Nitrate Limitation: how does it make red plants redder?

Nitrate limitation is the term used by advanced aquascapers to describe the strategy of unlocking the deeper reds in selected plant species that respond well to these conditions.

Nitrogen is the most abundant element used in plant tissue other than carbon/H2O. In many natural ecosystems, the availability of nitrogen determines the overall rate of plant growth. Nitrogen-rich soils are ideal for agriculture, for example.

For many aquatic plants, chlorophyll development is delayed when nitrogen is not available - this can cause plants that are red or orange to appear significantly redder. Species such as Rotala rotundifolia and its variants colorata/H'ra/etc, Rotala Goias, Hygrophila pinnatifida, H. araguaia, Ludwigia arcuata, L. brevipes - they only get really deep red under very low N conditions - using high light alone will not achieve the same effect.

Vibrant reds in Rotala rotundifolia species, even without the use of purple lighting. Nitrate limitation maximises redness to the tips.

Rotala rotundifolia red; you can see exactly at what point in its growth cycle nitrate limitation occurred in this tank; the leaves start to grow much redder.

A side by side comparison of Rotala H'ra grown in two different nutrient settings:

Nitrate limitation has less of an effect on the colouration of Ludwigia sp. red, which produces smaller growth forms when grown in lean conditions. Both plants here were grown from the same cutting in different tank nutrient regimes.

Ludwigia arcuata grows with narrower, redder leaves under nitrate limitation. When NO3 is rich, it grows wider leaves which are pale yellow in color. 

Nitrate limitation gives Ludwigia arcuata (right) a vibrant deep orange hue. 

Nitrate limitation has no strong impact on Alternanthera reineckii's coloration.

Nitrate limitation is achieved when all other nutrients are available in excess, but nitrogen is added at a controlled rate. This approach is often done by dosing less N in the water column but using a rich substrate to act as a back-up store for N. We do not want N to be completely depleted as this will result in stunted growth.

The other advantage of using controlled nitrogen levels is that it gives tighter growth forms and shorter inter-node distances for some stem plants. This is particularly desirable for the aquascaping crowd who wish to control plant bushes by pruning to match the contours of the hardscape. This is the main reason why most competition scapes in hardscape focused competitions use leaner dosing regimes. Slower growth rates also often mean more stability for the tank as a whole, as there is less trimming/replanting, slower overcrowding - a very common problem for aquarists.

If your aim is to grow plants at maximum rate, high N levels will achieve this. However, if your goal is aquascaping and controlling the aesthetic size and shape of plants, and deep reds for coloured plants, the N limited route will be more beneficial.

Hygrophila pinnatifida requires Nitrate limitation to get red. Even in bright light, the leaves will be olive green if NO3 levels are rich.

Some commonly asked questions:

When should I do the nitrate limitation approach? Is it suitable for all tanks?

You can do this if your aim is to grow plants more slowly and choose a combination of plants that grow well in lean conditions. This is usually done with rooted plants in rich substrate. You also need to watch out for conditions that become too lean and supplement with nutrients at the right time. Too lean conditions can lead to unhealthy plants in the long run, so enriching the soil is important if this method is to be used on a long-term basis. Certain species, such as Ludwigia pantanal, are more stable when the overall water column is enriched.

Isn't it problematic that the nitrate is 0ppm? 

The plants in these tanks are rooted in Aquasoil - so they can get nitrogen from the soil. Some N also leaches out of the substrate over time. Plants will adjust their growth to the nutrients available - so their growth rates will simply slow down when N is scarce. This often results in more compact, smaller forms that aquascapers find useful. However, it is also important to determine when the N source becomes too low in the long term, which can result in unhealthy plants. Depending on the species, some plants are better at scavenging low levels of nutrients than others, so overall plant selection in such tanks needs to be considered.

The vast majority of plants will do just fine in nitrate-limited conditions, despite the fear mongering of people who prefer a higher dosage regime. Most of our 2hr Aquarist show tanks run with 5ppm or less NO3 in the water column. Many tanks, like this one, measure 0ppm NO3 by the end of the day.

Which plant is healthier? Those grown in high NO3 or low NO3?

Those grown at higher N levels will usually be more robust; prolonged low N will make plants more fragile if done for long periods. Some species are better at scavenging low nutrient levels than others, so the overall choice of plants in the tank needs to reflect this. For the Rotala rotundifolia (RR) picture above, there is virtually no difference in plant health for RR above, as RR does quite well in lean conditions.

Other than color and growth rates, what else does nitrate limitation do?

Stems that produce side shoots will produce fewer side shoots under low nitrate conditions, making them more vertical and less bushy. When nitrogen is abundant, stems will produce more side shoots and become bushier more quickly. For some species, such as Ludwigia glandulosa and Ludwigia sphaerocarpa, low conditions cause the plant to bend over and try to root more into the substrate - whereas in rich conditions these plants grow more vertically.

For some species, lower N conditions produce smaller plant forms, making them attractive for aquascaping.

How do I do Nitrate Limitation safely, without risking deficient plants?

For rooted plants use a rich substrate and continue to enrich it periodically with new ammonia rich aquasoil or APT Jazz. Ammonia contains plenty of Nitrogen; it binds to soil and makes nitrogen available to rooted plants. As long as your substrate has available nitrogen, letting water column Nitrate levels completely bottom out to zero is fine as rooted plants can feed from the substrate. 

You can aim for a slight nitrogen limitation rather than a very steep nitrogen limitation by adding nitrates to the water column in small but regular amounts. For example, APT 3/ Complete aims to produce a slight nitrogen limiting effect but still contains good amounts of N. Whereas APT zero aims to produce a steeper nitrate limiting effect as it contains no nitrogen. In tanks using APT zero, the nitrogen must come from the animals or, in the long term, from enriched soil.

Would you say there is correlation between plant tolerance for N stressing and PAR?

Tolerance to low N status depends on the plant species. The colouration of N stress also depends on the plant species - some plants react strongly, others not at all. For RR, even if you have fairly strong lighting and high NO3, it is often not as red as tanks that have significantly lower light and no NO3. Other plants are a bit in between... for example the Red Eriocaulon grows more red under low NO3, but if you blast it with a lot of light it will also be quite red.