In writing this I hope to illustrate that using soil substrate offers a range of possibilities and that it is 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.
The hybrid-energy method uses soil but it can also take full advantage of CO2 injection, T5 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 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 emphasizes 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.
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 utilize, and the grit just adds extra structure so the 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 centers 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 soil. 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 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 fertilizers 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.
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 utilize.
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 the relatively intense lighting needed to compliment the higher rates of photosynthesis it drives. 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.
The type of aquarium lighting and its spectrum output isn’t really that important since plants aren’t too choosy, but avoid bulbs with outputs at the extremes of the visible spectrum. In essence the colour temperature of the bulbs you choose depends more on personal aesthetics than their affect on photosynthetic efficiency.
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 will probably be the only parameter that you can vary to achieve a balanced system; but around 8 - 12 hours is the often quoted standard.
A siesta period simply involves turning your tank light off for 2 to 4 hours in the middle of the photoperiod. It is a good idea in a tank without CO2 injection since in simple terms it prevents CO2 depletion in the water column allowing higher levels of photosynthesis and therefore plant growth. This in turn, helps plants maintain dominance over algae. It also reduces the energy input which means cheaper electricity bills.
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.
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 one to two weeks should be fine in most cases. If your stock levels are low to moderate water changes will be required even less frequently still, about 50% of the water every 3-6 months, but not necessarily all at once. 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.
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.
It’s a good idea to allow just enough surface agitation to prevent the formation of surface biofilms, which restrict surface water gas exchange, and to allow adequate oxygenation for healthy fish and shrimp. Don’t forget plants also need oxygen as do some beneficial bacteria. Surface agitation will drive off valuable CO2 but when it comes to meeting your plants demands for carbon, soil substrates have an additional advantage, but more on that later; for now back to filtration.
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.
Whatever rate of turnover you decide to choose I would strongly advocate 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 fishes, and can even prevent algal growth and kill harmful microorganisms.
Well that’s the basic components explained all we need to do now is put them together. 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. Any deeper and there is a danger that the soil will 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, heavy metals and hydrogen sulphide may reach levels toxic to aquatic life.
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 minimizes any soil disturbance and resulting turbidity during aquascaping.
Pool Filter Sand/Gravel
Cap the whole lot with about 3 cm of well washed gravel or coarse sand, sloping to perhaps 4cm – 5cm at the back. I use 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.
The oxidized 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.
Soil as a source of CO2
The benefits of soil substrates are so much greater than the sum of their parts, not least is their ability to retain nutrients, and offer some protection against algal blooms, albeit indirectly. But also, as I mentioned earlier, soil substrates have an additional advantage when it comes to meeting your plants demands for carbon. The decomposition of organic matter by bacteria in soil substrates releases CO2. In many freshwater bodies this decomposition produces far higher levels of CO2 than can be accounted for by atmospheric equilibrium alone.
Fertilization 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.
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. Research shows that given relative concentrations in the water column many aquatic plant species will preferentially uptake most of their nutrients from the substrate, 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.
Inorganic nutrient dosing
Although many plants will uptake nutrients through their roots others prefer to absorb them through 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 of that recommended for high-energy tanks, but it also accompanies at least one substantial water change of around 50%. The dose is small enough that ready made liquid nutrient formulations like TNC Complete are economical for me to use.
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. 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. 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 of 25% to 50% up to four times a month to prevent excess nutrient build up.
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.
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 nutrients from the water column, which not only increases plant growth rate, it also helps to combat algae.
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 mineralization.
Mineralization 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 mineralization.
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.
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 stabilize 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 stabilize 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.
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.
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.