As you may know, Calcium (Ca) and Magnesium (Mg) always go together when amending soil, because one aids in the absorption of the other.

Scouring the internet for Calcium:Magnesium application ratios will yield suggestions ranging from 2:1 to 8:1. Like everything in gardening though, it's worth running your own experiments and developing your own ratios.

butternut squash and yellow flowers hanging on trellis
Squash like this butternut require a lot of calcium to grow. Without it, fruits may shrivel up and die before ever attaining full size (blossom end rot).

For calcium I grind up eggshells in a cheap old coffee grinder. Some folks also soak their eggshells in white vinegar before using in the garden.

For magnesium I grab a bag of epsom salt.

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Garden Uses for Organic CalMag

Once you've got your CalMag mixed, it can be sprinkled on top of beds and planters as an amendment. It can also be worked into the top few inches of soil (through raking or tilther).

Keep in mind that your homemade organic CalMag mix is organic and will take a number of months to begin to break down and become bioavailable.

Another use for CalMag is as a compost or worm bin amendment. By adding CalMag to your compost you can begin the process of breaking it down further and becoming more bioavailable. CalMag added to a compost bin can also eventually end up in your seedling mix, potted plants, and garden beds. For this reason I like infusing CalMag into my soil at the very beginning via my compost.

It's also worth noting that worms have gizzards which rely on gritty particles to help them grind their way through food. Eggshell powder makes a great source of grit, which is another reason to add your CalMag straight to your compost.

Regardless of whether you choose to add your CalMag to your compost pile or straight to your garden beds and planters, you'll still see the benefits in vigorous strong plant growth and fewer deficiencies.