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.

Myco-WS, for example, supplies mycorrhizal fungi and Trichoderma. The mycorrhizal fungi alone are the kind that would take seasons to build up naturally in a tilled bed. TinyTeam adds a broader community of beneficial bacteria and fungi straight into the root zone.

SeaBrix is a foliar input, sprayed on the leaves, brewed from kelp, fish hydrolysate, moringa and AminoK. Compost cannot do this. Leaves have their own microbe community, and feeding it directly through a foliar spray supports the plant's natural resilience.

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 (Myco-WS, TinyTeam), feed the plant directly during growth (SeaBrix, AminoK), and support flowering and fruit set when the plant needs it (RealCal).

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 here: build healthier soil →

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