TOP MYTHS
Will deep soil become anaerobic?
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There is widespread belief that if one uses deep soil more than a few inches deep, it will result in an anaerobic substrate & create harmful gases (that consume oxygen).
The reality is that it principally depends on whether the soil is rich in materials available for decomposition. Soil that is too rich in organics (principally compost in commercial context) can get anaerobic quickly with depth as active decomposition consumes all oxygen. However, choosing a soil with highly degraded organics and lower organic content as a whole (only 10-20%) allows us to have very deep soil layers without issues. Hence, soil selection determines how deep we can go - compost should be avoided or used only in thin layers (an inch or less) near the surface (where there is more access to oxygen). Low organic substrates, or large grain sized substrates (such as aquasoil) can be layered thickly without issues.
In addition, plants channel oxygen down to their root zones and prevent most planted areas from going deeply anaerobic. 
Many technical hardscapes stack aquasoil high in the back to get an elevated horizon line. The coarse grain and highly degraded organic content of aquasoils prevent compaction or severely anaerobic conditions,
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How does nature survive without a limit on soil depth?
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Swords & cryptocorynes are root feeders and require root fertilisation |
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Somewhere along the hobby came the myth that plants with large root systems were "root feeders" while stem plants with smaller root systems fed through their leaves. Many plants with large root systems have large root systems because they grow in fast flowing or seasonal waters and the large root systems serves as anchors to resist being washed away, for other plants roots serve as a storage system. There is little correlation between the size of a plant's root system and nutrients absorption through the substrate or water.
In fact, most easier plants in the hobby can be grown without a substrate, hydroponic style, as long as there are enough nutrients in the water column ( this includes many species of hardy swords & crypts). Pickier species may do better with root fertilization, where certain nutrients may be assimilated more easily (PO4, NH3, Fe etc). Alternanthera reineckii, Eriocaulons, Pogostemon helferi are examples where growth is significantly accelerated by root fertilization. All plants, (aquatic or terrestrial) can take in nutrients through their leaves. Most aquatic plants have adaptations that enable better absorption of nutrients through their leaves. Submerged leaves often lack the external protective tissues in land plants to limit water loss. The epidermal (outermost) layer usually shows very little sign of cuticle formation. Surface cells are able to absorb water, nutrients and dissolved gases directly from the surrounding water while the internal system of tubes (xylem) which normally transports water & nutrients from the roots to all parts of the plant is often greatly reduced. That being said; root feeding often works well for many plants because we can introduce a higher concentration of nutrients at the root zone via root tabs. This is useful since most hobbyists aren't comfortable with dosing extremely high level of nutrients into the water column. We may not want to dose ammonia (which is very effective as a nitrogen source for plants) into the water column, but containing it in the substrate works well. Those that do sufficient water column dosing quickly realize that many common "root feeders" such as swords and crypts can grow well even purely with water column dosing. Substrates can also hold nutrients in an available form for plants through their cation exchange capacity (CEC); deeper substrate with slightly anaerobic conditions can reduce Fe3+ into the more usable Fe2+ format. |
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Joe Harvey's experiment below shows growing out a tank with only 1 side having root fertilisation, (osmocote tabs) while having good all round water column fertilisation. Both swords and crypts grew equally well on the side without additional root fertilisation.
Source: Joe Harvey |
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