The essential plant nutrients and their deficiency symptoms
The 17 elements every plant needs, the rules that make an element essential, what each one does, and how to read deficiency symptoms.
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A plant builds itself from seventeen chemical elements. Take away any one of them and it cannot complete its life cycle, however much of the others it has. These are the essential nutrients, and knowing them, their forms, their jobs and the signs of their shortage is basic to soil fertility, crop production and plant physiology alike.
This is one of the most examined topics in agriculture anywhere: classification questions, "which element is the centre of chlorophyll", and matching deficiency diseases to nutrients appear in entrance tests and professional examinations year after year.
What makes an element essential
The criteria were set out by Arnon and Stout in 1939, and they are still the textbook definition. An element is essential if:
- the plant cannot complete its life cycle (from seed to seed) without it;
- its role cannot be filled by another element; and
- it is directly involved in the plant's metabolism, as part of a molecule or an enzyme, not merely improving conditions around the plant.
Some elements help certain plants without meeting all three tests. These are beneficial elements: sodium (in some C4 and CAM plants and salt-tolerant crops), silicon (strengthens rice, sugarcane and other grasses against lodging and disease), cobalt (needed by the rhizobia in legume nodules, and by animals), and selenium (needed by animals, not plants).
The seventeen, grouped
| Group | Elements | Taken up mainly as |
|---|---|---|
| From air and water | carbon, hydrogen, oxygen | CO2, H2O, O2 |
| Primary macronutrients | nitrogen, phosphorus, potassium | NO3- and NH4+; H2PO4- and HPO42-; K+ |
| Secondary macronutrients | calcium, magnesium, sulphur | Ca2+; Mg2+; SO42- |
| Micronutrients | iron, manganese, zinc, copper, boron, molybdenum, chlorine, nickel | Fe2+ and chelates; Mn2+; Zn2+; Cu2+; H3BO3; MoO42-; Cl-; Ni2+ |
Carbon, hydrogen and oxygen make up roughly 95 per cent of a plant's dry weight. The remaining fourteen come from the soil. Macronutrients are needed in large amounts (typically 0.1 to 5 per cent of dry matter); micronutrients in tiny amounts (from a fraction of a part per million for molybdenum and nickel to around 100 parts per million for iron). "Micro" refers to the quantity, not the importance: a zinc-deficient crop can fail as completely as a nitrogen-deficient one.
Nickel was the last element added to the list, in the late 1980s, after it was shown to be needed for the enzyme urease.
What each nutrient does
| Nutrient | Main roles | Typical deficiency |
|---|---|---|
| Nitrogen | proteins, chlorophyll, nucleic acids | pale plant; yellowing from tips of older leaves |
| Phosphorus | ATP and energy transfer, nucleic acids, phospholipids; root growth and flowering | stunted, dark green or purple older leaves, delayed maturity |
| Potassium | stomatal opening, enzyme activation, osmotic balance, sugar transport | scorched margins of older leaves, weak stalks, lodging |
| Calcium | cell walls (calcium pectate), membranes, cell division | dead growing points; blossom-end rot of tomato, tip burn of lettuce, bitter pit of apple |
| Magnesium | centre of the chlorophyll molecule; enzyme activation | yellowing between veins of older leaves |
| Sulphur | amino acids cysteine and methionine; oil content of brassicas | uniform yellowing, often of younger leaves |
| Iron | chlorophyll synthesis, cytochromes, ferredoxin | interveinal chlorosis of young leaves, common on calcareous soils |
| Manganese | water-splitting in photosynthesis (oxygen-evolving complex) | interveinal chlorosis; grey speck of oats, marsh spot of peas |
| Zinc | enzymes; synthesis of the auxin precursor tryptophan | small leaves, short internodes ("little leaf", rosetting); khaira of rice; white bud of maize |
| Copper | plastocyanin, enzymes of lignin formation | dieback, white or twisted leaf tips of cereals, poor grain set |
| Boron | cell walls, pollen tube growth, sugar transport | dead buds; hollow stem of brassicas; heart rot of beet; cracked or corky fruit |
| Molybdenum | nitrate reductase, and nitrogenase in nodules | nitrogen-deficiency symptoms in legumes; whiptail of cauliflower |
| Chlorine | water-splitting in photosynthesis, osmotic regulation | wilting, bronzing (rare in the field) |
| Nickel | urease | leaf-tip necrosis from urea build-up; "mouse-ear" in pecan |
Reading the symptoms: mobility in the plant
Where on the plant a symptom appears first is the single most useful clue, and it depends on whether the plant can move the nutrient out of old tissue to feed new growth.
- Mobile in the phloem: nitrogen, phosphorus, potassium, magnesium (and chlorine). Deficiency shows first on older, lower leaves.
- Immobile or poorly mobile: calcium, boron, iron, manganese, zinc, copper, and sulphur in most crops. Deficiency shows first on young leaves, buds and fruit.
Calcium is the clearest example. It moves to leaves and fruit only in the transpiration stream and is hardly redistributed afterwards. A tomato fruit transpires little, so a few days of drought or uneven watering leave its tip short of calcium and it develops blossom-end rot, even in a soil rich in calcium. The remedy is steady watering as much as calcium.
Two further distinctions help: chlorosis (yellowing) between green veins points to magnesium (old leaves) or iron, manganese and zinc (young leaves), while yellowing of the whole leaf points to nitrogen (old leaves) or sulphur (young leaves).
Symptoms are clues, not proof
Many things mimic deficiency: waterlogging, cold, herbicide injury, root disease, nematodes and drought. Two nutrients can be short at once, and one can induce another's deficiency: excess phosphorus can induce zinc deficiency, heavy potassium can induce magnesium deficiency, and liming can induce manganese or boron deficiency. By the time a symptom is visible, yield is often already lost (hidden hunger comes first). Confirm with soil tests and plant tissue analysis, compared against critical concentrations: the level in a specified plant part, at a specified stage, below which yield falls.
Liebig's law of the minimum
Justus von Liebig's principle, popularised in the nineteenth century, states that growth is controlled by the scarcest resource, not by the total supply. It is often drawn as a barrel whose staves are the nutrients: water can be held only up to the shortest stave. Adding more of an already sufficient nutrient raises nothing until the limiting one is supplied. Modern agronomy treats it as a simplification (nutrients interact, and responses curve rather than stop abruptly), but it remains the right first question: what is limiting this crop?
Reading a fertiliser label
Fertiliser grades are written as N–P2O5–K2O by convention, not as elemental phosphorus and potassium. How much of each element is in a 50 kg bag of a 10–26–26 compound?
- Nitrogen: 50 × 0.10 = 5.0 kg N.
- Phosphate: 50 × 0.26 = 13.0 kg P2O5. Phosphorus is 2 × 30.97 ⁄ 141.94 = 0.436 of P2O5, so 13.0 × 0.436 = 5.7 kg P.
- Potash: 13.0 kg K2O. Potassium is 2 × 39.10 ⁄ 94.20 = 0.830 of K2O, so 13.0 × 0.830 = 10.8 kg K.
Mixing up oxide and element is a frequent slip in exams and in comparing recommendations from different sources.
Key points
- Seventeen elements are essential: C, H, O; N, P, K; Ca, Mg, S; Fe, Mn, Zn, Cu, B, Mo, Cl, Ni.
- Essentiality (Arnon and Stout, 1939): needed to complete the life cycle, irreplaceable, directly involved in metabolism.
- Magnesium is at the centre of chlorophyll; molybdenum is in nitrate reductase and nitrogenase; nickel is in urease.
- Mobile nutrients (N, P, K, Mg) show deficiency on old leaves; immobile ones (Ca, B, Fe, Mn, Zn, Cu, S) on young leaves and growing points.
- Classic disorders: blossom-end rot (Ca), whiptail (Mo), khaira and little leaf (Zn), hollow stem and heart rot (B), grey speck (Mn).
- Fertiliser grades are given as P2O5 and K2O; P = 0.436 × P2O5, K = 0.830 × K2O.
What the exams ask
- School-leaving and university entrance papers: the list and its grouping, uptake forms, the element at the centre of chlorophyll, and matching named deficiency disorders to their nutrients.
- Degree and research-fellowship entrances: criteria of essentiality, beneficial elements, the enzymes that need molybdenum and nickel, mobility and where symptoms appear, nutrient interactions.
- Licensing and professional examinations for agronomists: diagnosing deficiency in the field, critical concentrations and tissue testing, and fertiliser calculations between oxide and element.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Is silicon an essential nutrient?
Not by the strict definition, because most plants complete their life cycle without it. It is beneficial, strongly so for rice and sugarcane, where it stiffens stems and leaves and reduces disease and pest damage.
Why do some yellow leaves appear at the top and some at the bottom?
Because of mobility. Nutrients the plant can move, like nitrogen and magnesium, are taken from old leaves first, so the bottom yellows. Nutrients it cannot move, like iron and sulphur, are missing from new growth, so the top yellows.
Can foliar sprays fix a deficiency?
Often, especially for micronutrients needed in small amounts, such as zinc, iron, manganese and boron, and as a quick rescue. Macronutrients are needed in quantities too large to supply fully through leaves, so they are mainly applied to the soil.
Sources
- Marschner, P. (ed.) Marschner's Mineral Nutrition of Higher Plants, 3rd edition. Academic Press, 2012.
- Arnon, D. I. and Stout, P. R. "The essentiality of certain elements in minute quantity for plants with special reference to copper." Plant Physiology 14, 371–375 (1939).
- Brown, P. H., Welch, R. M. and Cary, E. E. "Nickel: a micronutrient essential for higher plants." Plant Physiology 85, 801–803 (1987).
- Taiz, L., Zeiger, E., Møller, I. M. and Murphy, A. Plant Physiology and Development, 6th edition. Sinauer, 2015. Chapter 5, Mineral nutrition.
- Havlin, J. L. et al. Soil Fertility and Fertilizers, 8th edition. Pearson, 2014.
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