What is a fertiliser?

Soil science

What is a fertiliser?

By Joe, Founder of Dr Forest · June 2026

Strip away the marketing and a fertiliser is a plain thing: food for the part of a plant you can't see it feeding, the roots, working away in the dark.

A fertiliser is any material you add to soil or to a plant to supply the nutrients it needs to grow. Most of a plant is built from air and water. Carbon, hydrogen and oxygen make up roughly 95% of its dry weight, and the plant gets all three for free through its leaves and roots. The remaining 5% or so is a set of mineral nutrients pulled up through the roots from the soil. When the soil runs short of those minerals, growth slows, leaves pale and yields drop. A fertiliser tops them back up.

The three numbers on the front of every bag, the NPK, tell you how much nitrogen, phosphorus and potassium are inside. That is the core of it. Everything else, organic against mineral, granular against liquid, is detail layered on top.

So what is a fertiliser, really?

The textbook definition is dry but useful: a fertiliser is a material, natural or manufactured, applied to supply one or more plant nutrients. The key word is nutrients. A fertiliser is not the same as a soil improver. Compost, leaf mould and well-rotted manure mainly improve the structure and life of the soil, holding water, feeding worms and microbes, opening up heavy clay. They do carry some nutrients, but at low concentration. A fertiliser is the concentrated end of that scale: its job is to deliver nutrients in a known amount.

The reason any of this matters comes down to a rule from the 1840s that still holds. Growth is limited by whichever nutrient is in shortest supply, not by the ones you have plenty of. Pour on nitrogen all you like; if the plant is short of potassium, potassium is what caps the growth. A good fertiliser is really an exercise in not letting any one nutrient become the bottleneck. That is why a balanced feed often beats a single heavy dose of one element.

Figure 1 · Where a plant's body comes from

Most of a plant is air and water. Fertiliser handles the rest.

The 17 nutrients a plant needs to complete its life cycle, grouped by where each one comes from.

From air & water · about 95% of dry weight

CHO

↓  the rest is drawn up through the roots  ↓

From the soil · 14 mineral nutrients · about 5% of dry weight

Primary (the NPK on the bag)

NPK

Secondary

CaMgS

Micronutrients (eight)

FeMnZnCuBMoClNi

The 17 nutrients every plant needs

Seventeen elements are currently considered essential, meaning a plant cannot complete its life cycle without them. Three of those, carbon, hydrogen and oxygen, come from air and water and account for the vast bulk of a plant's body. The other 14 are mineral nutrients, taken up as dissolved ions through the roots, and these are the ones fertiliser exists to supply.

They split into three tiers by how much the plant uses. The primary macronutrients, nitrogen (N), phosphorus (P) and potassium (K), are needed in the largest amounts and are the three a plant most often runs short of. The secondary macronutrients, calcium (Ca), magnesium (Mg) and sulphur (S), are needed in smaller but still meaningful quantities. Then come the eight micronutrients, used in tiny traces but no less essential: iron, manganese, zinc, copper, boron, molybdenum, chlorine and nickel. A trace shortage can stall a plant just as hard as a major one.

Figure 2 · The 14 nutrients fertiliser supplies

What each one does, and how a shortage shows up

The mineral nutrients drawn from the soil, with the classic sign of a deficiency.

Nutrient	Group	Main job in the plant	A shortage often shows as
Nitrogen (N)	Primary	Leaf and shoot growth, green colour	Older leaves yellow first, weak growth
Phosphorus (P)	Primary	Roots, establishment, flowering	Dull, sometimes purple older leaves
Potassium (K)	Primary	Flowers, fruit, water use, hardiness	Scorched, browning leaf margins
Calcium (Ca)	Secondary	Cell walls, new growth structure	Distorted new tips, blossom-end rot
Magnesium (Mg)	Secondary	Core of the chlorophyll molecule	Yellowing between the veins of older leaves
Sulphur (S)	Secondary	Proteins and enzymes	Pale, uniform yellowing of new growth
Iron (Fe)	Micro	Chlorophyll formation	Yellowing between veins of the youngest leaves
Mn, Zn, Cu, B, Mo, Cl, Ni	Micro	Enzymes, hormones, pollination, nitrogen use	Varied: distortion, spotting, poor set

A quick reading trick sits inside that table. Mobile nutrients such as nitrogen, phosphorus, potassium and magnesium can be moved around inside the plant, so when they run short the plant robs its old leaves to feed the new ones, and the oldest leaves show the damage first. Immobile nutrients such as calcium, sulphur and iron cannot be shifted once placed, so a shortage shows up in the newest growth. Where the yellowing appears tells you which half of the table to look at.

What the three NPK numbers mean

NPK stands for nitrogen, phosphorus and potassium, in that order. A bag marked 6-6-6 contains 6% nitrogen, 6% phosphate and 6% potash by weight. The first number is straightforward elemental nitrogen. By long-standing convention the second is reported as phosphate (P₂O₅) and the third as potash (K₂O), which is just the traditional way the industry writes phosphorus and potassium. It is worth knowing because it means the P and K numbers slightly overstate the pure element, but every bag uses the same convention, so like compares with like.

Figure 3 · Reading the bag

A 6-6-6 label, decoded

The three numbers are percentages by weight. The rest of the bag is organic matter and carrier.

So a higher set of numbers does not automatically mean a better feed. It means a more concentrated one. A 6-6-6 and a 20-20-20 carry the same balance; the second is simply stronger per gram, which is why mineral feeds usually carry bigger numbers than organic ones. The number you actually want depends on the plant and the season, which is where the next question comes in.

Organic and mineral fertilisers

Every fertiliser falls into one of two broad camps, and the difference is less about the nutrients themselves than about how they are delivered. A nitrogen atom is a nitrogen atom whatever its source. What changes is the packaging and the speed.

Mineral fertilisers, often called synthetic or chemical, supply nutrients in a soluble, ready-to-use form. They act fast and the numbers on the bag tend to be high. The trade-off is that they feed the plant rather than the soil, wash through quickly, and at heavy rates can scorch roots or push soft, sappy growth. Organic fertilisers supply nutrients locked inside plant or mineral matter that soil life has to break down first. They release slowly, feed the soil and its microbes as well as the plant, and are far more forgiving of a heavy hand.

Figure 4 · Two ways to feed

Organic against mineral, factor by factor

A general comparison. Individual products vary, but the pattern holds.

Factor	Organic	Mineral
Release speed	Slow, over weeks to months	Fast, often within days
Feeds	The soil and its life, then the plant	The plant directly
Longevity	Long, with a buffering effect	Short, leaches readily
Scorch risk	Low, forgiving	Higher if over-applied
NPK numbers	Lower, gentler	Higher, concentrated
Effect on soil structure	Builds it over time	Little, can degrade it

This is the camp Dr Forest sits in, by design. Our range is built from organic ingredients, blended as multi-input feeds so that the secondary nutrients and trace elements come along for the ride rather than being left out. The thinking is simple, and it is the line the whole brand is built on.

Feed the soil, and the soil feeds the plant.

You will also see fertilisers sold as natural plant food. For most gardeners that is the same idea as organic: nutrients from natural sources rather than manufactured salts. It is the phrase a lot of people search for, and it describes the Dr Forest approach well, feeding the living soil rather than just the plant standing in it.

Granular, liquid or slow-release?

Form is the practical layer on top of all this. Granular feeds are scattered and worked into the soil or used as a top dressing; they last and suit a base feed at planting. Liquids are diluted and watered or sprayed on; they act quickly and suit a mid-season boost or a quick correction. Slow-release products, organic feeds included, hand their nutrients over gradually as soil life or coatings break down, which keeps the supply steady and cuts the risk of a flush followed by a famine. Most growing years use a mix: a granular base in spring, a liquid top-up when plants are working hardest.

How much, and when

More is not better. Past the point a plant can use, extra fertiliser either washes away, where it becomes a pollution problem, or builds up as salts that pull water back out of the roots and scorch the plant. The honest answer to "how much" is to follow the rate on the pack and lean towards the lower end if you are unsure. You can always add a second feed; you cannot easily take one back.

Timing follows the plant. Feed when it is actively growing, spring through summer for most, and ease off as growth slows into autumn. And before you reach for any feed at all, it is worth knowing your soil pH, because a plant in the wrong pH cannot take up nutrients no matter how much you give it. We cover how to test and adjust it in our guide to raising and lowering soil pH.

A fertiliser, then, is no mystery. It is a way of putting back the handful of mineral nutrients a plant draws from the soil, in a form and at a time the plant can use. Get that right, keep the soil alive, and most of the rest of gardening looks after itself.

Common questions