What is a fertiliser?

What is a fertiliser? A guide to essential plant nutrients

By Joe, Founder of Dr Forest · April 2026

A fertiliser is any material added to soil, water or directly to plants to supply one or more of the seventeen essential plant nutrients.

Plant nutrition runs on seventeen elements. Fourteen of them come from soil and have to be replaced as crops, rain and time remove them. The other three (carbon, hydrogen and oxygen) come from air and water, and the plant takes care of those itself. Everything we call a fertiliser, from a heap of finished compost to a bag of granulated NPK, is a delivery system for some combination of those fourteen mineral nutrients.

A complete fertiliser supplies the three primary macronutrients (nitrogen, phosphorus and potassium) plus some combination of the other eleven mineral nutrients. The numbers on the front of a fertiliser bag, the familiar three figures like 10-5-5 or 4-3-6, give the percentages of those three primary macros by weight. Everything else is listed elsewhere on the label, when it is listed at all.

This article sets out what a fertiliser actually is and what each of the seventeen essential nutrients does in the plant. It covers the deficiency symptoms each nutrient produces, the difference between mobile and immobile nutrients (which is the practical diagnostic shortcut every gardener should know), and how to read both the bag and the leaf when something goes wrong.

What a fertiliser actually does

In the simplest terms, a fertiliser supplies a soluble or microbially accessible form of one or more essential plant nutrients to a plant's root zone, where the plant can take it up and use it. Some fertilisers also work foliarly, sprayed onto leaves where small amounts of nutrient are absorbed directly through the leaf surface.

Soil is the working substrate of plant life. It supplies fourteen of the seventeen elements a plant needs, holds water and air at the roots, and provides a habitat for the soil microbes that mineralise organic matter into plant-available form. Soil reserves are not bottomless. Every harvest takes away minerals locked into the leaves, fruits and seeds removed from the plot. Every winter rain leaches the soluble nutrients further down the profile, sometimes below the root zone entirely. Over years and decades, this one-way drain depletes soil to the point where plants no longer have what they need.

Fertilisers reverse that drain. They replace specific nutrients, balance ratios, correct documented deficiencies, and in some cases supply organic matter and microbial substrate alongside the minerals. A finished compost is a fertiliser. So is a bag of pelleted chicken manure. So is a bottle of liquid seaweed concentrate, a granular NPK blend, a quarry-mined polyhalite, and a hose-end feeder of dilute Epsom salts. The product label does not always say "fertiliser", but anything supplying any of the seventeen essential nutrients qualifies.

A few practical distinctions worth knowing. Fertilisers can be solid (granular or powdered) or liquid (pre-mixed concentrate). They can be slow-release, like coated granules or rock-mineral sources releasing over months and years, or quick-release, like soluble salts and liquid feeds available within hours of watering in. They can supply a single nutrient (sulphate of potash for K) or several together (a 10-5-5 NPK blend covers all three primary macros). They can be applied to soil, where the bulk go, or sprayed onto leaves as a foliar feed for fast uptake of trace elements.

The 17 essential plant nutrients

Plants are picky about which of the periodic table's elements they actually need. By long-standing botanical convention an element only counts as "essential" if three criteria are met: the plant cannot complete its life cycle without it, no other element can substitute for it, and it has a specific role in plant metabolism. By those rules, only seventeen elements qualify, and they are the same seventeen for every higher plant species on earth.

Three are taken from air and water rather than soil. Carbon comes from atmospheric CO₂. Hydrogen and oxygen come from water. Most gardeners never think about these because the plant takes care of them itself: photosynthesis turns atmospheric carbon and water into sugars, and the rest of the plant follows from there.

The other fourteen come from soil, and these are the ones a fertiliser exists to manage. They are split by the quantity in which plants need them. Six are macronutrients, used in the largest amounts. Eight are micronutrients (also called trace elements), used in tiny quantities, sometimes just parts per million in plant tissue. The macronutrients are themselves split into primary (nitrogen, phosphorus and potassium, the famous NPK) and secondary (calcium, magnesium and sulphur).

Every essential nutrient has a specific job. The next two sections set out what each one does, what its absence looks like on the leaf, and the natural sources gardeners can reach for to supply it.

The six soil-derived macronutrients

The three primary macros (NPK) are needed in the largest quantities, are most commonly deficient in cropped soils, and are what the three numbers on a fertiliser bag refer to. The three secondary macros (calcium, magnesium, sulphur) are needed in similar absolute quantities, but most natural soils carry workable reserves of them and chronic deficiency is less common. All six show classic, recognisable deficiency symptoms when they do run short.

Two patterns are worth noting. Nitrogen, magnesium and potassium deficiencies all start on older leaves because those nutrients are mobile inside the plant. When supply is short, the plant pulls them out of older tissue to feed the new growth. Calcium deficiency shows up first on new growth because calcium does not move once it is laid down. Any shortfall affects what the plant is currently trying to build, which is why blossom-end rot appears at the bottom of a developing tomato fruit and not on the leaves around it.

The mobility pattern is the single most useful diagnostic shortcut in plant nutrition, and there is a dedicated section on it further down.

The eight trace elements

The eight micronutrients are needed in tiny quantities, in some cases just a few parts per million in plant tissue. Tiny quantities does not mean optional. A plant short on iron, zinc or boron is a plant that cannot do its full chemistry, and the visible symptoms can be just as severe as a primary macro deficiency. Most micros are immobile in the plant, which means deficiency symptoms appear first on young leaves and growing tips.

The "rare deficiency" notes are worth taking seriously. Chlorine and nickel are technically essential but appear missing only in unusual soils, almost always with cropping histories that have selectively stripped them. Most gardens never need to think about either. The trace elements that cause real practical problems in UK gardens are iron (the classic interveinal yellowing of acid-loving plants on alkaline soils), boron (fruit set issues in pumpkins and tomatoes), manganese (interveinal chlorosis on calcareous soils), and zinc (small bunched leaves on a few susceptible crops).

Mobile vs immobile: where the symptoms appear

Some nutrients move freely inside the plant. Some do not. Once iron or calcium is laid down in a leaf, it stays there. The plant cannot take it back to feed new growth. Nitrogen, magnesium and potassium are the opposite. When supply runs low, the plant cannibalises older tissue to keep building new growth, pulling those mobile nutrients out of yellowing lower leaves and shipping them upwards.

The practical consequence is that you can read a deficiency by looking at where the symptoms appear.

This is the single most useful question to ask when a plant looks unhappy: are the symptoms on old leaves or new ones? Ask that first, and you have ruled out half the candidates before looking up the colour of the chlorosis.

From there, the colour and pattern of the discoloration narrow it down further. Even yellowing across older leaves usually means nitrogen. Interveinal yellowing (the green veins remain visible while the leaf flesh turns yellow) on older leaves usually means magnesium; on young leaves it usually means iron. Yellow margins scorching to brown on older leaves usually means potassium. Distorted, brittle, or hollow new growth usually means calcium or boron.

Beneficial elements

There are elements that are not formally classed as essential but that benefit some plants under some conditions. The five most studied are silicon, cobalt, selenium, sodium and vanadium. They are usually grouped under the umbrella of "beneficial elements" or "trace minerals" in soil-building contexts.

Silicon strengthens cell walls, improves drought and pest resistance, and activates secondary metabolite production. It is required as essential by some plants, particularly grasses, rice and members of the Equisetaceae, but officially classed as beneficial rather than essential for most. Volcanic rock dust is the most concentrated natural source.

Cobalt is required by the rhizobial bacteria that fix nitrogen in legumes. It is essential to those bacteria and beneficial to the host plant.

Selenium is readily taken up by plants although they do not strictly require it. Selenium-rich crops carry that selenium into human nutrition, which matters in regions where soil is low in selenium. UK soils generally are.

Sodium is beneficial for some C4 plants (sugar beet most notably) and for halophyte species that have evolved to tolerate or use it.

Vanadium has been studied as a low-level biostimulant for fruit set and as an alternative to molybdenum in some nitrogen-fixation systems.

These are the elements that come up when soil scientists and serious gardeners talk about "trace minerals". They are typically supplied by broad-spectrum mineral amendments such as basalt rock dust, seaweed, and humic acid extracts rather than by targeted fertilisers.

How to read a fertiliser label

The three numbers on the front of a fertiliser bag, the familiar 10-5-5 or 4-3-6, give the percentages of the three primary macronutrients by weight. They are always in the order N-P-K. A 10-5-5 fertiliser is therefore 10% nitrogen, 5% phosphorus, 5% potassium by weight. The remaining 80% is made up of carrier material, secondary nutrients, organic matter, and (in some blends) inert filler.

One oddity. The phosphorus and potassium figures are not quite the elemental values. By long-standing fertiliser convention, the "P" number is phosphorus pentoxide (P₂O₅), which is roughly 44% elemental phosphorus by weight. The "K" number is potassium oxide (K₂O), which is roughly 83% elemental potassium. So a 10-5-5 fertiliser actually contains about 10% N, 2.2% P and 4.2% K when measured as the pure elements. Don't worry about the conversion in everyday use. Every bag in every garden centre uses the oxide convention, so the numbers compare directly across products.

A higher first number (N) means a feed weighted towards leafy growth: lawn feeds, leafy crops, early-season vegetative push. A higher middle number (P) is sometimes called a "bloom" or "rooting" feed, although the case for high-P fertilisers in established soil is weaker than the marketing suggests, since most UK soils already hold workable phosphorus reserves. A higher last number (K) is a fruiting and finishing feed, used on tomatoes, fruit trees, and any crop where sweetness and storage life matter.

Secondary nutrients (calcium, magnesium, sulphur) and trace elements are sometimes listed elsewhere on the bag, sometimes in a separate analysis panel, and sometimes not at all. The absence of a printed value does not necessarily mean absence of the nutrient (most natural fertilisers contain trace elements as a matter of geology), but it does mean the manufacturer is not guaranteeing a specific amount.

Organic, synthetic and mineral

Fertilisers are commonly split into "organic" and "synthetic", and the framing is treated as a moral question when it is in fact a practical one. Both types work. They behave differently. The third category, natural mineral amendments, sits between the two.

A synthetic fertiliser supplies its nutrients as soluble mineral salts manufactured through industrial chemical processes. Ammonium nitrate (a source of nitrogen), potassium chloride (potassium), and triple superphosphate (phosphorus) are typical examples. The nutrients are immediately available to the plant, which is why synthetic fertilisers act fast.

An organic fertiliser, in the gardening sense, is one whose nutrients are bound into organic matter such as compost, manures, plant-based meals, hydrolysed proteins, or seaweed extracts. The nutrients release more slowly, over weeks to months, in step with microbial activity in the soil.

A natural mineral amendment is mined rather than manufactured. Polyhalite, rock phosphate, lime, gypsum and basalt rock dust all fall into this category. They supply nutrients in slow-release mineral form. They behave more like organic fertilisers in their release kinetics while supplying purely mineral nutrition.

Most serious gardeners and growers end up using a combination. Composts and plant-based meals for general feeding and soil biology. Mineral amendments for the slow background remineralisation. Liquid feeds (which can be either organic or synthetic) for in-season adjustment when a crop needs a quick boost. Synthetic single-nutrient fertilisers tend to play the smallest role in this kind of programme, used mainly for documented deficiencies that need rapid correction.

The practical takeaway

Most plants in most gardens are not deficient in dramatic ways. They are running borderline on something, usually a trace element or a secondary macronutrient, and the result shows up as lower yield, less robust flavour, and more disease pressure than there ought to be. The diagnostic framework matters because the difference between a mediocre crop and a good one is often a single missing element. Read the leaves. Read the bag. When in doubt, build the soil.

Frequently asked questions

Reference works