The standard recommendation is to use new soil when repotting houseplants or planting containers in spring. This seems to make a lot of sense. Containerised plants grow in very little soil, and giving them fresh stuff should be good for them; besides, the nutrients in the old soil have been used up, and it may contain diseases.
What is potting soil? In most cases, there is very little soil in potting soil. The main ingredients are usually peat moss, decomposed wood or coir (ground coconut husk). These materials produce a very light mix with lots of air for plant roots, contain few nutrients and tend to be weed-free.
Potted plants need to be fertilized for several reasons. The amount of soil is small and does not provide many nutrients for a crowded root system. They also dry out quickly and are watered much more often than plants growing in the ground. This extra water flushes nutrients out of the pot. The amount of nutrients in new and old soil is about the same, so replacing it will not eliminate the need for fertiliser.
Extra watering can cause a problem in old soil if you have hard water. Hard water contains a lot of calcium and magnesium, which will combine with fertiliser to turn into an insoluble material that can be seen as a white deposit on the outside of clay pots. This is very similar to the white coating you find in your kettle, which is calcium and magnesium left behind because of evaporation. Even if you can’t see it in your container, it is forming over time.
Some plants, notably orchids, are very sensitive to these salt deposits. If the potting media contains too much, roots stop growing. This is one of the reasons that orchids should be repotted every two years and the old media should not be reused for orchids. Most plants are not this sensitive to salts.
Are diseases a problem? It is true that old potting soil can contain bacterial and fungal diseases. If your plant was unhealthy, these diseases can infect your new plant, but old soil from healthy plants is unlikely to transmit diseases to new plants. Replacing the soil with new uninfected soil really does not help. If the disease spores were in the old pot, they are floating around in the air and will quickly find your new soil. Healthy plants are able to fend off these types of infection.
There is also talk about soil getting old. What does this mean? In nature, soil has been around for millions of years. How old does it need to get before you need to replace it? The organic matter that makes up 99% of soilless potting soil will decompose over time. As this happens, the material is turned into nutrients and CO2, and it contains more fine particles. There will be less of it over time, but the idea of it getting too old to grow plants has no basis.
In most cases, old potting soil can be reused for many years.
This article is Myth #88 from Garden Myths: Book 1, available in the UK online from Amazon here: https://www.amazon.co.uk/Garden-Myths-Mr-Robert-Pavlis/dp/1542465222/ref=sr_1_1?dchild=1&keywords=garden+myths&qid=1605707945&sr=8-1
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