I have spread eggshells in my garden several time this season, can I over do it?
The “how much is too much” question for any soil amendment largely depends on the composition of your soil. A soil test can tell you a lot about your particular growing conditions. The following link to the Ontario Ministry of Agriculture and Rural Affairs (OMAFRA) explains more about soil testing and links to accredited lab testing facilities in Ontario:
Eggshells are a source of calcium, which is a micro-nutrient necessary for strong cell growth. Calcium is alkaline, so adding it can have the effect of raising soil pH. However, adding eggshells, unless they are ground to a very fine powder, will probably not have a major impact as the calcium locked up in the shells will take a long time to break down.
Here is an excellent overview from Dave’s Garden on the role of calcium, and other ways to add it:
Incidentally, coarsely crushed eggshells are said to be a deterrent to slugs – as long as the leaves of the plant don’t touch the ground beyond the eggshells, acting as an on-ramp for the tricky critters. The worst that can be said of this method is that the shells show up white against dark soil.
As with all additives to the soil more is not necessarily better. Calcium is readily washed from the soil by rain or irrigation, however in this case the calcium not readily avaiable because the egg shells take so long to break down.
I hope this helps.