From: ted.held at us.henkel.com on 2006.05.15 at 16:31:15(14194)|
List members with an interest in submerged or emerse aroids should probably know that anaerobic substrate conditions in these situations is the norm. When disturbed, such substrates have very strong odors naturally associated with anaerobic organism activities. The smell is usually a high-sulfur sort and will be familiar to anyone who has spent time treading through muck in swamps and such. Naturally, if additional materials, such as dead insects, fish, or other animals are also present, the odor could ramp up from there. Of course, many on this list are aficionados of bad smells, so maybe this is not a deterrent.
Specializing in the genus Cryptocoryne as I do, which are south Asian aquatic and swamp species for the most part, I have long experience with different substrate mixes under waterlogged conditions. For Crypts, at least, here are a few conclusions I have made, based on semi-scientific experimentation.
1. Even substrates consisting of mineral sands and gravel will turn anaerobic in a day or two unless extraordinary efforts are made to prevent it.
2. If water through-circulation is attempted in an effort to keep the substrate aerobic, the plants will perform decidedly worse. These species actually seem to prefer this hostile environment, perhaps because of the relative ease of obtaining iron (this is speculation).
3. Adding fertilizer in the form of commercial NPK materials, or "tabs", is usually detrimental. Those wishing to supply NPK are advised to do so as a foliar spray. For aquatic or semi-submerged species, this may well be accomplished by having fish present.
4. Such nutrient as you might add is best supplied in the form of leaf mold or peat moss or similar partly decayed organic matter. The level of NPK-type nutrient here is almost zero. But the beneficial effect seems to be one of pH stability on the acidic side and perhaps some moderating or buffering of either beneficial or poisonous chemicals within the unseen matrix. In addition, there is the possibility of beneficial microbial flora, which may be best supported by having organic matter present.
5. Pure organic substrates suffer from the inability to dissipate gas bubbles (methane, hydrogen sulfide), a natural product of decay. As a consequence, such plantings can exhibit a tendency to float, possibly tipping over your plant. Best to intersperse judicious amounts of inorganics, such as sand.
6. Really anaerobic conditions are undesirable. To that end, supplying a gas and water permeable container, such as a terra cotta pot, allows slow exchange of chemical species, both in and out. In larger systems, such as a pond, this may not matter as much. As mentioned above, free water and gas exchange to the root zone is a bad idea. But a complete lockup is also undesirable.
7. Highly nutritious organic matter, such as fresh compost, or materials including manure, are too "hot" for these systems.
8. As time passes, any organic matter in your mix will degrade. Consequently, replanting on a periodic basis is a good practice. In large systems, top dressing might be feasible as an alternative.
9. Naturally, the nature of waterlogged soil will vary greatly in the wild. You may have a coarse organic component, or almost completely smooth muck. Your inorganic fraction may be impermeable or permeable. You may have sand mixed with some organic matter or an organic-rich layer over a relative hard-packed sand layer. This is the fun part: trying to create a combination from all these variables that will suit your plant.
While these observations apply to Cryptocoryne plants, it could be that they are more generally applicable.
I am now in the first stages of experimenting with a European idea of using semi-decomposed leaf litter from natural bodies of water as a starting medium. I understand that there may be a microbial component of success with certain species, meaning symbiotic species that assist in nutrition. In a year or two I should know if this idea is successful. Certain European Crypt growers are currently enthusiastic about leaf litter.
If anyone on the list knows of authoritative studies of waterlogged soils for horticultural purposes I would appreciate them sending me references. I also welcome well-meaning observations and criticisms of what I have said here.