Similar to terrestrial plants, aquatic plants require a supply of nutrients to grow well. This has been well studied in terrestrial science, and the chemical elements necessary for growth can be grouped into two major groups:
Notable macro nutrients
CARBON makes up the majority of plant mass, followed closely by nitrogen, then potassium and the rest of the macro nutrients. Plants use more than 20 times Carbon by mass compared to Nitrogen. This is why CO2 injection is such a big impact factor in plant growth outcomes. So a tank that is rich in NPK, but poor in carbon for example, will not be aided by pushing NPK levels ever higher if the carbon limitation was not broken.
NITROGEN is found in chlorophyll, nucleic acids and amino acids; component of protein and enzymes. Lack of nitrogen can enhance red coloration in certain plants by delaying production of chlorophyll. Nitrogen availability has significant impact on plant growth rates. Extreme restriction of nitrogen can cause growth rates to slow to a halt, and for plants to become brittle and delicate over long term.
PHOSPHORUS is an essential component of DNA and RNA, which play critical roles in cell membranes; also plays a major role in the energy system (ATP) of plants. In the tank context, PO4 is often under-dosed. Plants with good supply of phosphorus are more robust and some will display better coloration.
POTASSIUM plays a major role in the metabolism of the plant, and is involved in photosynthesis, protein synthesis. It is a highly mobile nutrient. Overall plant health deteriorates when it is lacking. Common symptoms include yellowing, pinholes, brittle leaves.
MAGNESIUM is part of every chlorophyll molecule which plants use for photosynthesis to generate food.
Notable micro nutrients
IRON in the form of both Fe2+ and Fe3+, are absorbed by plants, but plants would need to expand energy to reduce Fe3+ to 2+ before absorption. Iron is responsible for formation of enzymes involved in chlorophyll production and when a plant is Fe deficient, chlorophyll production is reduced, which results in the characteristic symptoms of chlorosis in leaves. (yellowing of leaf tips).
MANGANESE is involved in plant photosynthesis process, and deficient Mn levels results in reduced photosynthesis and growth. Sub-optimal levels could also result in curled leaves (but many other factors can also give this). This is one example where missing/deficient micro-nutrients will affect overall plant growth - and uptake of subsequent macro nutrients such as NPK. Micro-nutrients are thus very important, even though they are only required in trace amounts.
BORON, ZINC, COPPER, MOLYBDENUM These are all important for plant growth albeit in tiny amounts.
OTHERS Other macro nutrients include sulphur, oxygen, hydrogen; these are usually readily available in tap water or soil. Certain species may see improved growth from additional micro-nutrients; silica (Si), Vanadium (V), Selenium (Se), Cobalt (Co). These are not classified as essential- their usage in plants is not fully understood, though terrestrial studies show certain crops show increased yield when they are available. Many trace mixes include these. The micro traces Silica, Chlorine are common in tap waters and most substrates.
Using 'deficiency charts' - or rather why not to
These charts are derived from agriculture-diagnostics and they are terribly in-accurate for aquatic plants. Many different issues can give rise to yellowing leaves or stunting. Insufficient CO2 causes many symptoms that look similar to common 'nutrient deficiencies' including the premature fall off or yellowing of leaves, loss of color and stunning of leaf tips. This means that many folks diagnose their issues wrongly if they just try to make symptoms with what such deficiency charts depict.
Nutrients are also synergistic - none work in isolation. To find out what is lacking in a tank requires analysis of the holistic approach in managing the tank - not just by trying to spot a yellow leaf here or there.
Most of the time people use these charts to confirm existing biases. These charts also causes nutrient tunnel vision - the idea that all and any poor growth is a result of nutrient-related issues. Where in reality there are a ton of other issues that can affect plant health. This includes CO2 saturation levels, water parameters (KH, especially), overall tank stability, organic waste levels, substrate quality, whether one uses the correct pruning methods for certain species etc. When people keep thinking that it's nutrient related - especially when they are already dosing... then they keep looking for wrong causation which cause them to never solve their problems.
There is also a big gap between sufficiency and optimality. For example, to prevent phosphate deficiency symptoms from showing up for a certain plant, it may requires 0.1ppm of phosphates dosing a week. However, to grow it to great form may require 3ppm + a week instead. We should aim for the later (to grow plants great) rather than just the bare minimum for plant survival, and the gap between those levels can be huge. These charts will not bring you on that path to growing great plants.
"It is a chronic newbie-intermediate urge to get focused on deficiencies, even when they're following EI (!!). I used to think all my issues were lack of something or another and when you find charts like this, it temporarily confirms existing biases. You figure your issue is K deficiency. You add K2SO4 but nothing gets better. Then what? Then it's time to focus on other stuff. I've found many clever ways to kill and stunt plants, especially Rotala and Ammannia. Deficiencies are low down on the list. Poor maintenance and poor CO2 are big reasons." - Vin kutty
"Grow the plants, not the deficiencies. The method is deceptive as it is simple." - Tom barr
These glossostigma elatinoides are grown under slight nitrate limitation. Yellowing outer edges of leaves are a hint, but it can only be confirmed by examining tank parameters and dosing methodology.
Diagnosing issues - First step; rule out as many factors as possible by having a regular, holistic dosing schedule
Start with a good baseline holistic nutrient dosing regime (check out the nutrient dosing section). Having a regular regime is necessary for analysis - if you are tweaking every other day, there is no baseline on which to base one's observations on.
Next, optimize your CO2 levels. Drop checkers are not an accurate gauge of CO2. It is not as simple as tuning the injection rate higher - this is probably the hardest part to get right for most aquarists, as it involves tuning flow patterns, gaseous exchange, re-thinking diffusion methodology. CO2 levels has a tremendous impact on plant growth and health - this is especially so for difficult species.
Next, check that you do not have extreme water parameters. One easy way to rule this out is to check out folks in your country/area that are using the same tap water. Are they able to keep sensitive livestock & plants? If they are chance are your tap water isn't that bad. Check GH/KH readings and find out whether you are growing plants suitable for that range.
Then for further optimization, tweak nutrients one at a time.
To get a better estimation of what is a happening in a tank one needs to know the existing parameters and therefore what is likely to be the cause. i.e. If you have 50 ppm of potassium in the water, and still get yellowing holes in leaves, the issue is definitely not K related. If you are dosing more than 0.1ppm chelated Fe a day in a tank - you can safely rule out iron deficiency. Nutrient issues are really not that hard to rule out. This is largely because plants will self-regulate their growth to match what is available; lean conditions do not give rise to deficiency immediately - it just results in slower growth. True deficiencies take time to develop. The hard part for most aquarists is accepting that more often than not, poor growth is caused by non-nutrient factors. This is also why many aquarists can spend years tweaking their dosing schedules, but still end up with pretty mediocre looking plants by end of the day. Focusing on non-nutrient factors such as plant husbandry or CO2 is as important as having a holistic nutrient dosing approach.
Using plants as indicators; Fast growers vs slow growers
Fast growing plants (such as most stem plants), especially colored stems, are good indicators of recent / present conditions. Pay attention to the size, color and form of new leaves. Even for fast growing plants, deficiencies take awhile (a few days to week+) to set in. Melting and fast deterioration of plants is never nutrient related.
Are new leaves larger/consistent size (good) with old growth or smaller/stunted (usually a CO2 issue, or possible macro nutrient insufficiency)
Is coloration consistent or uneven (may hint at immobile nutrient issues)
Are the leaves smooth & well formed (good) or twisted (Inconsistent CO2, ammonia, KH, excessive fe/trace issues)
For old growth you can look for
Are they much paler/yellow or with pinholes compared to newer growth (possible mobile nutrient (NPK) deficiency, but also often caused by poor or unstable CO2)
Discoloration; veins are green but leaf tissue is pale/yellowish (could be Mg deficiency)
Very quick deterioration of old growth even though new growth seem perfectly formed - hints that unstable tank parameters
Persistent algae on old leaves; tank cleanliness issue, other stress factors, tank's biological maturity may need more time
Slow growing plants such as Anubias and Bucephalandra are not good gauges of recent conditions, but are great as long term indicators because their leaves last a long time. They are a good test for general tank stability, not just in terms of parameters but hobbyist habits as well.
Fast growing stems like these few varietals of Rotala macrandra need consistent all round fertilization to maintain their vibrant color.
These Anubias show signs of long term magnesium deficiency. To get it to this state requires extended period (months) of magnesium deprived growth.
Bucephalandra species are a good test of long term consistency and dosing methodology as they grow slowly. Leaves last many months; fluctuations in tank regime and poor/unstable nutrient/CO2 levels causes pre-mature shedding of old leaves as the plant continually need to re-program its enzymes to match current conditions.
Without optimal CO2 levels, diagnosing nutrient issues becomes difficult
CO2 is required for many picker species to grow in good form. Many plant issues are tied to poor CO2 or poor CO2 stability - without a solid baseline, it makes it much harder to diagnose issues. Both the Alternanthera reineckii below can be receiving the same amounts of nutrients, but poor CO2 levels and less than ideal light spectrum causes the one on the left to be in poor form - this issue isn't nutrient related at all.
Similarly, the Alternanthera reineckii below is not suffering from nutrient deficiency. It is instead suffering from toxicity - either by too heavy dosages of traces, or other harsh chemicals.
For an example that show why concepts are not so cut and dry; Rotala rotundifolia and its variants turn red when Nitrogen is limited. N limitation delays chlorophyll production in new leaves in Rotala rotundifolia. When done in a controlled manner, this does not hurt the plant and does not cause premature deterioration of old leaves. However, new growth will appear significantly redder.
In the above photograph, you can see the exact point where the tank became N limited. (It's where the new leafs on Rotala rotundifolia become red rather than green).
The Pale new tips (sometimes even white as below) is often associated with Iron deficiency. This may also be caused by other mechanisms that interfere with immediate growth. Iron is an immobile nutrient, as are many trace elements and calcium. Surprisingly (or not) CO2 is by far the biggest impact factor on new growth - most stunting of new tips occurs due to sub-optimal CO2. CO2 has to be present consistently during the light window as photosynthesis occurs.
Changing nutrient dosing approaches
If you suspect a certain nutrient is lacking in the tank you can dose additional amounts of that nutrient and see if it makes an impact. It takes a week to see changes with certainty and for some slower growing or previously stunted plants, adaptation to a new regime can take weeks.
The irony of nutrient dosing is that many tanks actually stabilize and do better with less. So for folks that are used to very rich dosing - reducing the amounts dose may be surprisingly fruitful.
Additional dosing face diminishing returns. In a tank that is very Nitrogen limited, the first 5 to 10ppm of NO3 added will affect growth/uptake rates a lot but additional dosing will have a much smaller effect. Where true deficiencies specifically are concerned, it should not take a huge amount of dosing to affect change. This is because plants would have slowed down their growth rates/metabolism to match the lean conditions. (not much growth = very little nutrient draw)
Extreme ratios also tend to ineffective. If you are only dosing 10ppm of Nitrates in a week for a particular tank (and there are no other sources of Nitrogen). It is extremely unlikely that it would consume say 40ppm of Potassium. So if you have suspected K deficiency - it would be ruled out for sure if you are dosing 40ppm a week of K.
Growth forms do change with nutrient levels. Richer conditions generally give larger, more robust forms. Robust plants transfer more easily to other tanks/conditions and have more energy for adaptation changes.
However, higher NO3 levels means less reddish pigmentation for species that get redder under low N conditions. Lean conditions also gives smaller or more compact forms for some plants.
If you are increasing dosing to try for faster growth (and bear in mind both the fact that there are diminishing returns - and that more doesn't always means better), these are the top end range of values I would not exceed in a week;
NO3 - 50ppm
PO4 - 10ppm
K - 50ppm
Fe - 4ppm
Just for comparison the nutrient dosage per week for our farm tank above is as follows:
As you can see, it does not take all that much to fulfill the nutrient requirements of a very high light, densely planted tank.
Head here to learn more about optimizing CO2 levels.