. There aren't too many grasses that can survive those conditions for very long. Instead of trying to grow grass, why not plant a shade loving ground cover such as vinca (periwinkle) or pachysandra? You'll still see lots of green under the tree, but you'll also get a bonus of seasonal flowers.
I'm afraid I'm not familiar with the soda and soap recipe, so I can't help you there.
Best wishes with your landscape!
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