Eggshells are calcium-rich organic material that decompose well when mixed into or layered on top of soil. Roses require heavy amounts of organic material to grow to their ultimate potential. Eggshells can be tilled directly into soil or added to compost or manure. Eggshells add nutrients, can help to stabilize the pH level and can act as a natural deterrent against pests.
Plants, including roses, require large amounts of calcium in order to thrive. In terms of quantity needed, calcium ranks as a secondary nutrient behind nitrogen and potassium in plant nutrient needs. In plants, calcium helps to maintain the chemical balance of soil, increases metabolic rate and helps to neutralize cell acid. Egg shells are composed of about 95 percent calcium and therefore can provide roses with their full calcium requirement. There are no studies indicating harm caused by calcium overdose on plants, so tilling eggshells both into the soil and layering on top of the soil can be done simultaneously.
Aside from calcium, eggshells contain lime and protein. Lime is added to soil when it is too acidic (low pH level) to neutralize the acidity and raise the pH level. Roses grow best at a moderate pH level of about 6.5. Lime adds magnesium to the soil and helps increase aeration and drainage. Most commercial fertilizers contain lime.
Eggshells can be added to home compost piles to add calcium and lime to the mix. Eggshells act as an alkalizer when added to compost and do not change the nitrogen-to-carbon ratio. Home compost can be tilled into rose soil or crushed eggshells can be mixed into a commercial fertilizer or manure. Roses greatly benefit from large amounts of organic matter, the best ratio is 50 percent organic material to 50 percent soil.
Crushed eggshells can also be scattered on top of the planted rose bed as a mulch. The nutrients seep into the soil with every rain or watering. To hide the eggshells, a second layer of bark or moss mulch can be layered on top. Eggshells placed on top and exposed to the air have a double use; aside from adding nutrients to the soil they act as a deterrent to slugs and cats. The eggshells lacerate a slug's underside, so slugs that crawl across broken eggshells turn back.