Amendments for indoor growing: naturally boost your substrates

Amendments are those little extras that transform a basic potting mix into a champion substrate. In indoor growing, they improve soil structure, optimize water retention, and provide progressive nutrition to your plants. Unlike liquid fertilizers that act quickly, amendments work deeply over several weeks.

Whether it's to lighten overly compact soil with perlite, enrich with organic matter using compost, or improve retention capacity with vermiculite, each amendment plays a specific role in the ecosystem of your growing space.

Why use amendments in indoor growing

Indoor cultivation places particular constraints on your substrates. In a closed pot, the soil compacts quickly, oxygenation decreases, and nutrients deplete faster than in open ground. This is where amendments become essential.

Perlite, those small expanded white beads, creates air pockets in the substrate. The result: roots breathe better and drainage drastically improves. A potting mix with 20% perlite maintains an aerated structure even after months of intensive watering.

Organic amendments like vermicompost or composted manure release their nutrients gradually. No more concentration spikes that can burn roots – here, it's gentle nutrition spread over 6 to 8 weeks. Vermiculite, on the other hand, acts like a sponge: it can retain up to 5 times its weight in water while remaining aerated.

For growers who recycle their substrates, amendments help regenerate tired potting mix. A well-thought-out blend revives exhausted soil and avoids having to buy new substrate for each cycle.

How to choose your amendments according to your needs

The choice of an amendment primarily depends on the problem to be solved. Is your potting mix waterlogged and roots rotting? Head for perlite or expanded clay pebbles. Granulometry 2-6mm for optimal drainage without small particles rising to the surface.

If nutrition is more of a concern, opt for organic amendments. Vermicompost provides balanced NPK (around 1-1-1) plus a host of beneficial microorganisms. Composted chicken manure, richer in nitrogen (3-2-1), is better suited for the vegetative growth phase.

To improve water retention without sacrificing aeration, vermiculite remains essential. It absorbs excess water during heavy watering and releases it when the substrate dries out. Practical when you go away for the weekend and can't water.

Coco fiber, technically an amendment, excels at lightening heavy potting mixes. With a neutral pH around 6.5 and excellent retention capacity, it blends perfectly with traditional substrates in proportions of 20 to 30%.

Frequently asked questions

Which amendment should I choose to improve substrate aeration in indoor growing?

Perlite remains the undisputed champion for aeration. These small expanded volcanic beads create lasting air pockets in the substrate. Aim for 15 to 25% of the total volume of your mix for a noticeable effect. Expanded clay pebbles also work very well, especially in 4-8mm granulometry. They are heavier than perlite but do not rise to the surface during watering. For tight budgets, coarse river sand (2-4mm) will do the trick, even if it makes the substrate heavier.

How do I properly mix amendments into my potting mix without the risk of over-dosing?

The secret is to respect the proportions and mix dry before moistening. For perlite or vermiculite: maximum 25% of the total volume. For organic amendments like compost: 10 to 15% is largely sufficient. First, mix all dry elements in a large container, then gradually moisten while stirring. Avoid exceeding 30% total amendments, otherwise you risk completely unbalancing the physical properties of the substrate. A potting mix containing 40% various amendments no longer resembles a potting mix.

What is the difference between chicken manure and vermicompost for my plants?

Composted chicken manure is richer in nitrogen (NPK around 3-2-1) and acts faster, ideal for boosting vegetative growth. Be careful with the dosage: 5% of the maximum volume, otherwise beware of root burn. Vermicompost, gentler with a balanced NPK around 1-1-1, primarily provides beneficial microorganisms and enzymes. It improves soil structure in the long term and can be used up to 15% of the volume without risk. For a complete cycle, vermicompost is more versatile, while chicken manure excels only during the growth phase.

When and in what quantity should amendments be added during soil preparation?

Amendments should always be mixed before planting, never on an already established substrate with roots. Prepare your mix 2-3 days before use so that everything stabilizes. For organic amendments, respect these dosages: vermicompost 10-15%, composted manure 5%, homemade compost 10%. For mineral amendments: perlite 15-25%, vermiculite 10-20%. If you use several amendments, proportionally reduce each dosage. A substrate with 15% perlite + 10% vermicompost + 5% manure gives excellent results without unbalancing the mix.

How to maintain amended substrate between two growing cycles for reuse?

After harvest, remove old roots and pass the substrate through a coarse sieve to eliminate large debris. Add 5% fresh compost or vermicompost to restart biological activity. If the substrate has compacted, incorporate an additional 5% perlite. Let it rest for 2 weeks while maintaining slight humidity – microorganisms need time to recolonize the environment. Avoid reusing the same substrate more than 3 times: beyond that, even with amendments, the physical structure degrades and health risks increase.