Soil Biology for Houseplants: Microbes, Humics, Kelp — What They Actually Do

If you’ve ever tipped a plant out of its pot and thought, “Huh… it’s just brown stuff,” this one’s for you.

When we talk about “good soil” for houseplants, we’re not really talking about one thing. It’s not just the brown bits, and it’s not just the biology. It’s the structure (air and water balance), the chemistry (nutrients, pH, salts) and the life inside it – the microbes, the by-products of their work (like humic substances), and the biostimulants (like kelp) that help the whole system tick along.

Indoors, your plants are growing in a closed little world. No worms migrating in from the neighbour’s garden, no leaf litter drifting down from a gum tree. Technically, most “soils” in pots are soilless potting mixes – bark, coir, perlite and friends – but the biology we’re aiming for behaves a lot like a miniaturised living soil.

If you don’t actively build that biology in your pots… it’s mostly not happening.

Where This Fits in the Plant Runner Method

At Plant Runner, we think about plant problems (and solutions) in a specific order:

1. Light → Is there enough light for this plant to actually grow?
2. Water → Are you watering in a way the pot and plant can handle?
3. Mix → Is the potting mix giving roots air, structure and a stable home?
4. Feed → Are you topping up nutrients and supporting soil life?

We always start with light and water. No amount of fancy biology can make up for a plant in a dark hallway getting drowned every three days.

This guide zooms in on the last two steps – Mix and Feed – because that’s where soil biology really lives. We’ll keep it grounded in the Plant Runner Method so you know not just what microbes, humics and kelp do… but when to care about them.

Potting Mix vs “Living Soil”: Why Biology Matters in a Pot

Out in a garden bed, plants plug into a huge underground network: fungi, bacteria, insects, organic matter constantly breaking down. It’s messy, dynamic and self-renewing.

In a pot, it’s a very different story:

• You’ve got a finite mix in a plastic, ceramic or concrete container.
• Water and nutrients can only move so far before they hit the edge.
• Every time you water, some nutrients leave through the drainage holes.
• Repotting every 1–2 years disrupts whatever biology has built up.

Most bagged potting mixes are designed to be clean and consistent, which is great for avoiding nasties but also means they’re often pretty low in biology to begin with. Left alone, the mix slowly becomes:

• Less structurally stable (collapsing and compacting over time)
• Less able to hold nutrients without them flushing straight through
• Less hospitable to roots (poor aeration, hydrophobic pockets, weird watering behaviour)

Biology is what stops your pot from becoming “dead substrate” with a plant just hanging on top. Your goal indoors isn’t to recreate an entire forest floor – it’s to nudge your potting mix in that direction so roots can actually relax and do their thing.

Meet the Microbes: The Tiny Workers in Your Pot

Think of soil microbes as your plant’s unseen support crew:

• Bacteria – quick-turnover decomposers that break down organic matter, cycle nutrients and form protective biofilms around roots.
• Fungi – long, thread-like networks that help move water and nutrients through the mix and literally glue particles together into nice, crumbly structure.
• Other oddballs (actinomycetes, protozoa, etc.) – they help break things down, recycle nutrients and keep the whole system balanced.

Roots don’t just sit there passively; they “chat” with microbes by leaking sugars and other compounds. In return, beneficial microbes:

• Break down organic matter into plant-available nutrients
• Help hold onto nutrients so they’re not lost with every watering
• Influence root growth and architecture
• Contribute to that “fresh soil” smell and feel

Not every microbe is a hero, but when you have a diverse, well-fed community, the good actors tend to dominate and support healthier plants.

Where Soil & Microbe Booster Fits (Mix Step)

In the Plant Runner Method, Soil & Microbe Booster is your “Mix upgrade”. It’s how you turn a standard potting mix into something much closer to a living soil system.

It’s built on two key ingredients:

• Frass – insect manure from black soldier flies
  • Provides a gentle, slow-release source of nutrients (a modest NPK plus trace elements)
  • Introduces and supports beneficial bacteria and fungi
  • Contains chitin (from shed exoskeletons), which can act as a signal that encourages plants to prime their own in-built defence systems and general resilience
• Biochar – a charcoal-like material with loads of internal pores
  • Gives microbes long-term “real estate” to live in
  • Can help the mix hold both water and nutrients
  • Helps improve structure so roots get more air and less compaction when used at sensible rates in potting mixes

Blend Soil & Microbe Booster through a good indoor mix and you’re helping shift it from “inert-ish substrate” towards a living, buffered, microbe-rich system.

In practice, that usually means plants that:

• Cope better with the odd over- or underwatering
• Make better use of the fertiliser you’re already adding
• Often get a bit more life out of their mix before you’re forced to repot again

You’re not eliminating the need to repot forever, but you are helping the mix stay functionally “alive” for longer – which ticks the “Mix” box of the Plant Runner Method very nicely.

Humics: The Dark, Crumbly Magic Behind Healthy Soil

“Humics” is a catch-all term for humic and fulvic acids – complex organic compounds formed as plant and animal material fully breaks down. You can think of them as:

• A sponge – helping hold onto water and nutrients
• A chauffeur – helping micronutrients stay soluble and move to where roots can actually grab them
• A buffer – smoothing out pH swings and salt spikes that can otherwise stress roots

In potting mixes, humic substances help to:

• Contribute to higher nutrient-holding capacity (cation exchange capacity, or CEC)
• Help keep trace elements like iron, zinc and manganese more available to plants
• Support microbial activity by giving them complex carbon compounds to chew on

So even if your fertiliser has the “right” NPK on the label, you can still run into:

• Yellowing new growth because iron isn’t available
• Crispy tips because salts build up near the root zone
• Plants that seem “fed” on paper but just won’t kick into gear

Humics are the behind-the-scenes crew that help nutrients behave themselves and actually get into the plant without causing drama.

Kelp: Liquid Seaweed & Stress Support

Kelp (seaweed extracts) gets thrown around a lot, but what is it actually doing for your indoor plants?

Seaweed is packed with natural compounds that function as biostimulants. Instead of being a straight source of NPK, they work more like a wellness program for roots and foliage:

• Encouraging strong, branching root systems
• Helping plants ride out stress (heatwaves, minor drought, repot shock)
• Supporting general vigour, leaf quality and recovery after pruning or damage

You’ll sometimes see words like auxins, cytokinins and gibberellins – these are plant hormones or hormone-like compounds naturally present in kelp. You don’t need to memorise them. The takeaway is:

Kelp doesn’t just “feed” plants – it helps them use nutrients better and cope with stress more easily.

Where Organic Plant Food Fits (Feed Step)

In the Plant Runner Method, Organic Plant Food lives firmly in the “Feed” step – and specifically as a soil food.

Alongside its 13–3–8 NPK, it includes:

• Humic acids – to help improve nutrient retention and availability in the mix
• Beneficial microbes – to keep nutrient cycling ticking along
• Kelp – to support root development and stress tolerance

So instead of blasting your mix with a high-salt hit that plants have to tolerate, you’re:

• Topping up nutrients in a low-salt, root-friendly way
• Feeding the microbial community you’ve built with your mix
• Gently improving the soil environment over time

We like using Organic Plant Food as a regular “soil reset” – for example, once a month in place of your usual feed – to keep that underground engine humming.

Seaweed extracts have been shown in lots of crops to support root growth and stress resilience. For houseplants, you’ll usually notice the biggest difference when plants are bouncing back from something – a repot, a dry spell, a bit of heat stress. The growth that comes back is usually tighter, more balanced, and less “leggy desperation”.

How This Actually Looks in Your Pots

Let’s compare two imaginary Monstera deliciosa (because of course it’s a Monstera).

Plant A: Generic Mix, Generic Fertiliser

• Potting mix starts out fine, then slowly compacts and loses structure.
• Water starts pooling on top then shooting out the drainage holes.
• Fertiliser salts build up near the root zone, nudging pH around and occasionally burning tips.
• Microbial life doesn’t disappear, but it becomes sparse and unbalanced – more like a ghost town than a thriving city.
• Plant looks okay… but growth is a bit stop–start and recovery from stress is slow.

Plant B: Good Mix + Soil & Microbe Booster + Organic Plant Food

• Biochar and frass help maintain structure and water-holding.
• Microbes constantly break down organic matter and recycle nutrients.
• Humics contribute to better nutrient holding and availability.
• Kelp and microbes together support root health and steady top growth.
• Plant rides out the odd mistake (late watering, missed feed, a hot day) with less drama.

Same plant. Same house. Different underground world.

Using the Plant Runner Method with Soil Biology

When a plant’s struggling, run it through the Plant Runner checklist: Light → Water → Mix → Feed.

1. Light

• Is the plant within a couple of metres of a window?
• Can you comfortably read next to it during the day without turning a light on?

If not, move it before you touch the potting mix or fertiliser. Biology can’t fix a dark corner.

2. Water

• Stick a finger into the mix – what’s happening 3–5 cm down?
• Are you giving it a proper soak, then letting it drain and dry a bit, or just tiny sips?
• Is water pooling on top or racing out the drainage holes instantly?

Smooth out the watering rhythm first. Good biology likes the same thing roots do: a clear “soak, then breathe” cycle.

3. Mix (Soil & Microbe Booster)

Once light and water are in a reasonable place, look at the mix:

• When did you last repot? If it’s been a few years, the mix may simply be tired.
• Does the mix feel dense and compacted, or light and crumbly?
• Does water soak in, or bead on the surface and run off the sides?

At your next repot, use a quality indoor mix and blend Soil & Microbe Booster through it, focusing around the root zone. For plants you’re not ready to repot yet, sprinkle Soil & Microbe Booster as a top-dress and water it in to give the existing mix a biological boost.

4. Feed (Organic Plant Food)

• Check what you’re using to feed – is it a salty, synthetic hit or something biology-friendly?
• Swap one of your regular feeds each month for Organic Plant Food to top up humics, microbes and kelp.

That way, every time you feed you’re not just dosing leaves – you’re supporting the whole soil system.

Bringing It All Together

If you strip away the fancy words, soil biology for houseplants within the Plant Runner Method comes down to three simple ideas:

1. Microbes in a good mix help break things down, cycle nutrients and support root function (Mix).
2. Humics help your mix hold onto water and nutrients and keep them available (Mix + Feed).
3. Kelp (and other biostimulants) help plants use those nutrients more efficiently and cope better with stress (Feed).

Indoors, those things don’t just appear. You have to invite them in – after you’ve sorted light and water.

That’s why we created our Soil & Microbe Booster (to give your pots a biological engine and longer-lasting structure at the Mix step) and Organic Plant Food (to keep that engine running in a low-salt, biology-friendly way at the Feed step).

Light and watering still do the heavy lifting. But when the soil life is humming along underneath, your plants have a much bigger margin for error – and that’s when indoor gardening starts to feel relaxing instead of slightly terrifying.