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When I was a boy…cheap potting mix and a bag of wood chips were different things. Now, though, there doesn’t seem to be much of a difference…
The problem with buying these cheapest potting mix (I’ll not name the brand but pictured, its a general thing) is that it’s mostly wood chips. When you plant into it, the microbes that find themselves there and in the soil that it rests on, take nitrogen out of whatever nutrients are in there in order to break down the wood. That means that there’s less for the plants and their leaves go yellow.
Nitrogen is a crucial nutrient – it is used to make proteins, including the enzymes that bacteria use to break down nutrients into plant accessible forms. It’s most important role, though, is that it is part of DNA and RNA, without which, growth and life itself is impossible.
Lack of nitrogen shows itself as yellowing in the older leaves at first, as the plant reclaims it and moves it to new growth. Eventually though, a long term deficiency will yellow the whole plant.
The key when using this kind of cheap potting mix is the same as when you’re using woodchips as mulch. That is to add a nitrogen rich fertilizer such as very dilute chook poo before you use the mix. That’ll add the nitrogen needed to break down the wood and leave enough for the needs of the plants that you plant into it.