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

Soil substrate offers the aquascaper, or planted tank enthusiast, a range of possibilities across the energy spectrum, from the traditional...
  1. TimHarrison
    The Soil Substrate Planted Tank - a How to Guide...

    ...or Zen and an Introduction to the Art of Underwater Gardening with Soil or Dirt...
    ...or The Hybrid-Energy Approach.

    In writing this I hope to illustrate that using soil substrate offers a range of possibilities and that it's up to the individual to decide on the level of energy investment they are happiest with in order to achieve their goals. I also hope to illustrate that the “hybrid-energy” approach provides an alternative to the traditional low-energy, low-tech soil substrate tank and the typical high-energy, high-tech CO2 injected tank.

    But first let’s just put all that high-energy, low-energy gubbins in to context. Aquariums require our intervention to reach a healthy equilibrium. The greater the intervention the greater their inherent instability and the greater the investment in energy required to maintain them (for energy also read effort on your part). For instance, the CO2 route requires a relatively high energy investment not just in terms of adding CO2, but also nutrients, artificial substrates, powerful filtration and high output lighting etc. The soil substrate route on the other hand requires none of these and is therefore considered a relatively low-energy investment with minimal inputs required to achieve equilibrium. However, although using soil substrate in an aquarium is traditionally considered a relatively low-energy approach, it does not necessarily have to be the case; neither does it have to be low-tech.

    Walstad Aquascape

    The hybrid-energy method uses soil but it can also take full advantage of CO2 injection, LED lighting, powerful filtration, inorganic nutrients, and frequent water changes. And any one or all parameters can be altered according to the desired outcome and the amount of time, money, and effort the aquarist is willing to invest. But before I go on to describe the hybrid-energy methodology in greater detail I thought it would be helpful to define some of the key terms.

    What is a soil substrate?
    Put simply, a soil substrate consists of mineral particles, organic matter, precipitated inorganic matter and microorganisms. A far simpler definition is ‘the stuff in which plants grow’. This definition emphasises the biological importance of soil, and I think it is this fundamental importance that is so often overlooked by many aquarists.

    Soil substrates use potential energy already harnessed by nature and once your aquarium is full of water it takes advantage of life’s natural flows and cycles. Get it right and soil substrate tanks can produce stunning results on a par with their relatively higher energy counterparts, but with minimum effort and very little expenditure; although it takes a little while longer.

    Potting compost
    So much of using soil substrates is experimental, and for me that has always been part of the attraction. For instance, I have experimented with several soil substrate formulas in the past, with the aim of providing a nutrient level just high enough to aid good plant growth but low enough to prevent excessive release of nitrogenous compounds such as ammonium. One such formula that has proven successful is a blend of 20% loam, 10% grit and 70% sphagnum moss peat. In simple terms the loam peat mix holds nutrients that plants can utilise, and the grit just adds extra structure substrate is more conducive to root development, water movement, nutrient transference, and gas exchange.

    But equally you could use aquatic compost, the type readily available at garden centres for use in ponds. Other people have had success using John Innes number 3, or Miracle Gro’s Organic Choice Potting Mix. I have also had great success in the past using sphagnum moss peat on its own or mixed 1:1 with aquatic compost. However, a word of caution, local water chemistry can also play a role too; for instance peat can sometimes drastically reduce aquarium pH in soft water areas which in turn can lead to metal toxicity. But this can easily be remedied by adding a source of carbonate or bicarbonate such as powdered Dolomite to buffer the pH. By the same measure the lime in John Innes number 3 can raise pH and water hardness. But the resultant water chemistry issues aren’t normally a problem unless you’re a specialist and intend to keep or breed species with exacting environmental requirements.

    One other word of caution, avoid composts that have added inorganic fertilisers since they can prove toxic to fish and invertebrates. Also, avoid composts containing additives like perlite since it has an annoying tendency to rise to the surface every time the substrate is disturbed.

    My preferred mix 1:1

    Garden soil
    Garden soil has also been used with great success, however not all garden soils are created equal. Soils behave differently when they are submerged and usually this just means that sometimes they don’t work very well as an aquatic planting substrate; even though they work perfectly well in your garden. In many cases it is the least toxic soil which provides the best growing conditions. Garden soils can also contain traces of insecticide and herbicide and other substances that may prove harmful to aquatic life.

    Soil vs potting compost
    The advantage of using proprietary brands like John Innes is that they are guaranteed to be of consistent composition, and have been tried and tested and proven to be safe and work well as submerged substrates. This, in no small measure, is also due to their relatively high CEC (cation exchange capacity), which means that they have the ability to absorb and hold nutrients in a form plants can easily utilise.

    Of at least equal importance to the substrate is lighting, considerations are type, intensity and duration. In a traditional soil substrate tank the rate of photosynthesis is limited by the lack of CO2, so if that’s the route you intend to take there is no need to invest in relatively intense lighting. As a general rule 1 to 2 watts per gallon is perfectly adequate depending on the type of aquarium lighting you plan to use. For instance, if you are going to use T5 bulbs around 1 watt per gallon is a good place to start. I use T8 lighting, about 1.5 watts per gallon, which is perfectly adequate. LEDs are an increasingly popular choice and dimmable units will provide the greatest flexibility.

    Lighting duration depends largely on which source and intensity of aquarium lighting you chose. There are no hard and fast rules and a bit of experimentation is needed to get the optimum combination. This is important since in a soil substrate tank, without CO2 injection and nutrient dosing, lighting is usually the only parameter that you can vary to achieve a balanced system; but around 6 - 8 hours is the often quoted standard. In a newly set up tank it’s best to start with a photoperiod of just 6 hours to avoid algal outbreaks. Once the plants are grown in, and the tank becomes biologically stable, the photoperiod can be gradually increased if necessary.

    Spectrum (quality of light) is also important to the health of aquatic plants not just in terms of growth rate but in terms of plant morphology, reducing the impact of resource limitation, and triggering life-cycle processes. Therefore, to ensure plants get the quality of light they require most aquatic plant growers tend to use full spectrum bulbs with a colour temperature of around 6500 kelvin. Luckily we also find them aesthetically pleasing. It's a function of the fact that the photosynthetically active spectrum and the visual spectrum are one and the same give or take a few nm; which is a happy coincidence of evolution.

    Soil Substrate CO2 Aquarium: Lighting

    Siesta period
    A siesta period is something of a contentious issue. Put simply it involves turning your tank light off for 2 to 4 hours in the middle of the photoperiod to stop photosynthesis and allow CO2 levels to replenish. The theory is that your plants then take advantage of the higher CO2 levels when the light is switched on again, increasing photosynthesis and growth. This in turn could help plants maintain dominance over algae.

    Get the balance right between bioload and plant density and you can do without a filter completely. Plants are very efficient water purifiers and readily uptake the toxic waste products excreted by fish and shrimp, such as ammonium, and use it as a source of nutrients. I choose to use filtration because it allows me to safely increase the bioload, to something approaching tank capacity, it also comes in very handy should I inadvertently disturb the substrate whilst moving plants around.

    Water changes
    The same goes for water changes. Even if you chose to stock to capacity like I do, water changes will not be required anywhere near as frequently as say a CO2 tank using a eutrophic nutrient dosing method such as EI (Estimative Index); about 20% - 30% every other week or so should be fine in most cases. But it also depends on the species of fish you decide to keep and their feeding habits; some are not particularly well suited to lower energy tanks.

    However, I choose to change about 50% of the aquarium water once a week and use it as an opportunity to reduce the concentration of both dissolved and solid organic compounds in my aquarium. This may rob plants of a potential source of nutrients, but I prefer to dose with inorganic nutrients anyway and a very clean tank appeals to my sense of aesthetics, and it may also help to prevent algae. In a newly set up tank even more frequent water changes are often recommended, but it's usually not that essential in a low-energy setup. Nevertheless, I usually change about 50%, 2 or 3 times a week for the first 2 weeks. Again this is to reduce the concentration of organic nutrients, but in this instance those that are excessively released by newly submerged soil, which in turn may help to reduce the likelihood of an algal outbreak.

    Water flow
    Good circulation is essential to disperse waste products and to ensure that all plant leaves are bathed in a homogenous solution of nutrients and dissolved gasses. This will help to promote better growth and prevent algae. Good water flow also helps to drive nutrients in to the substrate where they will become available to plant roots. However, if the circulation is too vigorous your plants will have to invest more energy in repairing subsequent mechanical damage rather than in increasing biomass or growth. Water flow just adequate enough to produce a gentle swaying motion in most of the plants is considered optimum.

    Soil Substrate CO2 Tank - Flow

    Surface agitation
    Good surface agitation ensures adequate oxygenation, essential to maintain the health of critters. It also inhibits the formation of surface biofilms, which not only restrict surface water gas exchange but look unsightly. Don’t forget plants also need oxygen, as do some beneficial microorganisms especially those that break down organic matter. In a soil substrate tank this in turn can lead to the production of more CO2. In many freshwater bodies this decomposition produces far higher levels of CO2 than can be accounted for by atmospheric equilibrium alone. However, in our tanks it's perhaps best to try and maintain a stable CO2 concentration close to atmospheric equilibrium, and good surface agitation can help with this too.

    To filter or not to filter not only depends on your desired livestock levels and how densely you decide to plant, but also on the plant species you intend to grow. Plants with finely divided leaves such as Limnophila spp. for instance, will not thank you for allowing their leaves to become clogged with suspended sediment from substrate disturbance. So a good filter could prove an advantage especially if your livestock levels are relatively high. In a low-energy tank a filter that turns over about 5 times the tanks total volume per hour will suffice. However, having said that I tend to stick to the general rule of turnover for a planted tank, that is 10 times the capacity of the tank per hour. So for a 100 litre tank the filter should turnover about 1000 litres per hour. I use external canister filters since they have many advantages over internal filters, not least they are less conspicuous and have a larger and more versatile media capacity.

    Peat filtration
    Whatever rate of turnover you decide to choose give some thought to the use of sphagnum moss peat in the filter. Peat filtration releases HS (humic substances) which have a number of beneficial functions in freshwater aquaria. Not least of these substances is DOC (dissolved organic carbon) which is an important source of CO2. However, if you are going to use peat filtration I would recommend that you don’t’ try to use it in conjunction with charcoal filter media. Charcoal is widely used for chemical filtration and is not compatible with the use of peat, since it removes DOC and valuable nutrients.

    HS also keep micronutrients, such as iron, in solution and available to plants whilst at the same time safeguarding against metal toxicity. They are also beneficial for the reproductive health of fish, and can even prevent algal growth and kill harmful microorganisms.


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

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  1. TimHarrison
    Thank you rob :)
  2. rodahl
    Very informative. Thank you Tim.
      TimHarrison likes this.
  3. Keith

    Thank you for that fantastic very informative article it's certainly well worth reading.

      TimHarrison likes this.