Macronutrients are those a plant requires in high concentrations. There are six of these: nitrogen (N), potassium (K), phosphorus (P), magnesium (Mg), calcium (Ca), and sulfur (S).

Plants receive these nutrients either as standalone ions or as part of other compounds. Potassium, for example, is available to plants as potassium ions (K+), while nitrogen is only available to plants as ammonium ions (NH₄⁺) or nitrate ions (NO₃^-).

Elemental nitrogen (N) is about 2–5% by weight of dry plant tissue, depending on species. This doesn’t seem like much, especially when compared to the carbon and oxygen contents of 40–45% each (these are high as carbon and oxygen are major components of sugars, starch and proteins). Yet deficiencies may appear when this concentration falls below 1.5–2%. Nitrogen is the highest in concentration of all the macronutrients before deficiencies begin to appear, with the next highest being elemental potassium (K), at around 1–1.5%.

From The Nitrogen Cycle and What is and What Causes Nitrogen Drawdown? it was becoming more clear how nitrogen features so prominently in, well, everything. But why is this so?

To find out, let’s first explore all the places nitrogen appears in a plant, and from that, understand why it is so important. From this we can determine which deficiencies and toxicities arise, and wrap up with how to recognise the symptoms of both.

Photosynthesis

Nitrogen atoms are essential to the structure of chlorophylls, the compounds in chloroplasts which use light energy to split water into hydrogen and oxygen during photosynthesis. There are several different types of chlorophyll: chlorophyll a below is the one common to all oxygenic photosynthesisers.

Four nitrogen atoms (N) surround a magnesium (Mg) atom (and one reason why magnesium is an important macronutrient too). This chlorin ring structure at the top of this molecule is the photosensitive part and is common to all the chlorophylls. The long tail is a hydrocarbon [made of hydrogen (H) and carbon (C ) atoms only] which anchors the entire molecule inside the chloroplast membrane.

Just an aside whilst here, it is very common in chemistry to not write ‘C’ for the carbon atoms in structural formulae — as carbon exists in so many things this saves a lot of time (and printed space - it all adds up when some of these organic compounds are very large)! Thus when you see something like:

‘read’ it as:

And once you add the carbons in, you have to then add the hydrogens where appropriate (as above) so the valency for each carbon atom is always four…it’s all so tiresome!

Amino Acids and Proteins

Proteins are both the structural components in plants as well as enzymes (biological catalysts essential in many biochemical reactions). And proteins are made up of amino acids. Just as all chlorophylls have a common chlorin ring structure, so too do amino acids have a common structure, depicted generically as:

where the R group is the variable part (the letter ‘R’ was chosen as it is not used for any element in the Periodic Table and thus can’t be mistaken for one). You can see all twenty versions of the R group here, each of which makes a unique amino acid. (Fun story: once upon a time I could actually write every single one of these from memory — and tell you which RNA codons coded for each of these — not anymore!)

Every single amino acid has the nitrogen-containing amine (-NH₂) group in common, regardless of the R group (some of which also contain nitrogen).

Adenosine Triphosphate (ATP)

ATP, or adenosine triphosphate, is the molecule that provides the energy that drives many, many biochemical reactions. It is found in all life, from the most ancient bacteria, to plants, to animals. In A World Without Prokaryotes I linked to this image just as an FYI to show what cellular respiration is, deliberately not providing further explanation as it’s incredibly involved and wasn’t relevant to the discussion. (Though I may pick it apart one day for anyone who’s interested.)

I never mentioned ATP there, but here I link to that image again to make the point as to how vital ATP is in cellular functions. Look how many times it is needed, and just look at all those nitrogen atoms in the diagram below! (That is actually very significant when it comes to energy, and a reason all explosives are also very nitrogen-rich.)

Nucleic Acids

Nucleic acids are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), of which there are three types: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).

DNA codes for genes, mRNA translates those genes into amino acids, and tRNA transfers those amino acids to cellular structures called ribosomes, where rRNA assembles those amino acids into proteins and enzymes.

(Yes, that’s really all there is to it! DNA is nothing more than a set of instructions on how to assemble amino acids together. But it’s the proteins those amino acids then form that determine everything else and make an organism ‘go’.)

DNA is made of four bases: cytosine (C ), guanine (G), adenine (A) and thymine (T). RNA has the first three, but the fourth base is uracil (U) instead of thymine. Again, have a look at all that nitrogen!

[With reference to the diagram above: RNA usually exists as a single-stranded molecule folded back onto itself (folding not shown here), rather than the famous double-stranded helix of DNA.]

Growth and Reproduction

A growing plant uses proteins and enzymes to grow additional cells that then differentiate into roots, branches, leaves, flowers, fruits and seeds. These cells come from cellular division, which is where one cell divides in two to make two copies of itself, each of which creates another two copies, and so on. Each copy contains identical DNA, which itself had divided and copied itself into each cell during division.

The specialised sex cells (pollen and ovules) also form from (a different type of) cellular division which also requires additional production of DNA and proteins.

Thus growth and reproduction are also heavily dependent on nitrogen, simply because of the extra DNA and proteins that must be formed to make the cells.

From all this it’s pretty clear that nitrogen features prominently in all life processes of a plant! It is required for photosynthesis, amino acid and protein synthesis, DNA replication, growth and reproduction, and as a major component of the energy-providing ATP molecule.

Nitrogen Deficiency Symptoms

All these functions is why nitrogen is by far the most important macronutrient for plants, and it follows that a lack of nitrogen — even starvation — would limit these functions noticeably.

Yet it’s because nitrogen features in so many different processes that a nitrogen deficiency is surprisingly hard to diagnose. This is because these processes involve complex pathways to get from beginning to end, and a disruption anywhere along a pathway by some other, different, factor can lead to the same visual syndromes.

For example, stunted growth would be a very obvious first sign of a nitrogen deficiency, as this element is prominent in photosynthesis as well as growth. But stunted growth could also arise from a herbicide, toxicity in the soil (eg aluminium), the ‘wrong’ soil pH, a bacterial or fungal disease preventing transportation of nutrients in general, or simply some other nutrient deficiency(ies) altogether.

While these things should always be considered, and barring a soil and/or leaf analysis which would confirm nitrogen levels definitively, stunted, starved growth is still a common enough sign of a nitrogen deficiency to suspect one. Another strong sign is the presence of yellowing leaves, but even here other factors must still be considered, as yellowing leaves can also be caused by other deficiencies, soil pH, diseases, or simply too little or too much watering!

Nitrogen-deprived leaves yellow as they suffer from chlorosis, or the insufficient production of chlorophylls. This leads to a collapse of chloroplasts and lack of development of new ones. Chlorophylls are green, thus a lack of these reveal the yellow pigments (the carotenoids) also in a leaf. As nitrogen is a major component of chlorophylls, and chloroplasts are in every leaf cell, nitrogen deficiency usually shows as a uniform chlorosis across the entire leaf, which turns yellow or yellow-white. The leaf may further turn brown, die and fall off.

Other deficiencies, such as in potassium (K), magnesium (Mg), iron (Fe), calcium (Ca), and sulfur (S) can also produce yellow leaves, but there are two general differences that help distinguish these from a nitrogen deficiency.

The first difference is that potassium and magnesium deficiencies are both marked by chlorotic and necrotic (localised, dead) spots in older leaves, but appear at a very early stage. Chlorosis induced by a nitrogen deficiency is more even across the leaf, and necrosis of the leaf, or part of the leaf, only occurs at a very late stage of the deficiency.

The second difference is that while iron, calcium and sulfur deficiencies also result in yellow and pale leaves, the symptoms first appear in the younger leaves. A nitrogen deficiency begins in the older leaves at the bottom of a plant. Nitrogen is highly mobile in plants, thus that obtained from the degrading chloroplasts in lower leaves can be relocated to newer growing leaves higher up the plant which remain healthy and green. Unless, of course, the deficiency is so bad the entire plant succumbs.

Nitrogen Toxicity Symptoms

All fertilisers contain good amounts of nitrogen, as it is such a well-known and crucial macronutrient. Following the manufacturer’s directions is always recommended, for the same reason you should follow instructions on prescription medication — to avoid an overdose and minimise side effects. It can be very tempting to think if some is good then more is better, and over-apply. Often in the case of fertilisers this leads to nothing more than a waste of money and product, as any excess nitrogen compounds are either taken up by microbes in the nitrogen cycle (if applied as ammonium) or simply leached away before the plant can use them (if applied as nitrate).

It is still possible to create a nitrogen toxicity however, especially if applying large amounts of ‘hot’ manures that haven’t been aged or composted first. Nitrogen toxicity can also be a problem in hydroponics systems if incorrect fertiliser mixes are applied and/or the pH of the solution wasn’t monitored and adjusted.

High levels of nitrogen can led to ‘nitrogen burning’. This is when nitrogen salts (specifically ammonium salts) are so concentrated around the roots that they draw water out of the root cells via osmosis [a phenomenon whereby water flows from an area of higher purity (here, the root cell) into a region of lower purity (the soil) until equilibrium is established. This is also why you wrinkle when in salt water long enough!]. This creates a flow-on effect, whereby water enters the now dehydrated root cells from surrounding cells, and so on. Eventually water is drawn from the cells on leaf edges, which are the ‘end of the line’ with no further cells to draw water from. These edge cells, drained and dehydrated, will die and collectively create yellow or brown edges that make the affected leaves appear ‘burnt’. Flushing the soil with copious watering can correct this, but if salt concentrations were so high that root cells died then the plant is not likely to survive.

Another sign of nitrogen toxicity is overly-rapid, jungle-like growth, as the excess nitrogen finds a home in increased chlorophyll production and cellular division. Still another, related, sign of toxicity is a very noticeable darkening of the leaves from all the extra chlorophyll in all the extra leaf cells. These cells may be smaller and more packed in the leaf because of the accelerated growth, which increases the chlorophyll density compared to a similar area of a normal leaf.

Excess nitrogen fuels growth, and a nitrogen-rich plant will look lush and healthy on the outside, almost succulent. But this is at the expense of root and flower development, as all that nitrogen has to go somewhere first, which is into the stems and leaves which use it, and not the roots and flowers which don’t. Worse, excess nitrogen prevents potassium uptake, further exacerbating flowering and fruiting problems.

In short, nitrogen-rich plants tend to be top-heavy and unstable from the excess growth, with stunted roots and an inability to reproduce. The additional stem and leaf tissue produced also holds more water than usual, which makes them more attractive to mites and other pests.

If you’ve always known that nitrogen is ‘important’, but wanted more detail other than ‘for growth’, I do hope this post has helped! Having a deeper understanding of what makes nitrogen an essential macronutrient can only help understand the interactions of other nutrients too. And these we will cover one by one, with potassium the next one in line.