What is a fertiliser?

By Joe, Founder of Dr Forest · November 2025, updated September 2026 · Edited 24 Sep 2026 · 11 min read

Most of a plant is built from air and water. Fertiliser is for the mineral fraction the roots have to find.

A fertiliser is any material you add to soil or to a plant to supply one or more of the nutrients it needs to grow. I mix premium organic fertiliser by hand in Stockport, so treat the product notes with the scepticism you would apply to anyone selling you a bag. The definition still holds if you never buy one.

Carbon, hydrogen and oxygen come from carbon dioxide, oxygen and water. They make up the bulk of a plant's dry weight. The rest is a set of mineral nutrients pulled up as dissolved ions through the roots. 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 the bag, the NPK, tell you how much nitrogen, phosphate and potash you are holding.

What a fertiliser is

The textbook line is dry and still the right one: a fertiliser is a material, natural or manufactured, applied to supply one or more plant nutrients. The key word is nutrients. Arnon and Stout (1939) put the test in plant-physiology language: an element is essential if the plant cannot complete its life cycle without it, no other element can replace it, and it is directly involved in metabolism [3].

That is a different job from a soil improver. Compost, leaf mould and well-rotted manure mainly change the structure and the life of the soil: holding water, feeding worms and microbes, opening heavy clay. They do carry some nutrients, at low concentration. A fertiliser is the concentrated end of that scale. Its job is to deliver a known amount of nutrient. The longer argument on dry amendments lives in what are organic dry amendments.

The reason any of this still matters is simple. Growth is limited by whichever nutrient is in shortest supply. Pour on nitrogen all you like; if the plant is short of potassium, potassium is what caps the growth. A good fertiliser is 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.

The 17 nutrients a plant needs

Philip White and Patrick Brown, writing in Annals of Botany in 2010, put it this way: in addition to oxygen, carbon dioxide and water, plants require at least 14 mineral elements for adequate nutrition [1]. Gardeners count seventeen by adding carbon, hydrogen and oxygen to that list of 14. The paper's job is the minerals. Fertiliser's job is the same list.

The six required in large amounts are nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulphur (S). The eight required in smaller amounts are chlorine (Cl), boron (B), iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), nickel (Ni) and molybdenum (Mo). White and Brown list them in that order in the abstract. Nickel is not a curiosity. Brown, Welch and Cary showed it was essential in 1987 [2]. A shortage of a micronutrient can stall a plant as hard as a shortage of nitrogen. The dose is simply smaller.

Figure 1 · schematic illustrative

Where a plant's nutrients come from

Fourteen minerals from the soil, after White and Brown (2010). Carbon, hydrogen and oxygen from air and water sit outside the fertiliser bag.

A quick reading trick sits in that list. Some nutrients can be moved around inside the plant. White and Brown, citing Marschner (1995), note that K, Mg, N, P, S and Cl are transported readily in the phloem, while Mn and Ca are essentially immobile in the phloem of most species, and Fe, Zn, Cu and Mo sit in between [1]. When a mobile nutrient runs short, the plant robs old leaves to feed new ones, so the oldest leaves show the damage first. When an immobile one runs short, the newest growth shows it. Where the yellowing appears tells you which half of the table to look at. That leaf-reading job, for tomatoes, is why are my tomato leaves turning yellow.

Soil pH decides whether those minerals are actually available, even when they are in the soil. How to move pH is how to make soil more acidic, or raise it.

What the three NPK numbers mean

NPK stands for nitrogen, phosphorus and potassium, in that order. A bag marked 4-4-4 contains 4% nitrogen, 4% phosphate and 4% potash by weight. The first number is elemental nitrogen. By long-standing convention the second is reported as phosphate (P₂O₅) and the third as potash (K₂O). That means the P and K numbers slightly overstate the pure element. UK bags are labelled on that convention, so like compares with like.

Figure 2 · reading the bag

A 4-4-4 label, decoded

Our all-purpose bag, checked 6 September 2026: 4% nitrogen, 4% phosphate and 4% potash.

A higher set of numbers does not automatically mean a better feed. It means a more concentrated one. Crop bags change the ratio to match the plant. Tomato 3-4-6 from £11.50 is heavier on potash for fruit. Rose 5-3-5 from £11.50 holds more nitrogen for leaf and repeat flowering. Fruit and vegetable 4-5-6 from £11.50 leans phosphate and potash for yield. If you do not have a crop bag, 4-4-4 is the even feed. All of them are in the organic fertiliser collection.

Organic and mineral feeds

A mineral fertiliser is a salt or a mined concentrate. It dissolves. The plant can take it up the same week. That speed is useful in a greenhouse, and it is also how people burn roots. An organic fertiliser is made with organic ingredients. Release is usually slower, because soil life has to process part of it. I mix plant-based feeds where the recipe allows. Blood and fish meal are common garden-centre answers I leave out; All Purpose 4-4-4 and Bloom use bone meal for slow phosphorus.

White and Brown remind us that N, P and K are the three most often supplied as fertiliser because soils rarely hold enough of them, in a form roots can use, for a fast-growing crop [1]. That is true whether the bag is a mineral salt or a slow organic blend. The difference is the release curve and what else comes with the nutrients: organic matter, traces, and in our bags Yorkshire polyhalite as a potassium source on several crop feeds.

If you want the crop-specific tomato argument, that is best fertiliser for tomatoes. If you want a high-nitrogen grass or foliage feed, that is high nitrogen feed. Flowering mixes live on what is a bloom fertiliser. For "which bag do I actually pick", use which fertiliser do I need, then open the organic fertiliser collection.

Granular, liquid or slow-release

Granular slow-release is the default for UK borders and pots: one application at planting or in spring, then a top-dress later if the crop is hungry. Liquid is for a fast correction, or for containers you water often enough to wash a granular feed through. Foliar sprays are a leaf-level patch, useful for traces and for nitrogen when roots are cold or wet. They do not replace a soil programme.

Slow-release is not magic. It still has an NPK. It still obeys the shortest-nutrient rule. What it does well is stop you dumping a soluble salt every weekend and wondering why the leaves scorch. The liquids and the granules are both in the organic fertiliser collection, with the crop bags at the top.

For a soluble biostimulant rather than a high-NPK feed, seaweed extract powder mixes into water as a mild plant food.

How much, and when

Read the label on the bag you actually bought. Rates change with concentration and with whether you are feeding a lawn, a tomato, or a mixed border. As a working habit for UK gardens: feed in active growth, not in midwinter dormancy; water it in; do not stack a high-nitrogen liquid on top of a high-nitrogen granular "just in case". Extra nitrogen grows leaf at the expense of flower and fruit, and it grows that leaf soft.

If you are stuck between bags, start with 4-4-4 in mixed planting, switch to a crop ratio once you know the plant, and keep the single-nutrient minerals (potash, phosphate, magnesium, nitrogen) for a named shortage rather than as a weekly ritual. That whole set is the organic fertiliser collection: slow-release feeds, every ingredient named on the label.

In the organic fertiliser collection the crop blends sit first: tomato, rose, fruit and vegetable, chilli, strawberry, potato, lawn, bloom. Under them are the straight minerals, polyhalite, sulphate of potash, rock phosphate, kieserite, nitrogen. Use the blend when you want a programme. Use the mineral when a leaf or a soil test has named the gap. Do not run both at full rate in the same week unless you know why.

Related: Organic fertiliser collection · All-purpose 4-4-4, from £12.00 · Tomato 3-4-6, from £11.50 · Rose 5-3-5, from £11.50 · organic fertiliser collection · which fertiliser do I need

Name the shortage, read the three numbers, pick the bag that matches the plant. Then put it down at the rate on the pack and let the roots do the quiet work in the dark.

Frequently asked questions

Organic fertiliser in a UK garden

“Organic” on a fertiliser bag usually means the nutrients are held in plant, animal or mined materials that soil life must process — not that every bag carries a whole-product Soil Association mark. That is why a balanced organic fertiliser feels slower than a soluble blue liquid, and why cold soil stalls it. When you want one default bag for mixed beds and pots, use All Purpose 4-4-4. When you want to compare crop blends and single nutrients, browse organic fertilisers.

Sources

1. White, P.J. and Brown, P.H. (2010). Plant nutrition for sustainable development and global health. Annals of Botany 105(7):1073–1080. doi:10.1093/aob/mcq085
2. Brown, P.H., Welch, R.M. and Cary, E.E. (1987). Nickel: a micronutrient essential for higher plants. Plant Physiology 85(3):801–803. doi:10.1104/pp.85.3.801
3. Arnon, D.I. and Stout, P.R. (1939). The essentiality of certain elements in minute quantity for plants with special reference to copper. Plant Physiology 14(2):371–375. doi:10.1104/pp.14.2.371