The recipe is suggesting indoor household use only. I would use caution outdoors. There is no question that it would have an affect on the soils PH. I have no Idea what affect it would have on the microbes...might pickle them
The synthesis of alum can be veiwed here...
http://michele.usc.edu/classpages/chem1 ... -lab2.html
Her is some more infomation:
alum - a double sulfate of of ammonium or a univalent metal (such as sodium or potassium) and a trivalent metal (such as aluminum, iron or chromium): it is used as an astringent, as an emetic and in the manufacture of baking powders, dyes and paper; the commonest form is potash alum (potassium aluminum sulfate).
According to the National Food Safety Database, alum is used as an ingredient in baking powder and is used as a crisping agent in the production of pickles and maraschino cherries. It is used only in a soak solution and is washed off thoroughly before completing the recipe. It is also used to harden gelatin. Alum's medical uses are as an astringent, a styptic and an emetic. Some people say that placing a small piece of alum on a cold sore causes healing to take place much faster, but I wouldn't do that myself.
The USDA says that using alum to firm fermented pickles is unnecessary, and that food-grade lime can be used instead. But lime has its hazards too. Excess lime neutralizes or removes acidity and so must be washed out to make safe pickles, and the cucumbers must be rinsed and re-soaked in fresh water.