In a previous article, I wrote about limiting factors and how CO2, light, and nutrients all played an interconnected role in the growth of plants in the freshwater aquarium. Carbon is required by plants the most and makes up nearly 50% of a plant’s dry weight. The most efficient way to supply carbon to plants is to inject into the aquarium as CO2 gas. The other nutrients usually come in the form of a liquid fertilizer.
Before you conclude you have a nutrient deficiency, consider the more likely scenario that your CO2 levels are too low, or your alkalinity or general hardness are too high. Nutrient deficiencies happen over time, not over night. If you notice symptoms of deficiency cropping up within a week, especially on fast growing stem plants, it may be a sign of improper concentrations of CO2, alkalinity, or general hardness.
It is important to keep nutrient levels stable. If the concentration of nutrients changes frequently, plants will have to drop old leaves and regrow new ones to accommodate the new nutrient concentrations. If this happens frequently enough, plants will expend too much energy growing new leaves and not enough photosynthesizing. This will result in extra decaying tissue and unhealthy, stressed plants.
There are so many different nutrients responsible for several dozen unique functions within plant cells; to say you have a nutrient deficiency is an oversimplification. You must determine which nutrient is lacking so you can take steps to raise its concentration.
This is easier said than done. There are many symptoms of nutrient deficiency in aquatic plants. Most of these symptoms can be attributed to deficiencies in multiple nutrients. It's nearly impossible to tell from symptoms alone which nutrient is lacking. Symptoms of nutrient deficiency are a sign you have a deficiency more than an indicator of which specific nutrient is deficient.
Knowing what nutrient deficiencies look like, combined with knowing how much of each nutrient you are adding, and testing the water, if applicable, can help you determine which nutrient is limiting and allow you to take steps to rectify it.
Once you have determined which nutrients are deficient, you can then start slowly adding the proper fertilizers to bring those levels up. In the article below, I will detail the visual symptoms of many different nutrient deficiencies, the concentrations and ratios those nutrients should be at, and how to raise the concentration to eliminate limiting factors.
Plant Nutrients
There are two groups of nutrients, macro and micro. Macronutrients are those elements that plants require in large quantities. These are carbon, oxygen, hydrogen, nitrogen, potassium, phosphorus, sulfur, calcium, and magnesium. Micronutrients are the elements that plants still require, but in much smaller or trace amounts. These include, but are not limited to: iron, manganese, cobalt, boron, copper, molybdenum, zinc, and nickel.
Mobile vs Immobile Nutrients
Some nutrients are mobile within plant tissues. This means they can move from older to newer tissues as necessary. Calcium, sulfur, iron, boron, and copper are immobile. The rest are mobile. If a deficiency symptom is only prevalent in new leaves, and not older ones, it may be a sign the lacking nutrient is immobile.
Carbon | 25 to 40 ppm (CO2) |
Nitrogen | 5 to 20 ppm (nitrate) |
Potassium | 8 to 15 ppm |
Phosphorus | 0.05 to 1.5 ppm (phosphate) |
Sulfur | 1/3 to 3x phosphorus level |
Calcium | 10 to 15 ppm |
Magnesium | 5 to 10 ppm |
Iron | 0.05 to 0.1 ppm |
Macronutrients
Carbon, hydrogen, oxygen, nitrogen, potassium, phosphorus, sulfur, calcium, and magnesium are the major macronutrients. Nitrogen, phosphorus, and potassium should be in a ratio of around 6 to 1 to 8 respectively. Below, I've detailed each macronutrient’s recommended concentration, its role in plant biology, and symptoms of deficiency.
Carbon
Carbon is one of the most important nutrients to plant growth and all life for that matter. As carbon-based life forms, plants use carbon to grow new tissues and to respire by breaking down sugars to acquire energy.
The injection of CO2 gas is the most efficient form of supplying carbon to photosynthesizing plants. With normal atmospheric pressure, and no extra addition of CO2, most aquariums test at 1-4 ppm CO2. Natural systems with plenty of surface agitation and microfaunal respiration (bacteria breathing and respiring CO2) produce around 10 to 20 ppm of CO2. Aquariums with CO2 gas injection measure at 30 ppm and above.
CO2 absorption happens during the day when the light is on and plants are actively photosynthesizing. There is no need for CO2 injection at night.
The main symptoms of carbon deficiency are: (1) necrosis, (2) chlorosis, (3) stunted growth, and (4) twisted leaves. Necrosis is the death of plant tissues, turning it brown. Chlorosis is the yellowing of leaves as chlorophyll begins to die.
Yellowing leaves turning into necrosis
Oxygen and Hydrogen
While these two elements are certainly considered macronutrients, they are not normally included in a discussion on plant nutrients and fertilizers as oxygen and hydrogen are abundant in water. Plants utilize dissolved oxygen and hydrogen ions for respiration and photosynthesis.
As long as your water is not completely still, or is injected with too much CO2 (80+ ppm), plenty of oxygen and hydrogen ions should be present. These macronutrients are not present in any form of liquid fertilizer.
Nitrogen
The second most used nutrient in plant growth is nitrogen. It is mobile within the plant and is absorbed in the form of nitrate, ammonia, ammonium, and urea. Nitrogen is used to synthesize proteins and nucleic acids, a main component of plant growth. Measured in the form of nitrate, concentrations of nitrogen in the aquarium should be around 5 to 20 ppm.
A significant amount of nitrogen can come from fish waste in the form of ammonia, urea, and nitrates. Because nitrifying bacteria convert ammonia fairly quickly, a good nitrogen fertilizer can provide multiple forms of usable nitrogen to supplement nitrates, which plants have to spend energy to convert.
The main symptoms of nitrogen deficiency are: (1) necrosis, (2) chlorosis, (3) stunted growth, (4) small plants with large root systems, and (5) red new growth. When nitrogen is deficient, chlorophyll production is delayed and since chlorophyll is the reason plants are green, they take on a reddish hue instead. Reddish hues can also come from an abundance of other nutrients and from intense lighting. Reddish coloration from nitrogen deficiency is usually present in only the newest growth at the plant tips.
Possible nitrogen deficiency with red shoot tips. Because shoot tips are not stunted, it is more likely an abundance of pigment nutrients and light.
Potassium
Potassium is an important nutrient responsible for plant metabolism, photosynthesis, and protein synthesis. Potassium is highly mobile and moves through plant tissues easily. It is also highly reactive and consequently is not easily measurable. Its concentration should be around 8 to 15 ppm, although this amount can only be tested immediately after adding fertilizer. Make sure you add potassium separately if you use an all-in-one fertilizer that does not include potassium as it is a crucial macronutrient.
The main symptoms of potassium deficiency are: (1) necrosis, (2) chlorosis, (3) pinholes, and (4) brittle leaves. Pinholes are visible all over new and old leaves and their margins are usually yellow. Pinholes will start as brown spots and develop into full blown holes throughout the leaves.
Pinholes could be a possible potassium deficiency.
Phosphorus
Phosphorus is an essential component of DNA, RNA, membrane phospholipids, nucleic acids, and the energy systems of plants (ATP {adenosine triphosphate]). Phosphorus is mobile in plants and hover around a concentration of 0.05 to 1.5 ppm in the aquarium.
Like nitrogen, phosphorus is a major component of fish waste and fish food. Some of the plant requirements for phosphorus can be supplemented by the phosphates in fish waste and food. Phosphorus is still commonly under dosed and may need to be added via a trusted fertilizer as well.
The main symptoms of phosphorus deficiency are: (1) stunted growth, (2) smaller shoot tips, and (3) dark coloration in leaves. Leaf darkening usually happens in older leaves first as phosphorus is mobile.
Sulfur
Plants absorb sulfur in the form of a sulfate ion (SO4), usually added to the tank attached to another element like magnesium or potassium. Sulfur is essential in the production of chlorophyll, photosynthesis, respiration, the use of phosphorus, and disease resistance. Sulfur is easily stored by plants and can be found in concentrations anywhere between one third and three times that of phosphorus levels.
Sulfur normally doesn’t need to be dosed separately as many fertilizers utilize the sulfate ion in their mixtures. Just read your fertilizers label to ensure it contains some form of SO4, usually as MgSO4 or KSO4, magnesium or potassium sulfate respectively. Sulfur is immobile in plants; any signs of deficiency will be in new leaves only.
The main symptoms of sulfur deficiency are: (1) Chlorosis, (2) slow growth, and (3) short, spindly, and slender stalks. You may also notice darker leaves as phosphorus utilization also suffers when sulfur is deficient.
Calcium
In the aquarium, calcium plays a primary role in the general hardness of the water. The more calcium or magnesium ions present, the harder the water is. Most tropical plants, generally like softer water. Plants will pull calcium from the water to develop cell walls and membranes, absorb and convert nitrate, metabolize carbohydrates, all crucial components of cell division.
You can test for calcium directly, but as long as your general hardness (GH) remains above 2 degrees of General Hardness (dGH), your water should have a sufficient concentration of calcium. Calcium should exist at concentrations between 10 and 15 ppm. Calcium is immobile in plants; any signs of deficiency will be in new leaves only.
The main symptoms of calcium deficiency are: (1) stunted growth, (2) twisted new growth, and (3) pale color in new leaves.
Magnesium
Plants use magnesium as the core of the chlorophyll molecule and to help transport phosphorus through the plant. Magnesium is also used to activate specific enzyme systems required for energy expenditure.
Like calcium, magnesium is included in trace amounts in most all-in-one fertilizers, and should be present in sufficient quantities in tanks with a general hardness above 2 dGH. Magnesium should exist at concentrations between 5 and 10 ppm. Magnesium is mobile within plants and deficiency symptoms can be seen in old and new leaves.
The main symptoms of magnesium deficiency are: (1) chlorosis of leaves with leaf veins remaining dark, and (2) stunted growth.
Micronutrients
The following nutrients are still vital to plant health and growth, but are considered micronutrients, or trace elements because of the relative concentration required compared to macronutrients. They are iron, manganese, cobalt, boron, copper, molybdenum, zinc and nickel.
When the chemical composition of a dried plant is formulated, these nutrients make up fractions of a percent of the total dry weight.
Most macronutrients are available in a single fertilizer. Sometimes iron is offered separately, but usually includes the other trace elements as well. Make sure your dosing regimen includes a mix of trace elements, as you’ll see, they may be the lowest concentration, but without them, plants would not be able to complete even the most basic functions.
Iron
Much like magnesium, iron is used to produce chlorophyll and is essential in photosynthesis and enzyme systems, which are crucial to supplying plants with energy. In high lighting and CO2 injected systems, plants can grow fast if they have enough energy available. Iron is also present in many pigments.
Although it is only required in trace amounts, iron is considered a crucial nutrient and is often sold as a separate fertilizer to ensure its proper dosage. Iron is absorbed very quickly, but should be dosed to achieve a concentration between 0.05 to 0.1 ppm. Iron is immobile in plants and deficiency symptoms will only present in newer leaves.
The main symptoms of iron deficiency are: (1) necrosis, (2) chlorosis, (3) stunted growth, and (4) pale to white new growth. If the deficiency is long term, each new leaf will be lighter than the preceding one.
Lighter colored new growth could mean an iron deficiency.
Manganese
This element is crucial in the growth and development of plants and sustains metabolic roles.
The main symptoms of manganese deficiency are: (1) necrosis between leaf veins, and (2) chlorosis of young leaves with the smallest veins remaining green.
Cobalt
Cobalt’s main role is to aid in nitrogen fixation. Without cobalt, nitrogen can’t be uptaken by plants. It is functionally the same as a nitrogen deficiency.
The main symptom of cobalt deficiency is: (1) stunted growth.
Boron
Boron plays an especially vital role in plant physiology. It is responsible for reducing iron to a usable form by plant roots. Without sufficient boron, plants can’t uptake iron, magnesium, calcium, potassium, or phosphate, which can have devastating effects on plant health.
The main symptoms of boron deficiency are: (1) necrosis, (2) chlorosis, (3) stunted growth, and (4) twisted leaves.
Copper
Copper is a critical component of many enzymatic processes, leading to energy availability in growing plants.
The main symptoms of copper deficiency are: (1) dark color in new leaves, and (2) twisted leaves.
Molybdenum
Molybdenum (muh-lib-duh-num) is a component of two enzymes that convert nitrate to nitrite to ammonia so it can be used by the plant to synthesize amino acids. It is also crucial in the synthesis of phytohormones, and the detoxification of sulfite.
The main symptoms of molybdenum deficiency are: (1) necrosis of new leaves, (2) chlorosis, and (3) twisted leaves.
Zinc
Zinc is used to form chlorophyll and some carbohydrates, and the conversion of starches to sugars. Zinc can also help plants withstand cold temperatures, and is crucial in the formation of auxin, a molecule that regulates the elongation of plant stems.
The main symptoms of zinc deficiency are: (1) chlorosis, (2) smaller leaves, (3) shortened internodes, (4) spotted leaves. Nodes are the point on the plant stem where leaves grow from. Zinc deficiency can cause a shortened distance between these nodes as the stem grows.
Nickel
Nickel can easily become toxic in aquatic environments, but is required for plants to properly absorb iron.
The main symptoms of nickel deficiency are: (1) necrosis, (2) chlorosis, (3) stunted growth, (4) pale to white new growth.
Experiment
You may notice in the lists of symptoms for each nutrient, there are many which repeat. As I mentioned in the beginning of this article, it is nearly impossible to identify a deficient nutrient on symptoms alone. You must take into consideration all the factors that affect your aquarium. You must consider lighting, CO2 levels, and your dosing regimen together. Each of these may be added separately, but they affect your plants as one.
Keeping track of your dosing regimen is of paramount importance. You can’t begin to deduce what any deficiencies are until you know what you are adding, how much, and how often. Testing also helps, but can be more difficult for the trace elements and the nutrients that are rapidly absorbed.
There are many resources for the planted aquarist out there. I encourage you to explore them all. Just remember to not believe anything you read (including what you read here), until you see it corroborated by at least one, preferably two other sources.
Sources
Management of Aquatic Plants. EGLE Environmental Assistance Center. Michigan Department of Environment, Great Lakes, and Energy.
Photos
Boodleshire LLC 2022
https://www.bioleaf.co.nz/articles/nutrients/magnesium-deficiency-in-plants