Nico’s Nuggets: Understanding Grow Mediums & Additives

Dear Nico,
Thanks for the years of great grow advice. I am relatively new to growing (second cycle) and I am wondering about grow mediums. Can I add ground up eggshells and coffee grounds to my medium? I hear eggshells can add calcium and grinds are also beneficial. Hope this is not a dumb question! Thanks again, Devin F. via

Dearest Devin, thanks for writing in and having no fear! There are no dumb questions when it comes to educating yourself and your question is no exception. Soil additives and amendments can be a complicated matter.

To begin with, many growers amend their grow mediums and they do so for a variety of reasons, using a variety of additives. The most common reasons to amend soil is to either a) increase its nutrient value or, b) aid in aerating the medium. Most commonly, soil or soilless (peat/ coco/ sphagnum-based) mediums are amended. If the intent is to increase nutrient values in the medium, it is often recommended to use organic elements or minerals. Eggshells would fit this criteria and I have heard of people using them. However, there are some possible problems to consider with eggshells which is their inconsistency in breaking down and the possible bacteria they may carry that might be harmful to the beneficial bacteria we usually find in soil. If the intent is to enrich the medium with calcium, there are Calcium-Magnesium (“cal-mag”) supplements available that are relatively inexpensive. Barring that, my recommendation would be to use a softer additive that would be gentler on the medium (and roots!) rather than eggshells.

Perlite helps aerate mediums while retaining moisture.

As far as coffee grinds go, this is a new one for me. I’m not sure what the nutritional value would be there, though I am sure they breakdown into a couple of useful minerals. Still, I would not recommend the grounds, mostly because finely ground materials (even those used in standard horticultural such as coco-coir) tend to pack down tightly after watering and render mediums too compact for adequate air penetration into the medium. This is crucial because the plants’ roots actually breath in oxygen and use it quite extensively during the dark cycle. Common aeration additives are those that are organic but do not readily breakdown such as wood chips or hardened-expanded clay (HEC).

HEC is enert and excellent for a bottom drainage layer for potted plants.

Some growers also like to amend with perlite or vermiculate which are not only great for aeration, but also excellent at retaining moisture, helping to keep soil mediums moist throughout the day. Other additives, such as lava rock or stone pebbles are often added as a bottom layer in containers (under the primary medium) to both aid in aeration (from the holes in the bottom of the pots) and drainage. Remember, while moist grow mediums are good for plants, it is also wise to allow the medium to dry out completely once or twice a week which will aid in medium aeration.

Top photo: Healthy plants starts with healthy medium.

Thanks for reading everyone and remember: Grow… And help the world grow, too!

Got questions? Email ‘em over to Nico at and be sure to put “Nico’s Nuggets” in the subject line!

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