Egg shell fertiliser for plants

Research on the commercial scale of this subject is ongoing. However, it's possible to make this type of fertiliser at home by baking egg shells in the oven and then crushing them manually using a pestle and mortar. Recycling egg shells in the garden is a great way to reduce food waste while providing nutrients to plants. Bear in mind that, in such small quantities, it's unlikely your home-made egg shell fertiliser would make a significant impact on plants.

How do egg shells benefit plants?

Egg shells are principally made up of calcium, in the form of calcium carbonate, which is better known to gardeners as lime. A typical egg shell contains just over 2g of calcium, which makes up 95 per cent of the shell. Other beneficial nutrients and minerals contained in small quantities are phosphorus, magnesium, potassium, zinc, iron, and manganese. As a plant food, egg shells are therefore perfect for lime-loving plants such as brassicas, but are best avoided as a food for acid-loving plants, such as rhododendrons.

The simplest way to use egg shells to benefit plants is by composting, adding the shells to garden and kitchen waste in a compost bin. To prepare egg shells for using on plants or to put in a traditional compost bin, rinsing in water first before hand-crushing the shells is recommended because the traces of egg white left inside the shell attracts wild animals, such as rats. There’s no need to wash the shells if putting in a wormery or a rat-proof compost bin – simply crush the shells by hand before putting them in your kitchen waste caddy.

Although placing crushed egg shells around plants is often mentioned to keep slugs and snails at bay, recent studies have shown that they have little or no benefit as a deterrent.

Can you use egg shells on all plants?

Certain crop plants have a high calcium requirement and are more likely to benefit from the application of egg shells, particularly as this may help avoid certain problems. For example, a lack of calcium causes disorders such as blossom end rot in tomatoes, which shows as a dark brown patch on the fruit, and bitter pit on apples, where the fruit is pitted with tiny brown blotches. However, what must be borne in mind is that it is more commonly a lack of water that causes these disorders, rather than a shortage of calcium in the soil, as plants can only take up nutrients when dissolved in water.

Other crop plants to prioritise with a dose of egg shells include potatoes, cucumbers, peppers, courgettes, marrows and squash.

Avoid adding egg shells in quantity to lime-hating (acid-loving plants) which include blueberries, rhododendrons, pieris and azaleas.

Before adding lots of egg shells to the soil, it is advisable to carry out a soil pH test to determine its level of acidity or alkalinity.