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 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 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.
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. I also add water conditioner that contains beneficial bacteria in the hope that it will speed the process up.
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, in the forming sediment. The existing bacteria on plant roots will also help seed 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 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 etc; this is where the sensible use of a decent test kit comes in to its own.
Mutually inclusive processes
Overall levels of ammonia, nitrite, and nitrate always seem to stabilize within acceptable levels quite quickly, often within a few weeks, which is borne out by the experience of others. So although it can take up to 2 months before mineralisation is complete - and therefore for the soil to stabilise as "aquatic sediment" - it is not usually necessary to wait anywhere near that long before adding fish etc. In this respect it probably helps to think of mineralisation and tank cycling as two separate but mutually inclusive processes.
Adding fish and invertebrates
Gradually add your aquarium critters one or two at a time over the space of a month or so to ensure that the system is not overwhelmed by the increasing bioload. The nitrifying/denitrifying qualities of the soil combined with the water purifying ability of macrophytes will probably prevent ammonia and nitrite reaching toxic levels, even with a partially cycled filter, but there’s no harm in exercising caution. It’s also a good idea to observe your critters closely during the settling in period for signs of stress.
