What nutrients do aquatic plants need for growth ?

What do aquatic plants need?

Like terrestrial plants, aquatic plants need a supply of nutrients to grow well. This has been well studied in terrestrial science and the chemical elements necessary for growth can be divided into two main groups:

Macronutrients, together with carbon, make up ~96% of plant mass. Hence the term macronutrients. Many of the basic building blocks for plants come from air/water; hydrogen, oxygen. The rest of the elements come from soil (for terrestrial plants that's where the nutrients accumulate) and water (aquatic plants can take up nutrients through the water column).

Note that carbon makes up 45% of the dry mass, but 'naturally' dissolved carbon in tap water is usually low. Compared to carbon, other macro-nutrients are a small percentage of dry mass! Plants use 10 times more carbon by mass than all other macro-nutrients combined. This is why CO2 injection has such a big impact on plant growth results. Tank CO2 levels without CO2 injection hover around 2-3ppm as it reaches equilibrium levels with the atmosphere given the gas pressure laws of liquids. Natural waters that support plant growth have levels closer to 10-30ppm. With CO2 injection we can push these levels even higher, close to 40+ppm. Consequently, growth rates in an injected tank are 5-10 times higher than in a non CO2 injected tank. The nutrient requirements of a CO2-injected tank also scale up in a similar way.

Micro-nutrients are used in very, very small quantities. It is sometimes hard to imagine the difference in magnitude of these micro-amounts. For example, plants use 100 times more potassium than iron.

A typical plant dry mass composition is shown in the table below:

Liebig's law of the minimum is the principle that growth is controlled by the scarcest resource, meaning that providing an excess of other nutrients will not stimulate growth unless another critical variable is increased proportionately. This is why a complete fertiliser is so important.

Notable elements and common forms:

Nitrogen (Ammonia NH3, Nitrates NO3): Element most used by plants apart from carbon. Important growth regulator - nature is often nitrogen limited and plants are quick to take up new sources of nitrogen. By changing the N levels in the aquarium we can speed up or slow down growth rates. Some aquatic plant species become redder in colour at low N levels due to delayed chlorophyll development (Rotala rotundifolia & variants, Ludwigia arcuata/brevipes, Limnophila aromatica). In general, the N dosage in a tank should be kept stable to prevent plants from constantly reprogramming their growth rates, which leads to many problems. Animal waste will contribute a significant amount of N if the tank is well stocked.

Phosphorus (phosphates, PO4): Coloured plants become paler when PO4 is lacking. Tanks with a significant amount of livestock usually have quite a lot of PO4.

Potassium (K): Potassium is used in many essential plant functions. In regions where tap water does not contain potassium, plant tanks can quickly run into a variety of problems if K levels are inadequate. This is readily available in most commercial fertilisers (although not necessarily in optimal amounts). Livestock manure is not an optimal source.

Iron (Iron chelates, soils, Fe): Iron is an immobile nutrient (unlike NPK above) and plants cannot transfer Fe from old leaves to feed new growth. Therefore, Fe deficiency is first seen as yellowing of new leaves and poor pigmentation in coloured plants. Contrary to what most hobbyists think, providing excess iron will not stimulate extra pigmentation in red plants. What is more important is regular dosing to maintain sufficient levels.

Magnesium (Mg): Key molecule in chlorophyll. (above) Available quite commonly in tap water, but for regions where tap water has no Mg, dosing it regularly is critical. Many commercial fertilizer miss this in their formulation. 

Trace elements (Boron B, Copper Cu, Boron B..etc): Plants use only tiny amounts of these, but they do affect both plant coloration and growth form. In high speed growth rates of CO2 injected tanks, it is easy to get sub-optimal levels of trace elements. Required dosage is tiny, but should be done regularly.

How much of each element to dose is covered in the dosage guide here.

Nutrients from livestock waste? - It just ain't enough

The planted aquarium is a closed environment. Unlike a truly natural environment, there is no inherent cycle of nutrient replenishment where organic decomposition and mineral erosion return nutrients to the soil. Aquarium soil substrates in a planted aquarium can provide nutrients for a long time, but all substrates eventually become depleted. Nutrients that are soluble are removed with water changes; and depending on what is in your tap water - may or may not be replenished.

Depending on the fish, the waste looks good in theory, but is often incomplete. For example, no fish we know of produces chelated iron as waste.

In many planted aquariums, elements such as potassium and iron are usually lacking unless we dose them. Some aquariums try to rely on animal waste as a fertiliser. Organic waste also doesn't decompose cleanly - there are many by-products in the process, and having high levels of organic waste in the tank is a good trigger for algae blooms. Animal waste from heavy fish tanks can produce significant amounts of nitrates and phosphates. However, other elements such as iron, trace elements and potassium are likely to be at sub-optimal levels. Whilst some tanks will survive with the "no dosing" approach, they will never be the ones that get the best growth.

The 2Hr approach is always to strive for the more optimal scenario, rather than take chances with the bare minimum, hence we would highly recommend complete fertilizers that cover all angles nutrient-wise.

When aquatic plants get all access to all their nutrient needs - they can be grown more densely and exhibit better coloration. You won't see examples such as these in "no dosing" tanks. Even the humble Bucephalandra; one of the easiest aquatic plants to grow, show their full potential only when their needs are met comprehensively. Whether you are running CO2 or not, dosing makes a significant difference to plant health.

What happens when there is a lack of nutrients?

In general, when plants are not getting the nutrients they need, the first thing that happens is that growth slows down. In rooted species, energy is diverted to root growth in search of nutrients in the substrate layer. Then, depending on what type of nutrient is lacking, a mix of symptoms can occur - in some cases, leaves become paler or new shoots become whitish. Stems appear thin and leaves may be smaller than normal. When mobile nutrients such as NPK and Mg become too low, plants can take minerals from their old leaves and channel them into new growth. This leads to premature leaf drop or yellowing of older leaves.

Weak and unhealthy plants are the number one cause of algae. Therefore, feeding plants regularly to ensure their good health is vital to prevent algae.

Deficiency charts are not accurate and inexperienced hobbyists should be careful about jumping to the conclusion that all plant health problems are nutrient related. (It is more likely that problems are due to non-nutrient related factors).

We do not believe that dosing only when deficiencies occur is a good method as the plant will already be stunted and problems such as algae will already have occurred. Rather than waiting for deficiencies to manifest, an easy way to avoid deficiencies in general is to have a regular dosing regime of all the required elements. This is easily achieved with a broad spectrum liquid fertiliser. This is the standard approach of most successful planted tanks.

 Anubias barteri nana showing long term magnesium deficiency.

Go to this section to learn more about fertilization.