Root or water column fertilization?

Introduction

There are two approaches to fertilising: dosing liquid nutrient solutions directly into the tank with liquid fertilisers (water column dosing approach) and using substrate based fertilisers such as root tabs (root zone fertilising). While most plants can take up nutrients by either route, there are advantages to having nutrients in both locations.

Water Column Dosing

Water column dosing involves adding liquid fertiliser directly into the water column. This involves measuring out a certain amount of liquid fertiliser and pouring it directly into the tank water on a schedule - often daily for fast growing tanks and once or twice a week for slower growing tanks. Aquatic plant leaves readily absorb nutrients from the water column as most aquatic plants have aquatic adaptations such as reduced cuticle formation, which makes the leaves more permeable to dissolved minerals. For floating plants and plants that are not planted into the substrate, feeding from the water column is the plant's main way of obtaining nutrients.

Many aquatic plants have finely divided leaves to increase the surface area in contact with the water, allowing better gas exchange and mineral uptake.

The advantage of dosing liquid fertiliser is that it is very precise and consistent - we can measure down to parts per million (ppm) how much of a particular nutrient we want in the water and how regularly we add that amount. It's also easy to reset nutrient levels in the water column - by making large water changes. It is also easier to measure nutrient levels in the water column than in the substrate.

No reliance on root systems

Water column dosing is not dependent on the plant's root system. This is particularly useful when growing plants with damaged or immature root systems (such as freshly cut stems). For plants that are frequently topped and replanted, it is helpful to have accessible nutrients in the water column as their root systems need time to grow in.

All plants compete equally in the water column. However, each plant has more exclusive ownership of its own root zone.

In newly planted tanks where the plants have not yet rooted to the substrate, water column feeding will give the plants a good boost. For this reason, water column dosing should be applied to newly planted tanks rather than waiting until the plants have fully established their root systems.

Free for all buffet

The water column is an open buffet, so larger, faster, more aggressive growing plants will take what nutrients they can, leaving less aggressive growing plants with less. In a densely planted aquarium, faster growing plants will take most of the available nutrients and less aggressive plants will be disadvantaged. Similar to foliar feeding in terrestrial plants, feeding plants in this way underwater is very effective.

Some elements are easily absorbed by foliage, such as potassium, so plants can get all the potassium they need through the water column. Some elements are more reactive (PO4, Fe) and may precipitate from the water column over time, depending on your water chemistry. Reductive processes in the substrate zone make these more readily available to plants, so having these nutrients in the substrate zone can be more effective.

Substrate fertilization

Substrate fertilization involves mainly using nutrient rich soils (or Aquasoils) and enriching them over time using root tabs. As well as providing organic matter and a large surface area for microbial life, soils have a much better texture for fine root hair development than coarser inert substrates. Inert substrates accumulate organic matter over time which becomes binding sites for nutrients. Inert substrates mainly provide an anchoring base for plants, their ability to provide nutrients to plants is much less effective compared to soil based substrates. If you use inert substrates, your plants will be fed mainly by the water column.

Localized substrate zones

Unlike water column fertilisation, which is a free-for-all where all plants take what is available, plants have more ownership of their substrate zones. Some larger plants have very extensive root systems that readily invade surrounding areas, but for the most part most aquatic plants have small root systems that are locally confined. This means that locally available substrate nutrients are always available to the plant rooted in that location. In addition, this allows us to feed exact patches of plants by adding root tabs to specific areas. We can choose to root feed certain plants that we want to grow faster, while denying root feed to plants that are already growing too fast. Root feeding is therefore a technique for compensating for differences in growth in a layout to achieve a better aesthetic match between different species.

In this tank, root tabs are used to stimulate growth of slower growing species such as Hygrophila sp. Chai, while the substrate is not enriched for species that are already growing too fast, such as the background stems.

Providing ammoniacal nitrogen to plants

Plants prefer ammoniacal nitrogen as a nitrogen source to extracting nitrogen from nitrate because it is less energy intensive. While we could supply ammonium nitrogen to plants through the water column, ammonium is easily oxidised by nitrifying bacteria and can be toxic to aquatic life in large quantities. A much better strategy is to supply ammonia to plants through the substrate. Ammonium also binds to soil particles, making it readily available to plants while preventing its movement into the water column.

The presence of ammoniacal nitrogen in the substrate gives a big boost to plant growth rates and health. Plants with access to ammoniacal nitrogen grow faster and are more robust. This is one of the main reasons for fertilising the substrate rather than just relying on water column feeding.

Rotala macrandra type 4 grown with access to ammoniacal nitrogen (left) vs without (right).

Ludwigia pantanal with stunted crowns grown in a tank with high nitrate in the water column (20+ppm) but no ammoniacal nitrogen (left picture) VS Ludwigia pantanal grown in a tank with limited nitrate (0ppm residual in the water column) but with ammoniacal nitrogen in the substrate zone (right picture).

Fresh aquasoils contain higher levels of ammonia and this is why plants grow better in fresh soil than in depleted aquasoils. Most fresh aquasoils will significantly deplete their nitrogen supply within 3 to 6 months. Enriching the substrate with slow release root tabs such as APT Jazz, which slowly release ammoniacal nitrogen into the substrate, will make the substrate perform as if it was new.

Using root tabs in inert substrates is less effective than using soil substrates because soil has the ability to bind many mineral elements, including ammonia, and hold it in a form that is readily available to plants, whereas in inert substrates root tabs will only slowly leach into the water column. Root tab users who use tabs in inert substrates are fooling themselves that they are feeding their plants in any meaningful way - all that is happening is that the water column is being fertilised.

Moving nutrients into the substrate

When using rich Aquasoil, it is possible to dispense with the addition of nitrogen and phosphorus to the water column. This method of using only root-feeding plants allows you to deplete the water column of nutrients and reduce algae growth. This approach is particularly effective in hardcore tanks where you cannot rely on plant mass to defend the tank against algae.

In this Iwagumi setup, the carpet draws nitrogen and phosphorus from the aquasoil substrate below. We dose APT 1 into the water column, which contains all nutrients except nitrogen and phosphorus. By depleting the water column of these two trigger nutrients, the hardscape will remain pristine in the long term. Lower light levels are also used - this allows the tank to remain algae free despite the sparse plant mass available. This is a key management strategy for many Iwagumis that have a sand front or large rock formations.

Tired of getting green rocks? Substrate feeding plants N and P while keeping the water column level low for both nutrients will reduce algae spawning on hardscape.

The concept of moving N and P into the substrate to reduce the incidence of algae on hardscape can also be applied to other types of aquascape. Keeping the water column lean in N and P will reduce dust/green algae on glass and hardscaping.

Can I just do with water column or just do substrate dosing?

The main reason people think about pure water column dosing is to avoid using soil/aquasoil, which is expensive. Inert substrates are much less messy when it comes to replanting. For tank styles that rely heavily on frequent replanting, this may become the default choice due to replanting concerns.

While it is certainly possible to grow great tanks with inert substrates, it takes a lot more sensitivity, knowledge and effort to manage the exact water parameters (unless you are only planning on growing the simplest of plants) that work for a particular tank style than most people realise. Even then, some species may not be at their best.

If you rely purely on water column dosing - you can't, for example, reduce NO3 levels sharply to get rid of GDA - your plants will starve. Pure water column dosing methods invariably end up with a huge amount of monitoring and tweaking of water column variables on the part of the user. (Not to mention the disproportionate amount of time spent on forums looking for ways to tweak water chemistry.) Whatever time is saved on soil management is wasted on the above. A very rich water column also means that existing algae outbreaks are more severe. And not being able to manage algae problems is probably the number 1 reason why people quit the hobby.

On the other hand, relying solely on substrate means that water-soluble nutrients are rapidly depleted with each water change. Not all plants can be rooted - modern aquascapes use a lot of plants that grow on wood & rock. If you have 100% rooted plants and are diligent about keeping your substrate enriched, you can run planted tanks that are substrate only.

Water column fertilisation is a quick and effective method of supplementation. Plants have also been shown to be very efficient at taking up certain elements, such as potassium, from the water column.

This tank is grown using APT Complete in the water column, and APT Jazz in the substrate.

Combining root and water fertilisation - a balance of two systems - is by far the simplest and most effective way to grow plants and avoid algae. This is why every large scale successful commercial planting system out there is based on the two pronged approach.

Click here to read more on substrate maintenance 

Click here to read more on fertilization for planted tanks