What is the difference between a fertiliser and a soil improver?

These are two different jobs and confusing them produces both of the classic failures: a border that gets a bottle of feed every fortnight and has soil like dust, and a border buried under compost every year that still cannot produce a decent crop of runner beans. Most gardens need both, applied for different reasons on different schedules.

Step by step

1. Understand what a fertiliser does.Supplies nitrogen, phosphorus, potassium and sometimes trace elements in concentrated, plant-available form. It is measured in grams per square metre or millilitres per litre because a little goes a long way. It acts within days for a mineral product or weeks for an organic one, and then it is gone.
2. Understand what a soil improver does.Supplies bulk organic matter. It opens up clay, helps sand hold water and nutrients, feeds earthworms and soil microbes, buffers pH, and builds the crumb structure that decides whether roots and water can move. It is measured in cubic metres and 50mm layers because it works by volume.
3. Note that the improvers do contain nutrients — just not many.Garden compost is typically well under 1 per cent nitrogen. Spread 50mm deep across a bed that is a real contribution over a year, but it will not carry a heavy crop through a season the way a fertiliser will. Treating compost as a feed leads to hungry plants.
4. Note that some products do both.Well-rotted manure and chicken manure pellets sit in the middle — manure is bulky enough to improve soil and rich enough to feed, and chicken pellets are concentrated enough to be a fertiliser while still adding organic matter. Both are genuinely dual-purpose.
5. Identify what the label calls it.'Soil conditioner' and 'soil improver' on a bag mean it is sold on volume and organic matter, not nutrients, and it will usually have no NPK declaration or a very low one. A product with a stated NPK analysis and an application rate in grams per square metre is a fertiliser.
6. Apply each on its own schedule.Improvers: once a year, in autumn or late winter, as a 25 to 50mm surface mulch, indefinitely. Fertilisers: at the start of the growing season and during it, matched to what the plant is doing, and stopping in late summer.
7. Get the priority right for open ground.If you are only going to do one thing to a border, do the compost mulch. Over ten years it changes the soil permanently; ten years of liquid feed changes nothing that lasts past the season. Feeding is the correction on top of that, not the foundation.
8. Get the priority right for containers.Reversed. A pot has a fixed volume of compost and no soil biology to speak of, so structure declines and nutrients leach out fast. There, feeding is the primary activity and repotting into fresh compost is how you handle the structural side.

Tips

• Green-waste compost sold in bulk is a soil improver, not a feed, and is priced accordingly. It is usually the cheapest way to get organic matter onto a garden.
• Leaf mould is the purest soil improver there is — almost no nutrient value, superb for structure and water holding, and free if you have trees.
• Biochar, rock dust and clay-breaker products are marketed alongside improvers but do something different again. Their evidence base at garden scale is much thinner than compost's, which is close to unarguable.
• A soil that has had organic matter added for five years needs measurably less fertiliser to produce the same crop, because it holds nutrients rather than letting them wash through.

Questions people ask