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You can always buy those expensive online fertilizers (over $40 for a 3 lb pkg in some cases) but I have been using spent (used) coffee grounds for years. I even use it around rose bushes. It's simple to do, as you'll see below:
Spent Coffee Grounds
Believe it or not, those coffee grounds left behind from your morning cup of Joe have serious power.In fact, they are one of the best organic fertilizers around!
Coffee grounds are a great source of nitrogen and other trace elements for plants.Coffee grounds contain nitrogen and other trace minerals that help to build soil vitality. And vegetable plants, as well as perennial and annual flowers use that nitrogen to bloom and produce.To use, either work into the ground when planting, or spread grounds around the base of plants as they grow.
You can also dig a hole when planting something (trees, bushes, flowers, veggies, etc) and put a handful of coffee grounds in the bottom of the hole, cover with just a thin layer of soil and then plant your item and finish filling in the planting hole. It's better to have the grounds at the top of the plant so that when you water it will wash the nutrients down into the soil, but whatever works for you!
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Here's another cheap but good idea people could/should be using. Too many people put egg shells into their garbage disposals and that is a terrible idea. A plumber told us that years ago. He said the shells are just heavy enough so that when they are all ground up the water used while using your disposal will not wash them down into the pipes very well, so a lot of them lay at the bottom of your disposal and will eventually stink, and the ones that DO go down the drainpipes will come to a "curve" in the pipe and end up just laying there until they build up enough to "catch" other foods that sometimes get washed down without being well ground and then you've got a plumbing issue. And plumbers ain't cheap! Never were.
Good info re: using egg shells in your garden and keeping them out of your plumbing. Not to mention they will do YOUR garden projects no good at all if you put them into a garbage can, right?