container gardening, soil - Ask Extension
Burpee and other seed companies tell me NOT to reuse potting soil for a new season's "crop." Nowhere is there a suggestion what should be d...
Knowledgebase
container gardening, soil #892449
Asked February 08, 2025, 1:44 PM EST
Burpee and other seed companies tell me NOT to reuse potting soil for a new season's "crop." Nowhere is there a suggestion what should be done with the used last-season potting soil.
QQ.
1. Is it mostly moisture-maintenance differences that order me to use potting soil for my container garden instead of plain old soil???
2. Really? Should not reuse last season's soil for the new season, even if fortified with fresh compost???
In advance, thank you.
Baltimore City County Maryland
Expert Response
The recommendation to not re-use potting mix comes from the precaution of avoiding a repeat of any infections that may have occurred the prior year. Spores of relatively minor (like powdery mildew) and devastating (like Fusarium Wilt) diseases can overwinter on or in the soil and affect next year's plantings. Granted, such pathogens could also always blow-in from nearby areas, but having spores immediately surrounding the plant would increase the risk of disease.
Other reasons for not re-using potting mix include avoiding root stress and damage from a buildup in salts (residues left from both unused fertilizer minerals and minerals in the water source), plus maintaining good drainage. Over time, the components in potting mixes degrade and wear-down into smaller particles, which reduces the pore space between them that holds oxygen for root health, risking over-watering or at least poorer root growth. The rate of degradation can vary between potting mix ingredients (peat moss versus perlite, for example, the latter of which can disintegrate after multiple freeze-thaw cycles) and with how many microbes or other soil life is living in the container and gradually decomposing any organic matter in the mix.
Used potting mix can be added to a compost pile or spread in the garden anywhere it's useful (to fill-in ruts from downspout drainage, perhaps). Granted, to conserve materials and reduce costs, re-using it at least one or two seasons more may be ideal if the plant in the container(s) last year didn't suffer from any serious diseases like root rot. Adding about 50% new material (more potting mix, or some compost or more perlite, say) can help to improve drainage and dilute any salt residues present. Don't use too much compost because that will also subside over time, sinking the plants deeper into the container, as well as potentially drying out more frequently or, depending on other factors, retaining too much moisture for roots to stay healthy. You may need to experiment and see what works well for your situation, both in terms of plant care and what particular plants are being grown.
You generally do not want to use actual soil (sand, silt, or clay) for container growing, because it will tend to hold too much water and not give roots access to enough oxygen. Here too, plant tolerances will vary, so if you had a moisture-loving species like an Elderberry shrub in a large pot with a portion of the potting mix being a clay loam, it might be fine, whereas lettuce plants in the same situation might not be as adaptable. Since compost is not technically soil (it does not contain any mineral soil like the aforementioned sand, silt, or clay), it can be more useful in containers, but it ideally should not comprise the entirety of the media, just part of a mix of other ingredients that may include peat moss, coco coir, fine pine bark, perlite, vermiculite...etc.
Miri
Other reasons for not re-using potting mix include avoiding root stress and damage from a buildup in salts (residues left from both unused fertilizer minerals and minerals in the water source), plus maintaining good drainage. Over time, the components in potting mixes degrade and wear-down into smaller particles, which reduces the pore space between them that holds oxygen for root health, risking over-watering or at least poorer root growth. The rate of degradation can vary between potting mix ingredients (peat moss versus perlite, for example, the latter of which can disintegrate after multiple freeze-thaw cycles) and with how many microbes or other soil life is living in the container and gradually decomposing any organic matter in the mix.
Used potting mix can be added to a compost pile or spread in the garden anywhere it's useful (to fill-in ruts from downspout drainage, perhaps). Granted, to conserve materials and reduce costs, re-using it at least one or two seasons more may be ideal if the plant in the container(s) last year didn't suffer from any serious diseases like root rot. Adding about 50% new material (more potting mix, or some compost or more perlite, say) can help to improve drainage and dilute any salt residues present. Don't use too much compost because that will also subside over time, sinking the plants deeper into the container, as well as potentially drying out more frequently or, depending on other factors, retaining too much moisture for roots to stay healthy. You may need to experiment and see what works well for your situation, both in terms of plant care and what particular plants are being grown.
You generally do not want to use actual soil (sand, silt, or clay) for container growing, because it will tend to hold too much water and not give roots access to enough oxygen. Here too, plant tolerances will vary, so if you had a moisture-loving species like an Elderberry shrub in a large pot with a portion of the potting mix being a clay loam, it might be fine, whereas lettuce plants in the same situation might not be as adaptable. Since compost is not technically soil (it does not contain any mineral soil like the aforementioned sand, silt, or clay), it can be more useful in containers, but it ideally should not comprise the entirety of the media, just part of a mix of other ingredients that may include peat moss, coco coir, fine pine bark, perlite, vermiculite...etc.
Miri
