Soil Amendments and Living Soil: Worm Castings, Frass, Pre-Mix

Soil amendments are what you mix into a potting soil, or spread on top, to change how it drains, holds water or feeds the plant. This guide explains the soil amendments we sell, from worm castings and insect frass to Biobizz Pre-Mix, and the mycorrhizal products that bring fungi and bacteria to the roots. It shows which of our soils already contain them, when to add each, and what growers mean by living soil.

The soil amendments we sell

Perlite and vermiculite add no food; the amounts above for them come from our perlite and vermiculite guide.

Worm castings: Biobizz Worm Humus

Worm castings are what earthworms leave after digesting organic matter; growers add them for gentle food and soil life. The Biobizz Worm Humus page describes a fully organic worm cast from Holland and gives an inclusion rate of approx. 10-15%: part of the mix, not the mix itself.

Add them to a light soil. The Biobizz All Mix page says the main difference between it and Biobizz Light Mix is the worm castings and Pre-Mix, so All Mix already has both. Light Mix, or Plagron Lightmix Soil (nutrient content kept to a minimum, its page says), is the one to build up.

Biobizz Pre-Mix

Pre-Mix is for enriching a soil before you plant; our Biobizz feed chart guide names it and Worm Humus as Biobizz's own ways to do that. The Pre-Mix page describes an organic-based fertiliser, NPK 3-3-5, in a 5 L tub or 25 L bag. All Mix contains Pre-Mix, and its page suggests pre-wetting the filled pots 24-48 hours before potting up, which it says stimulates microbial activity and activates the Pre-Mix. Growers commonly do the same with a home-made mix.

Insect frass: Ecothrive Charge

Frass is insect droppings. The Charge page describes a Soil Association approved, 100% organic soil improver made from the droppings of Tenebrio molitor beetles - dried mealworm waste. It says Charge helps promote a healthy micro herd in soil and coco, contains chitin derivatives that help the plant defend against pests, bacteria and fungal disease, and makes its NPK 3/2/3 available over 3-4 weeks.

It comes in 500 g, 1.75 kg and 3.5 kg packs, with no rate on the page, so follow the pack.

Mycorrhizae and microbes

The Great White page says mycorrhizal fungi improve the roots' uptake of water and nutrients through networks of fungal filaments. The microbial products in our root products range include Great White, Plant Magic Oldtimer Granules, Ecothrive Biosys and Biobizz Microbes, whose page says they help break down organic matter. Our root stimulants guide compares what is in each, how it is used and which media it suits. The Great White and Oldtimer Granules pages say to get the product in contact with the roots, so add them at potting up.

Pick living or sterile, not both: the Oldtimer Granules page says peroxide-based products kill or severely inhibit micro-life, and Liquid Oxygen is a hydrogen peroxide solution (see our root rot guide).

Check the bag first

Several of our premium soils already contain an amendment: check before you add the same again.

Our which potting soil guide compares the soils and how long each one feeds.

Living soil: organic growing in pots

Living soil is what growers call a mix rich in organic matter and alive with microbes: you feed the soil life, and it feeds the plant. Castings, frass and Pre-Mix add organic matter. Beyond that:

• Feed the soil. Top-dress Charge as the plants grow (step 4 below). For a liquid feed, pick one that works through the soil: the Biobizz Grow page says it works by activating bacteria in the soil, and the Biobizz Fish Mix page that it can replenish old or poor soil with microbial life. Use one, not both, as our Biobizz feed chart guide says, and go by that chart rather than an EC meter.
• Water that spares the microbes. The Ecothrive Neutralise page says it neutralises the chlorine and chloramine in tap water, chlorine compounds that kill microbes, and that standing water for 24 hours is no longer an effective fix; Ecothrive recommend 1 drop per litre.
• Fungus gnats. Damp, organic-rich compost suits them, so let the surface dry between waterings (see our fungus gnats guide).

When to add each: what to do

1. Pick the soil. Pre-fed soils (All Mix, Batmix, Gold Label) already carry food; to add your own, start from a light soil.
2. Before potting into final pots, mix in Worm Humus at approx. 10-15%, Pre-Mix or Charge at the pack rate (both carry NPK, so as a rule of thumb start with one), and perlite if the mix stays wet. Pre-wet the filled pots 24-48 hours ahead. Keep seedlings and cuttings in a light mix in 1 or 2 litre pots first, as the All Mix page suggests (see our sowing seeds guide).
3. At every potting up, put Great White or Oldtimer Granules in the hole, where the roots will touch it.
4. As the crop grows, top-dress Charge at the pack rate and add Microbes as the Biobizz chart shows. Liquid feed: see our which nutrients guide.
5. In coco (see our coco coir guide), use the ones whose pages say coco: Charge, Oldtimer Granules, Great White and Biosys, plus Microbes, whose page says any medium.

Quick answers

What is living soil?

A potting mix rich in organic matter and alive with microbes. From our range, growers use worm castings, frass, Pre-Mix and mycorrhizal products.

How much worm castings should I add to soil?

The Biobizz Worm Humus page gives an inclusion rate of approx. 10-15% of the mix. All Mix already contains worm castings, so add them to a light soil instead.

What is insect frass used for?

As an organic soil improver. Ecothrive Charge is dried mealworm frass; its page describes slow-release NPK 3/2/3 over 3-4 weeks, beneficial micro-organisms and chitin derivatives, for soil and coco. Growers commonly mix it in before potting and top-dress more later, at the rate on the pack.

When should I add mycorrhizae?

At potting up, in the hole, where the product touches the roots. Great White can also go on seeds and cuttings, or in the feed.

More Growers' Guides

• Which potting soil?
• Root stimulants and rooting gels
• Perlite vs vermiculite
• Biobizz feed chart
• Sowing seeds indoors

Not sure what you need? Call the shop on 0117 971 3000 or pop in to the Bristol store and we'll help you pick.