Hi all,
You can construct your own substrate, by changing the proportion of the ingredients you can create growing media that have whole range of differing nutrient status.
If you look at the major constituents of natural aquatic sediments, you have sediments that range from totally inert (quartz rich bed-rock, cobbles, pebbles, gravels or sands) through to fine sediments that have high CEC, lots of organic matter and are nutrient rich in both total and available nutrients. These conditions are described by different terms "oligotrophic" = nutrient poor, "mesotrophic" = some nutrients & "eutrophic" = nutrient rich, but really they all grade together along a continuum with these 3 loci as convenient pigeon-holes. There are other factors that effect the plant is the substrate. Coarse substrates will normally be in either areas of high flow (upland streams, Rio Tapajos etc), or where the sediment supply in the water shed is very limited (Lake Malawi etc), oligotrophic sediments will usually be aerobic all the way through the sediment, but their base status can range from fully saturated with carbonates, to extremely base poor. Thick layers of fine sediments in eutrophic lowland depositional basins will have high "Redox" potential, which will also effect nutrient availability.
Personally I like my substrate on the oligotrophic side of mesotrophic and base poor, as I keep fish from base poor waters. This equates to a largely coarse silica sand substrate, with the addition of a slow release carbon-source (I use fairly robust dead leaves, Loquat, Oak, Camellia etc) and some clays (with a high CEC, but low in calcium carbonate) these can be either calcined (like cat litter or Seramis) or in their native state . This media layer will be largely aerobic, and I will aim to keep it as little disturbed as possible. You can think of this as the orchid, bromeliad, fern, succulent or alpine growing approach, it is aimed at growing plants with low potential growth rates in nutrient poor conditions.
If I wanted to create a more nutrient and base rich media, I would still have the same basic ingredients, but I would play with the proportions and grain size. The silica sand can become a finer grade, and I'd use one with some calcium carbonate content, I'd also lower the proportion used. The organic matter and clay constituent would form a larger proportion of the mix, and I would probably add them already naturally combined as a "brown earth" or "calcareous brown earth" soil, usually sourced from "mole hills". I would still try not to disturb the sediment, but in this case the substrate will show a stratification in to zones which are aerated (near the surface and in the rhizosphere), a fluctuating zone which may be either aerobic or anaerobic and an anaerobic, anoxic zone. This substrate will also produce CO2 from the oxidisation of the organic matter in the aerobic zone. This is aimed at growing plants with higher potential growth rates (Tomatoes, amenity turf, Dahlias etc) in conditions which are closer to optimal.
If you don't want to muck about making your own mix, you can just use 100% calcined clay, and add Osmocote as your fertiliser source, this should have the advantage of allowing you to replicate the mix used, and due to the high CEC & AEC and stable physical nature of the substrate it can be re-charged with nutrients via the water column.
All these substrates will become nutrient depleted over time if nutrients aren't added, the rate at which that depletion occurs will depend upon how tightly the nutrients are bound in the substrate and how much of them there were at the start. The other factor that will effect this is the potential growth rate of the plants used, "hungry", fast growing plants with access to aerial, or enhanced dissolved levels of, CO2 will deplete the substrate more quickly.
The factor limiting plant growth will usually be the level of one of the macro-nutrients, and in the majority it will be nitrogen that is the limiting nutrient.
cheers Darrel
