Can I reuse soil from my last grow?

Q: Can I reuse soil from my last grow?

Yes, you can reuse soil from your last grow if the plants were healthy. Break up the root ball, add fresh compost or worm castings, slow-release organic amendments and perlite, then keep it moist for 2–4 weeks before planting. Don't reuse soil from plants with root rot, wilt or root pests. Coco can be rinsed and re-buffered; living soil is reused in place, no-till.

Short answer

Yes, reuse soil from a healthy grow. Break up the root ball, mix in fresh compost or worm castings, slow-release organic amendments and perlite, and keep it moist for 2–4 weeks before planting. Don't reuse soil from plants with root disease or root pests. Grower practice, consistent with established soil-health principles.

Why

A finished run leaves the mix depleted, not ruined. The plant used up the starter charge, and the peat or coco has broken down a little, so the mix holds more water and less air ^[4].

Old roots are food. Left in the mix, they decompose and feed soil life, and keeping soil covered and biologically active is a core soil-health principle ^[1].

The danger is disease. Fusarium, Pythium and similar pathogens survive in old roots and media ^[2]^[3]. Soil from a sick crop carries them into the next one.

Living soil goes further: it isn't emptied at all. Stems are cut at the surface, roots stay, and the bed is top-dressed and replanted (no-till) ^[1].

The numbers

Step	Amount or time
Fresh compost or worm castings	About 20–30% of volume
Extra perlite or pumice	10–20% to restore air space
Slow-release organic amendment	Label rate for the volume
Rest, moist and warm	2–4 weeks before planting
Coco: rinse	To runoff EC below about 0.5 mS/cm, then re-buffer
Pot disinfection	10–15% household bleach solution ^[2]

What to do

Check the last crop. Any root rot, wilt or root aphids: bag and discard the soil.
Dry the old mix slightly, crumble it and pull out the large root masses.
Mix in compost or castings, perlite and a slow-release amendment.
Rehydrate, cover and leave it warm and moist for 2–4 weeks.
Plant, water with plain water, and feed only when lower leaves begin to pale.
Wash pots and trays. Wear gloves and eye protection with bleach, and never mix it with acids or ammonia.

Common mistakes

Reusing soil from a crop that wilted or had brown roots.
Planting seedlings straight into freshly amended "hot" soil.
Reusing the same mix for many runs without restoring air space.
Sterilising healthy soil and killing its biology.

Full article: Growing in soil
Answers: what is super soil · best soil for cannabis · when soil runs out of nutrients
Systems: living soil
Terms: top-dressing · no-till

An organic fertiliser supplies nutrients from plant, animal or mineral-rock sources that soil microbes must break down into ions before roots can absorb them.Perlite is a light, white, puffed volcanic glass added to growing mixes to increase drainage and air space; it is chemically inert and holds no nutrients.Decay of roots by pathogens such as Pythium, Fusarium, Rhizoctonia or Phytophthora, usually triggered by low oxygen, heat and waterlogging around the roots.Peat is partially decomposed sphagnum moss from bogs, the base of most potting mixes; it holds water and nutrients well but is acidic and needs lime.Coco coir is a growing medium made from processed coconut husk, prized for holding water and air at once; it binds calcium and magnesium unless buffered.A soil-borne fungal disease, chiefly Fusarium oxysporum, that blocks water-conducting tissue, causing one-sided yellowing, wilting and brown streaks inside the stem.A genus of oomycete water moulds, such as Pythium myriotylum and P. aphanidermatum, that cause brown, slimy root rot, especially in warm hydroponic systems.Living soil is an organic growing medium managed to sustain an active soil food web, so microbes and invertebrates release nutrients and the soil is reused across cycles.No-till growing reuses living soil from cycle to cycle without turning or replacing it, cutting out the old root ball and replenishing with top-dressed amendments.Air-filled porosity is the share of a container's volume that holds air rather than water after it has drained, a key factor in root oxygen and overwatering risk.Runoff is the water or nutrient solution that drains out of the bottom of a container after irrigation; growers test it to see what is happening in the root zone.EC (electrical conductivity) measures how well a solution conducts electricity, a proxy for total dissolved salts; growers read it in mS/cm to gauge nutrient strength.mS/cm (millisiemens per centimetre) is the standard unit of EC for nutrient solutions; 1 mS/cm equals 1 dS/m and 1,000 µS/cm.Aphids that colonise roots and the crown below the medium surface, on cannabis chiefly the rice root aphid (Rhopalosiphum rufiabdominale), causing slow, unexplained decline.A seedling is a young cannabis plant from emergence of the cotyledons until it has several sets of true leaves, typically the first 2-3 weeks.

Follow-up questions

Do I need to sterilise old soil before reusing it?

No, you don't need to sterilise old soil from a healthy grow, and doing so kills the beneficial organisms you want. Sterilising or pasteurising only makes sense if the last crop had root disease, and then discarding the soil is usually simpler. Instead, re-amend it with compost and nutrients, rehydrate it and let it rest moist and warm for 2–4 weeks.

Can I reuse soil from plants with root rot or pests?

Don't reuse soil from plants that had root rot, Fusarium wilt, root aphids or a heavy fungus gnat infestation. These organisms survive in old roots and media and will attack the next crop. Bag the soil and dispose of it away from the grow, then wash pots and trays and disinfect them with a 10–15% household bleach solution before reuse.

Can I reuse coco coir?

Yes, coco coir can be reused after a healthy run. Remove as much root mass as you can, rinse until runoff EC is below about 0.5 mS/cm, and re-buffer it with a calcium-magnesium solution before replanting. Root debris raises water holding and disease risk, so add some fresh coco and perlite, and don't reuse coco more than a couple of times.

Sources

USDA Natural Resources Conservation Service. Soil health principles (minimise disturbance, keep soil covered, maintain living roots, maximise biodiversity). nrcs.usda.gov/conservation-basics/natural-resource-concerns/

Utah State University Extension. Fusarium Wilt (hemp). extension.usu.edu/planthealth/ipm/notes_ag/hemp-fusarium-wil

Punja ZK, et al., 2019. Pathogens and Molds Affecting Production and Quality of Cannabis sativa L. Frontiers in Plant Science 10:1120. doi.org/10.3389/fpls.2019.01120

Raviv M, Lieth JH, Bar-Tal A (eds), 2019. Soilless Culture: Theory and Practice, 2nd ed. Elsevier (organic substrates and compost chapters). doi.org/10.1016/C2015-0-01807-5