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 those conditions.

Nitrogen is the element that is used in the largest quantity in plant tissue besides Carbon/H2O. For many natural ecosystems the availability of nitrogen determines the overall growth rate of plants. Nitrogen rich soil is great for farming for example.

For many aquatic plants, chlorophyll development is delayed when there is a lack of Nitrogen available - under this condition this can make plants that are red or orange appear significantly redder. For 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 truly 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 use of purplish lighting. Nitrate limitation maximizes redness down to the very tips.

Rotala rotundifolia red; you can tell exactly at which point in its growth cycle that Nitrate limitation occurred in this tank; where the leaves start growing much redder. 

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

Nitrate limitation has lesser impact on coloration for Ludwigia sp red, growing it in lean conditions causes smaller growth forms. Both plants here were grown from the same cutting in different tank nutrient dosing 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 supplemented at a controlled rate. This approach is often done by dosing less N in the water column, but using a rich substrate, which serves as a backup store for N. We do not want N to bottom out completely as this leads to stunting/stop of growth.

The other advantage of using controlled Nitrogen levels is that it gives tighter growth forms and shorter inter-node distance for some stem plants. This is particularly desirable for the aquascaping crowd which want to control plant bushes by trimming to match hardscape contours. This is the main reason most competition scapes in hardscape focused competitions use leaner dosing regimes. Slowing down growth rates often also mean more stability for the tank overall, as there is less trimming/replanting, slower overcrowding - which are very common issues for aquarists.

If you are intending to farm plants at maximum speed, high N levels will achieve that goal. Whereas if your goal is aquascaping and controlling the aesthetic sizing and form of plants, and deep reds for coloured plants, going the N limited route will be more advantageous.

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 it if your goal is to grow plants more slowly and choose a combination of plants that grow well in lean conditions. This approach is usually done with rooted plants in rich substrate. One must also look out for conditions becoming too lean, and supplementing with nutrients at the correct time. Overly lean conditions may give rise to unhealthy plants in long run, so enriching the soil is important if doing this method over long term. Certain species such as Ludwigia pantanal grow more stable with richer water column dosing as a whole.  

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

The plants in these tanks are rooted in aquasoil - so they can get Nitrogen through the soil. Some N also leaks from the substrate over time. Plants will calibrate their growth to match available nutrients - so their growth rates just slow down when N is scarce. This often give more compact, smaller forms which aquascapers find useful. However, one must also determine when N source drops too low in the long horizon, which can result in unhealthy plants. Depending on the species, some plants are better scavenging low levels of nutrients better than others, so overall plant choice in such tanks need to be taken into account.

The large majority of plants do just fine in Nitrate limited conditions, despite fear-mongering by folks that prefer richer dosing regime. Most of our 2hr Aquarist show tanks run with 5ppm or less NO3 levels in the water column. Many tanks measure 0ppm NO3 by end of day such as this one.

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

The one grown in higher N levels is usually more robust; prolonged low N makes plants more delicate if done for long run. Some species are better at scavenging lean nutrient levels than others, so overall plant choices in tank need to reflect that. 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?

Stem plants that sprout side shoots sprout less side shoots under low nitrate conditions, meaning that they grow more vertical and less bushy. When nitrogen is plentiful, stem plants sprout more side shoots easily, becoming bushier faster. For some species such as Ludwigia glandulosa and Ludwigia sphaerocarpa, lean conditions causes the plant to bend over and try to root into the substrate more - whereas in richer conditions these plants grow more vertical. 

For some species, lower N conditions produce smaller plant forms, which make 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 to have a slight nitrogen limitation rather than a very steep nitrogen limitation by dosing nitrates into the water column in small but regular amounts. APT 3/ Complete for example, aims to produce a slight nitrogen limitation effect, but still contains good amounts of N. Whereas APT zero aims for produce a steeper nitrate limitation effect as it does not contain Nitrogen. In tanks that use APT zero, Nitrogen must come from the livestock or enriched soil in the long run.

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

Tolerance for low N condition depends on the species of plant. With regards to N stress coloration, it depends on the plant species as well - some plants react strongly, some not at all. For RR, even if you have pretty strong lighting and high NO3, often it is not as red as tanks that have significantly lower light and no NO3. Other plants are somewhat in between... like the Red Eriocaulon grows more red under low NO3, but if you blast it with a lot of light, it is also pretty red.