Root or water column fertilization?

Introduction

There are two approaches to fertilization: dosing liquid nutrient solutions directly into the tank with liquid fertilizers (water column dosing approach) and using substrate based fertilizers such as root tabs (rootzone fertilization). While most plants can take in nutrients through both routes, there are advantages to having nutrients in both locations. 

Water Column Dosing

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

Many aquatic plant species have finely divided leaves to increase surface area contact with water, allowing better gaseous exchange and mineral assimilation.

The advantage of dosing liquid fertilizer is that it is very precise & consistent - we can measure down to the parts per million (ppm) of how much of a certain 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 doing large water changes. It is also easier to measure nutrient content in the water column than in substrates.

No reliance on root systems

Water column dosing has no reliance on plant root systems. This is especially useful when cultivating plants with damaged or immature root systems (such as freshly cut stems). For plants that are topped and replanted frequently, having accessible nutrients in the water column is helpful 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 their own rootzone.

In newly planted tanks where plants have not yet rooted to the substrate, feeding the plants through the water column gives them a good boost. This is why water column dosing should be done on freshly planted tanks, rather than waiting till plants have fully established their root systems.

Free for all buffet

The water column is an open to all 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 tank, faster growers will take in most of the nutrients available and weaker growers will be disadvantaged. In a similar fashion to foliage feeding in land plants, feeding plants this way underwater is very effective.

Some elements are easily absorbed through foliage, such as potassium, so plants can get all the potassium they require through the water column. Certain elements are more reactive (PO4, Fe) and may precipitate out of the water column after a period of time depending on your water chemistry. Reductive processes in the substrate zone make these more easily 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. Soils provide not only organic material and great surface area for microbial life but also have a much better texture for fine root hair development compared to coarser inert substrates. Inert substrates gather organic material over time that become 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 are using inert substrates, your plants will be mainly water column fed.

Localized substrate zones

Unlike water column fertilization which is a free for all buffet, as all plants snatch up what is available, plants have more ownership over their substrate zones. Some larger plants have very extensive root systems that invade surrounding areas readily, however for the most part, most aquatic plants have small root systems that are locally contained. This means that locally available substrate nutrients will always be available to the plant rooted in that location. Furthermore, 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 plants that are already growing too fast. Root feeding is thus a technique to even out grow differences in a layout to get better aesthetic alignment of different species.

In this tank, root tabs are used to boost growth for slower growing species such as Hygrophila sp. Chai, while keeping the substrate unboosted 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, rather than extracting nitrogen from nitrates as it less energy intensive. While we could give plants ammoniacal nitrogen through the water column, ammonium is easily oxidized by nitrifying bacteria and can be toxic to aquatic life in large amounts. A much better strategy to feed plants ammonia through the substrate. Ammonium also binds to soil particles, making it readily available to plants while preventing its slippage into the water column. 

Having ammoniacal nitrogen in the substrate gives a large boost to plant growth rates and health. Plants with access to ammoniacal nitrogen grow faster, with more robust forms. This is one of the main reasons to do substrate fertilization rather than just depend on water column feeding.

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

Ludwigia pantanal showing stunted crowns grown in tank with plenty of nitrates in the water column (20+ppm) but without ammoniacal nitrogen (left picture) VS Ludwigia pantanal grown in a nitrate limited tank (0ppm residual in water column), but with ammoniacal nitrogen in the substrate zone (right picture). 

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

Using root tabs in inert substrate is less effective than using soil substrates as soil has the ability to bind with many mineral elements including ammonia, and hold it in a form that is easily accessible by plants whereas for inert substrates root tabs will just slowly leech into the water column. Root tab users that use tabs in inert substrates are fooling themselves that they are root feeding their plants in any meaningful manner - all that is being done is that the water column is being fertilized.

Moving nutrients into the substrate

When rich aquasoil is used, it is possible to skip dosing nitrogen & phosphorus in the water column entirely. This method of purely root-feeding plants allows one to drain the water column of nutrients and mitigate occurrence of algae. This approach is especially effective in hardscape heavy tanks, where one 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. Draining the water column of those two trigger nutrients allows hardscape to be spotless over 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 diminishes algae spawning on hardscape.

The concept of shifting N & P into the substrate to reduce incidence of algae on hardscape can be applied to other styles of aquascapes as well. Keeping the water column lean in N & P reduces dust/green algae on glass and hardscape.

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

The main reason folks think about pure water column dosing is to avoid using soils/aquasoils which is expensive. Inert substrates are much less messy when replanting occurs. For styles of tanks that rely very 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 requires much more sensitivity, knowledge and effort to manage the exact water parameters (unless you are just on planning on growing the easiest of plants) that works for a certain tank style than most folks realize. Even then some species may not show their best form.

If you are depending purely on water column dosing - you can't for example, drop NO3 levels steeply to get rid of GDA - your plants will starve. Pure water column dosing methods invariable ends up with a huge amount of monitoring and tweaking water column variables on the user's part. ( Not to mention disproportionate amount of time on forums searching for ways to tweak water chemistry) What time saved in managing soil is wasted on the above. Having a very rich water column also means existing algae outbreaks are more severe. And failure to manage algae issues is possibly the number 1 reason for folks quitting the hobby.

On the other hand, depending purely on the substrate means that water soluble nutrients get depleted quickly with water changes. Not all plants may be rooted as well - modern day aquascapes make a lot of use of plants that grow on wood & rock. If you have 100% rooted plants and are diligent on keeping your substrate enriched, you can run planted tanks that are purely substrate fed. 

Water column fertilization is a fast and effective method of supplementation. Plants have also shown to uptake certain elements such as potassium, very efficiently from the water column. 

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

Combining root fertilisation with water fertilisation- a balance of two systems- is by far the easiest, more effective method of growing plants and avoiding algae. This is why every large scale successful commercial planted system out there is based on the two pronged approach.

Head here to read more on substrate maintenance 

Head here to read more on fertilization for planted tanks