Can Compost Replace Fertiliser? How to Use Compost to Feed Your Plants

Compost is one of the most complete soil amendments available to gardeners, but it is often misunderstood as simply a soil improver rather than a fertiliser. In truth, good compost does both — and in many situations, regular compost applications can significantly reduce or even eliminate the need for additional fertiliser. The key is understanding what compost provides and where its limits lie.

What Nutrients Does Compost Contain?

Good garden compost contains all three primary nutrients — nitrogen, phosphorus and potassium — along with calcium, magnesium, sulfur and a wide range of trace elements. The NPK values are relatively low compared to a concentrated synthetic fertiliser (roughly 1-0.5-0.5 for typical garden compost), but the nutrients are released steadily over months as microbial activity continues. Unlike bagged fertilisers, compost also supplies organic matter that improves soil structure, water retention and drainage.

The Benefits Beyond Nutrients

The real value of compost extends well beyond its nutrient content. A well-made compost feeds the entire soil ecosystem — bacteria, fungi, nematodes, earthworms and other organisms that are essential for nutrient cycling. These organisms break down organic matter into forms plants can use, fix atmospheric nitrogen and help suppress soil-borne pathogens. In other words, compost does not just deliver nutrients; it builds the system that delivers nutrients.

How to Use Compost as a Fertiliser

For vegetable beds, dig or fork in a 5–10 cm layer of finished compost before planting each season. This is your primary background feed. For established beds, apply compost as a mulch around plants — 5–7 cm deep — and let rain and earthworms work it into the soil. For containers, replace or supplement potting compost each season since nutrients deplete quickly. For lawns, a thin top-dressing of fine compost worked in after scarifying improves soil fertility gradually.

When Extra Fertiliser Is Still Needed

Compost alone may not be enough for heavy-feeding crops or for plants in active fruiting. Brassicas, sweetcorn, courgettes and tomatoes in the ground can exhaust soil nitrogen faster than compost alone replenishes it. Container plants definitely need additional feeding through the season. For these situations, use compost as the base and supplement with targeted liquid feeds or granular fertilisers when the plant is at its hungriest growth stage.

Making Better Compost for Higher Nutrient Value

The nutrient content of compost varies with what went in. Compost rich in green material (grass clippings, vegetable peelings, fresh weeds) tends to be higher in nitrogen. Adding comfrey leaves, wood ash (for potassium) and seaweed to your heap improves the overall nutrient profile. Avoid composting only brown, woody material — it produces a good soil conditioner but is low in nutrients.