08-29-2009, 04:23 PM
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Although I have never personally used sand [not yet--I have a bag of HD play sand sitting in my fishroom, intended for a future project creating an authentic SA Rio Nego biotope], many on here do and plants grow very well. Every plant authority I have so far come across in my research and monthly magazine articles mentions sand as one substrate but recommends fine grain gravel as the best for plants; the reason is a concern over the compaction than can more easily occur with sand (being finer grain) than gravel. As you are aware of certain aspects of that issue, Herky, I'll confine my comments to how this affects plants.
In nature, the substrate in a watercourse is warmer than the water, by about one degree [Peter Hiscock, Encyclopedia of Aquarium Plants]. This creates a convection current that continually moves water down through the substrate, warming it slightly; the warmer water rises, cooling as it leaves the substrate. The water contains nutrients that the plant roots absorb. There is a complex bacteria cycle occurring in the substrate, both aerobic and anaerobic, in partnership with the plant roots which absorb nutrients, including nitrogen from the anaerobic activity, and in turn release oxygen to feed some of the aerobic bacteria.
This natural convection does not normally occur in an aquarium, so the substrate has to be open enough to allow circulation in order to provide nutrients to the roots. A fine substrate like sand will more easily compact, halting the movement of oxygen and nutrients with the water and creating "dead spots" or anaerobic cnditions. This can cause plant roots to be starved and rot. Ensuring the sand remains free is imortant. The depth of the substrate also enters into this; it must be sufficient to anchor the plants, particularly larger plants like Echinodorus (swords) which have large root systems. But the deeper the substrate, the more likely it will tend to compact. The choice of plants also enters this equation. Plants with large root systems will be much more affected by this that will plants with insignificant or no substrate roots, and these also tend to compact the substrate--just pull up a large established sword plant to see evidence of this.
There are many plants less dependant upon the substrate for nutrients. Stem plants develop roots all along the stems and thus absorb nutrients directly from the water and not via the substrate. Anubias, Java Fern, and floating plants also work this way, since their roots are anchored to objects like wood and rock and not buried in the substrate. Rooted plants like Vallisneria and Sagittaria apparently do well in sand, provided it is not allowed to compact. There is also evidence that plants absorb nutrients through their leaves [see Daina Walstad, Ecology of the Planted Aquarium].
Several on this forum use sand in planted aquaria, so they will have the best advice as to how the above issues are handled.