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The Soil Substrate Or Dirted Planted Tank - A How To Guide - Part Two

In the first instalment of this two part article the basic components were explained. Part two looks at how these components need to be put...
  1. TimHarrison
    Soil depth
    Start by placing your slightly moistened soil in the bottom of the tank to a depth of around 2cm, you can slope it up toward the back to 4cm or so if desired. I've achieved excellent results with deeper soil but there is a danger it may become too anaerobic. Aquatic sediments are anaerobic by nature and macrophytes have evolved to grow in them but if the sediment is too devoid of oxygen plants have to work harder to uptake nutrients from them. Further, hydrogen sulphide may build up to levels that inhibit root development and therefore plant growth. However, hydrogen sulphide is unlikely to harm aquarium critters since it is quickly oxidised to harmless sulphates in the presence of oxygen.


    Soil retainer
    I usually allow a 1cm - 2cm gap around the edges, for cosmetic reasons and also to discourage unsightly algal growth up against the glass, especially blue-green algae or cyanobacteria as it is correctly known. Once this has been done I will cover the soil with a gravel tidy or soil retainer, but this isn’t essential. A soil retainer is a sheet of fine plastic mesh, the type usually used for greenhouse shading. Plants will naturally extend their roots down through the sand cap and soil retainer and in to the soil substrate below, often in a matter of days. The phenomenon is known a geotropism and occurs in response to gravity. The advantage of using a soil retainer is that it minimises any soil disturbance and resulting turbidity during aquascaping, and ongoing maintenance.

    Capping substrate
    Cap the whole lot with about 3 cm of well washed gravel or sand, sloping to perhaps 4cm – 5cm at the back. A good choice is pool filter sand grade 6/14 or sand with an average particle size of about 3mm. Pool filter sand is composed of inert silicates that will not affect water chemistry. The size and rounded shape of the grains prevents compaction allowing water movement, nutrient transference, and gas exchange. This allows for a healthy oxidised microzone. That said, I've used everything from coarse gravel to fine sand and all to good effect too, so there is no need to compromise aesthetics for function. However, if the sand is too fine it effectively seals off the soil from the water column stopping nutrient transference, so additional fertiliser dosing will eventually become necessary.


    Oxidised microzone
    The oxidised microzone is the very thin highly aerobic surface layer of soil that interfaces with the water column. It is of huge importance in a soil substrate tank since it supports the hive of microbial activity needed to neutralise toxic substances and unlock nutrients.

    Fertilisation and nutrients
    Given the correct selection of slow growing plants such as Aunbias spp. it is not strictly necessary to add nutrients to lower energy soil substrate tanks, other than those provided by tap water and fish food. Fish food contains all the essential elements required for healthy plant growth. But I am not suggesting that you leave fish food to rot in your tank. If you feed your fish very well, what they don’t absorb they excrete in the form of small inorganic compounds, or in other words the nutrients that plants can use. Shrimp and snails also help by breaking down organic matter, including fish mulm, in to smaller particles and bacteria do the rest, unlocking even more nutrients.

    Soil degradation?
    Nevertheless, this often begs the question…surely the soil will degrade over time as nutrients are steadily depleted by vigorously growing plants? The answer is not necessarily. The soils I have recommended are largely composed of clay and peat and have naturally high CECs as they contain particles that readily attract and bind nutrients to them. Plants are then free to uptake the nutrients through their roots, in particular iron and other trace elements, and phosphorus which is rapidly absorbed. So providing your cap of sand is of a thickness and grade that allows adequate water movement and nutrient transference your soil substrate should retain enough nutrients to keep your plants happy almost indefinitely.

    Walstad Nature Scape

    Inorganic nutrient dosing
    Plants can also absorb nutrients through their leaves. Therefore, in addition I dose my low energy tanks with nutrients to supplement those derived through fish excretion. Typically, the weekly dose I use is around one fifth to one tenth of that recommended for high-energy tanks. The dose is small enough that ready made liquid nutrient formulations like TNC Complete are economical for me to use. This relatively low dosing regime also means that regular water changes are not needed. Instead simply missing a dose every so often, about once a month or two will suffice.

    Dry salts
    Similarly, dry salts can also be used particularly if a more economical alternative is required for larger aquariums and/or higher doses. The standard regime, for say a 20 gallon low energy tank, is to dose once every week or two with the following; 1/4 teaspoon of GH booster, plus 1/8 and 1/32 of a teaspoon of KNO3 (potassium nitrate) and KH2PO4 (monopotassium phosphate) respectively. The ratios can be scaled up or down to suit any size of tank. The above dosing regime presupposes that macrophytes in lower energy tanks grow 5 to 10 times slower than in higher energy setups, and it also assumes that “fish food” indirectly contributes about 80% to 90% of the nutrient load.

    Bioavailable organic carbon
    It is also possible to achieve even greater growth rates by dosing with bioavailable organic carbon and doubling or trebling the above nutrient doses. This is considered by many a high energy route, and to maintain a healthy equilibrium this dosing regime will also require larger and more frequent water changes; typically 25% to 50% once a week to remove the metabolites of increased photosynthesis and respiration, and to reset nutrient levels.

    Similarly, there is absolutely no reason why a soil substrate shouldn’t prove beneficial as a planting medium in a higher energy setup as well. There isn’t any reason why soil substrate couldn't be used with eutrophic nutrient dosing, such as EI, and CO2 injection. After all many higher energy enthusiasts already use mineralized substrates, and the use of potting compost is not that much of a leap of faith when all things considered.

    CO2 & Soil

    Soil synergy
    In short there are many synergistic benefits to using soil substrate alongside nutrient dosing methods. For instance, soil substrates with a high CEC will also attract and bind inorganic nutrients added to the water column, where they will also be made available for root uptake. The high nutrient content of soil substrates can also act as a safety net, buffering against the occasional missed nutrient dose.

    The aerial advantage
    Floating plants such as Salvinia spp. and others that grow emergent or floating leaves, like Aponogeton natans. have the aerial advantage. The aerial advantage allows plants to harness relatively high concentrations of atmospheric CO2, and take advantage of much higher rates of CO2 diffusion; diffusion of CO2 in water is very slow by comparison. Plants with emergent leaves can also take advantage of higher light intensity which combined with greater CO2 uptake results in higher levels of photosynthesis and rapid removal of dissolved organic nutrients from the water column, which not only increases plant growth rate, it also helps to combat algae. Further, emergent plants are generally better at oxygenating the root zone, or rhizosphere, than submerged plants; this helps create an environment more conducive to healthy growth.

    Soil/sediment metamorphosis
    Newly submerged terrestrial soil goes through a number of chemical and biological changes before it becomes stable aquatic sediment. During these changes organic matter is broken down to form inorganic molecules, or the nutrients that plants can use; this process is often referred to as mineralisation.

    Mineralisation of a submerged soil usually releases ammonia and other chemical compounds in to the water column where they can reach levels that are toxic to fish and invertebrates; but rarely to plants so it is usually safe to plant a newly set up tank immediately. The use of macrophytes as water purifiers is well documented, so apart from adding instant interest, planting heavily from the outset will help to reduce ammonia and other chemical compounds to non-toxic levels. The plants will also often benefit from the additional nutrient load and CO2 given off during mineralisation.

    Soil Substrate CO2 Tank

    Self-cycling method
    I have always found that the ammonia given off during mineralisation is more than adequate to cycle a filter so now is the time to hook one up. This self-cycling phenomenon is in effect fishless cycling but without the hassle of dosing ammonia, or adding fish food and suffering the subsequent consequences of phosphate build up. There is also far less water testing involved.

    Rate of mineralisation
    Mineralisation can take up to 2 months to complete, but the actual rate is determined by a number of factors such as the organic content of the soil, water and soil chemistry, and microbial activity. Planting heavily from the outset also helps to reduce the length of time it takes for newly submerged soil to stop giving off ammonia since macrophytes release O2 and organic compounds through their roots which will greatly increase microbial activity, and therefore nitrification and denitrification. The existing bacteria on plant roots will also help inoculate the sediment and perhaps further speed the process on its way.

    Soil equilibrium
    Eventually an equilibrium is reached and the soil substrate will actually start to absorb ammonia/ammonium from the water column where it will undergo nitrification. When levels of ammonia, nitrite, and nitrate stabilise within acceptable levels it’s a sign that denitrification is also well under way. If Nitrate levels are still a little high a substantial water change is usually all that is required to make the tank habitable to fish.

    Mutually inclusive processes
    Overall levels of ammonia, nitrite, and nitrate always seem to stabilise within acceptable levels quite quickly, often within a week or two. So although it can take up to 2 months before mineralisation is complete it is not usually necessary to wait anywhere near that long before adding fish. In this respect it probably helps to think of mineralisation and tank cycling as two separate but mutually inclusive processes.

    Walstad Jungle Scape

    Mineralised top soil (MTS)
    An alternative to allowing soil to mineralise in situ is the use of MTS (mineralised top soil) or mineralised potting compost. MTS is thought to bind more bioavailable nutrients, and give your plants a better start whilst preventing the excessive release of organic nutrients. Excessive organic nutrient release combined with too much light, can lead to algal outbreaks. Further, MTS is often considered less prone to disturbance during scaping and subsequent maintenance. The internet provides a wealth of information on how to mineralise soil, but the methodology after Aaron Talbot is perhaps the most widely used. It simply involves a process of repeated soaking, rinsing and drying, typically four cycles long. Eventually, the soil is sifted to remove large particles and achieve a fine grained well sorted substrate.

    Finally, powdered clay can be added. Its flocculating properties help bind the soil particles and its high CEC and iron content benefit plant growth. Powdered or pelleted Dolomite can also be added as a source of the nutrients magnesium and calcium, and if necessary to buffer pH, and similarly potash can be added as a source of potassium. However, mineralising soil is a lot of messy work, and on balance I prefer to do it in situ, mainly because it's far less labour intensive but also because of the synergistic benefits already discussed further above. Not least of these is the evolution of CO2 as the organic matter in the soil decomposes, and as also previously mentioned, it’s a convenient way to cycle a tank.

    Walstad Nature Scape: Flowering Aunbias

    Suitable plants
    There are many aquatic plant species that will grow vigorously and thrive in just soil alone and whatever additional nutrients tap water and fish food have to offer, and for years without showing any signs of nutrient deficiency. And there are even more that will benefit from the addition of water column nutrient dosing. But growing plants successfully is also about choosing those that best suit your unique aquarium conditions. One approach is to plant as many different species as possible and then let them fight it out. That way it soon becomes obvious which plants thrive in your unique aquarium conditions and which ones to avoid in future. The following low-energy plant list is by no means definitive but it should give you a good place to start; most are very easy to grow:

    Anubias spp.
    Aponogeton natans/crispus
    Bolbitis heudelotii
    Ceratopteris spp.
    Cryptocoryne spp.
    Echinodorus spp.
    Egeria densa
    Fontinalis antipyretica
    Hygrophilia spp.
    Lilaeopsis brasiliensis/novae-zelandiae
    Ludwigia repens/natans
    Microsorium pteropus. var.
    Pistia stratiotes
    Salvinia auriculata
    Sagittaria spp.
    Taxiphyllum spp.
    Vallisneria spp.
    Vesicularia dubyana

    Give it a go!

    I hope this article has informed, and inspired even the most dedicated dyed in the wool technophiles amongst you to set up a hybrid-energy soil substrate planted tank. Why not let nature do some of the hard work for a change? You can still use most of your gizmos, and at the very least you will create a fascinating complement to your high-energy setups. At the very most you might even become a full blown convert. And for those of you new to the hobby or returning after an absence, consider the method before you take the high-energy plunge. Honestly, it really isn’t rocket science and once the basic principles are grasped the benefits are there for the reaping. If despite all you are still determined to set up a high-energy tank why not give some thought to using soil substrate anyway? Finally...the Zen bit, if you decide to give the hybrid-energy soil substrate method a go I hope you enjoy the journey as much as I have so far.


    Further reading:
    The Barr Report
    Walstad (1999) Ecology of the Planted Aquarium. USA: Echinodorus Publishing.

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    Keith, MarcelM and Jason like this.


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  1. TimHarrison
    Thanks Keith:)
  2. Keith

    Part 2 is also fantastic reading its full of very useful information.

      TimHarrison and MarcelM like this.
  3. Keith

    Part 2 is also fantastic reading its full of very useful information.

      TimHarrison likes this.
  4. MarcelM
    Who was it again who said!? The journey is more important than the destination... :)

    Thanks Tim! Reading this makes me feel like captain Kirk again... Boldly go where Marcel hasn't gone before!

    If soil works in a garden and in most natural ponds öfcourse" for longer as we are on this nut, it must work in a tank as we know it as well. Never thought of it putting it in there.... Commercials?? I guess so..

    Beam me up snotty!...
      Keith and TimHarrison like this.
  5. TimHarrison
    Being essentially lazy, if I ever get around to scaping a big tank again it'd be a variation on this theme too...it'd be what I've tried to describe above by coining the term "hybrid energy".
    Basically, I'd use soil substrate, plant with "easy" to "medium" difficulty plants and inject just enough CO2 to turn the drop checker dark green.
    Lighting would be relatively low intensity, as would fertz dosing.
    This in turn means I'd get away with less frequent and substantial water changes.
    Overall, that'd give me slowish growing but healthy plants, and relatively low maintenance, and reduced utility bills; at least compared to the usual fuel injected tank;)
  6. Keith

    If I was starting up again (which I never will) that is a method I would consider.

      TimHarrison likes this.