Which substrate should I choose?
Choosing your substrate is perhaps the most important part of the process. What kind of jarrarium are you going for? Do you want an au natural jar that mimics a pond? Are you going for a finely crafted piece of art, and the jar is just the medium? Are you primarily concerned about caring for some healthy plants? Your goal determines the substrate.
If you want a little piece of a pond you should, of course, just use pond mud. Gather it by scooping a couple handfuls from shallow water. Transport it in any fashion; just make sure it has a little water covering it. Mud from a natural pond is rich with nutrients, bacteria, and invertebrates that make up the foundation of a healthy ecosystem. It also inevitably carries the seeds of life – eggs of tiny zooplankton like water fleas (Daphnia) and traces of algae that will eventually propagate.
If you want a decorative centerpiece, you should just use sand or gravel. It harbors less life, which keeps a jar cleaner, and doesn’t cloud the water. You can gather sand from a beach or simply use pea gravel from a garden store. Aquarium gravel or sand is also fine. If you choose one of these substrates, you will lack some of the stability that an organic substrate provides – your jar will be less resilient in terms of maintaining essential chemical levels. That doesn’t mean that this type of jar is invalid, though!
Finally, for the aquatic horticulturists among us, you need to do a combination of both of the above. Regular potting soil capped with sand will allow your plants the freedom to grow roots while remaining securely anchored to the bottom. It is important that you include a little bit of pond mud to inoculate the soil with the appropriate bacteria and invertebrates. This substrate paradigm balances the pragmatism of mud and the aesthetics of sand.
Take into consideration the needs of the plants and animals you want to include. Many kinds of aquatic plants don’t uptake nutrients through their roots – they merely use them for anchoring themselves to the bottom. Those plants wouldn’t mind a non-organic substrate like rocks or sand. Other plants, of course, do want dirt. Most of the animals we will discuss in this guide don’t require soil of any kind. However, some (like snails) are most comfortable when they have the ability to burrow under the substrate a little bit. The substrate won’t be the difference between life and death in such an instance, but it could make your little buddies happier.
If you choose to omit soil or mud the most impacted group will likely be the micro-organisms. Many of them live part of their life cycle in the top couple inches of mud. You’ll miss out on seeing thread-like polychaete worms wiggling around, but the overall health of your jar is unlikely to be affected. Your plants might grow slower, and chemical levels will fluctuate a little more. You will definitely see more accumulation of debris, too. If you don’t use mud you may prevent some “biofouling” (that brownish green gunk you find on docks and such, which is mostly algae). In my opinion, the pros of using pond mud outweigh the cons though – I recommend it!
Alright, we’ve got dirt. Next is learning about the water part of your jar!