Much like humans plants need calcium. Plants use calcium to build cell walls and membranes. Without the proper supply and intake of calcium, plants can develop conditions such as blossom end rot. Plants take in calcium along with water through their roots, so the calcium has to be in a soluble form in order to be useful.
Use eggs in your garden to add calcium
A great DIY method to add calcium to your garden is to grind eggshells and mix into your soil at the end of your growing season. They will break down and leave behind plenty of calcium for the next planting season. For most in the US, you would put them in during the fall. For southerners like me, they go in in the summer when a limited variety can grow in the garden. If you are composting at home egg shells can go right into your compost. Egg shells would be a green in your compost. Wash dry and grind in a coffee grinder for best composting or adding directly to the soil.
Add calcium to your garden with gypsum
Gypsum can be added to the soil if calcium is lacking and in need of acid. Gypsum is used in soil that has been farmed for many years and has built up too much salt. Most home gardens do not need this. Gypsum can affect how plants take up other nutrients, such as phosphorus, magnesium, copper, and zinc. Gypsum will last several months.
Use limestone to add calcium to your garden.
Ground limestone is most commonly recommended to increase calcium in the soil. Some limestone provides calcium only and others provide magnesium as well. Either spread over the soil then tilled into the soil. Apply the limestone in the fall at least 30 days before any fertilizer. Limestone will lower soil acidity. Limestone will last for several years.