Compost vs biological inputs: what is the difference?

"If I already use compost, do I really need a biological product?"

We get this question every week. The answer is: probably yes, and the reason is that compost and biological inputs do related but different jobs. This guide explains the difference in plain language so you can decide what your garden needs.

What compost actually is

Compost is broken-down organic matter. Plant material, kitchen scraps, manure, leaves, all decomposed by the microbes that live in your compost heap. Good compost is dark, crumbly, smells earthy and stays warm in the middle while it is finishing.

Compost does three things for soil: it adds organic matter that improves structure and water-holding, it supplies a slow trickle of nutrients as it continues to break down in the bed, and it brings some living biology with it.

Compost is a good thing. Every garden should be making or buying some.

Where compost falls short

The biology in compost is the biology that grew on whatever you composted. A heap built from grass clippings has a different microbe profile to a heap built from leaves and kitchen scraps. Most home compost is heavy on bacteria and light on fungi, because the things that feed soil fungi (woody material, deep roots, old leaves) are usually scarce in a kitchen-scrap heap.

Compost also takes a long time to make. Three to six months for a proper batch. A small home heap might only produce 50 to 100 kg of finished compost a year, which is enough for a fraction of a typical food garden.

And compost gives you a generalist microbial community. It does not specifically target the partnerships that matter most for vegetable roots, like mycorrhizal fungi or the bacteria that fix nitrogen at the root.

What biological inputs add

Biological products supply specific, concentrated microbes and microbial food, targeted at the partnerships that matter for plant growth.

Beyond PSP, for example, supplies a starter charge of beneficial bacteria, fungi (including mycorrhizal fungi), kelp extract and humic substances. The mycorrhizal fungi alone are the kind that would take seasons to build up naturally in a tilled bed. The kelp extract supplies trace minerals and growth hormones that compost rarely contains in useful amounts.

SeaBrix™ is a foliar input, sprayed on the leaves. Compost cannot do this. Leaves have their own microbe community, and feeding it directly through a foliar spray supports the plant's natural defence against fungal diseases.

AminoK™ is an amino-acid foliar. It gives a stressed plant a quick source of building blocks for new growth. Compost does not do this either.

When to use which

Use compost as the bulk of your soil-feeding programme. Top up beds between plantings. Mulch with finished compost in autumn. Add to potting mixes.

Use biological inputs to do what compost cannot do: deliver a strong, specific microbial boost at planting (Beyond PSP), feed the plant directly during growth (SeaBrix, AminoK), and inoculate the soil with targeted microbes that are slow to build up naturally (TinyTeam).

The two work together. Compost is the bread. Biological inputs are the yeast. A home garden gets the best result from running both.

A simple test

If your beds get a good helping of compost twice a year, plants grow reasonably and you are happy with the result, you may not need a biological product yet. Carry on.

If your soil is tired, sandy or compacted, if you struggle to grow good crops despite adding compost, if you are starting a new bed on poor ground, or if you want to step up the result from what you are doing now, a biological starter is the fastest lift you can give the soil.

Start with a Patch Pack. It is the easiest way to add a biological boost to a garden that already uses compost. Shop the Patch Pack →

Further reading:
A Beginner's Guide to Feeding Your Soil | How It Works